ALBERT

Description: In this study, we present a new synthesis of GPS velocities for tectonic deformation within the Tibetan Plateau and its surrounding areas, a combined data set of ~1854 GPS-derived horizontal velocity vectors. Assuming that crustal deformation is localized along major faults, a block modelling approach is employed to interpret the GPS velocity field. We construct a 30-element block model to describe present-day deformation in western China, with half of them located within the Tibetan Plateau, and the remainder located in its surrounding areas. We model the GPS velocities simultaneously for the effects of block rotations and elastic strain induced by the bounding faults. Our model yields a good fit to the GPS data with a mean residual of 1.08 mm a –1 compared to the mean uncertainty of 1.36 mm a –1 for each velocity component, indicating a good agreement between the predicted and observed velocities. The major strike-slip faults such as the Altyn Tagh, Xianshuihe, Kunlun and Haiyuan faults have relatively uniform slip rates in a range of 5–12 mm a –1 along most of their segments, and the estimated fault slip rates agree well with previous geologic and geodetic results. Blocks having significant residuals are located at the southern and southeastern Tibetan Plateau, suggesting complex tectonic settings and further refinement of accurate definition of block geometry in these regions.

Description: Using an up-to-date global plate rotation model, applied to the endpoints of preserved major spreading ridge isochrons, we have calculated the explicitly reconstructable length-weighted mean global half-spreading rate (HSR), ridge length and area production as a function of time since the end of the Cretaceous Normal Superchron at 83.0 Ma. Our calculations integrate uncertainties in rotation parameters and chron boundary ages with the partial sampling uncertainties arising from progressive subduction of older oceanic lithosphere and its preserved spreading record. This record of directly reconstructable oceanic ridge production provides a well-constrained baseline that can be compared to reconstructions that include the largely unconstrained extrapolated histories of entirely subducted oceanic plates. The directly reconstructable global mean HSR has not varied by more than ±15 per cent about an average rate of 28.4 ± 4.6 mm a –1 since 83 Ma. No long-term secular trend is evident: a maximum global mean half-rate of 32 ± 6 mm a –1 occurred from 33.1 Ma to about 25.8 Ma, with minima of 26 ± 5 mm a –1 between about 56 and 40.2 Ma, and 24 ± 1 mm a –1 since 3.2 Ma. Only this most recent interval has a rate that differs significantly (at ±2) from the long-term mean. The global, reconstructable ridge length at 56 Ma decreases by less than 15 per cent relative to the modern ridge system; by 83 Ma it has decreased by 38 per cent. These relatively high preserved ridge fractions mean that the estimated uncertainty due to partial sampling stays roughly equivalent to the estimated rotation model uncertainties, allowing long-term spreading rate variations of 〉20 per cent since the Late Cretaceous to be ruled out. In contrast, prior to 83 Ma too little oceanic lithosphere is preserved to reliably reconstruct global spreading rates.

Description: We explore thermal convection of a fluid with a temperature-dependent viscosity in a basally heated 3-D spherical shell using linear stability analyses and numerical experiments, while considering the application of our results to terrestrial planets. The inner to outer radius ratio of the shell f assumed in the linear stability analyses is in the range of 0.11–0.88. The critical Rayleigh number R c for the onset of thermal convection decreases by two orders of magnitude as f increases from 0.11 to 0.88, when the viscosity depends sensitively on the temperature, as is the case for real mantle materials. Numerical simulations carried out in the range of f  = 0.11–0.55 show that a thermal boundary layer (TBL) develops both along the surface and bottom boundaries to induce cold and hot plumes, respectively, when f is 0.33 or larger. However, for smaller f values, a TBL develops only on the bottom boundary. Convection occurs in the stagnant-lid regime where the root mean square velocity on the surface boundary is less than 1 per cent of its maximum at depth, when the ratio of the viscosity at the surface boundary to that at the bottom boundary exceeds a threshold that depends on f . The threshold decreases from 10 6.5 at f  = 0.11 to 10 4 at f  = 0.55. If the viscosity at the base of the convecting mantle is 10 20 –10 21  Pa s, the Rayleigh number exceeds R c for Mars, Venus and the Earth, but does not for the Moon and Mercury; convection is unlikely to occur in the latter planets unless the mantle viscosity is much lower than 10 20  Pa s and/or the mantle contains a strong internal heat source.

Description: Subducting oceanic lithosphere is an example of a thin sheet-like object whose characteristic lateral dimension greatly exceeds its thickness. Here we exploit this property to derive a new hybrid boundary-integral/thin sheet (BITS) representation of subduction that combines in a single equation all the forces acting on the sheet: gravity, internal resistance to bending and stretching, and the tractions exerted by the ambient mantle. For simplicity, we limit ourselves to 2-D. We solve the BITS equations using a discrete Lagrangian approach in which the sheet is represented by a set of vertices connected by edges. Instantaneous solutions for the sinking speed of a slab attached to a trailing flat sheet obey a scaling law of the form V / V Stokes  = fct(St), where V Stokes is a characteristic Stokes sinking speed and St is the sheet's flexural stiffness. Time-dependent solutions for the evolution of the sheet's shape and thickness show that these are controlled by the viscosity ratio between the sheet and its surroundings. An important advantage of the BITS approach is the possibility of generalizing the sheet's rheology, either to a viscosity that varies along the sheet or to a non-Newtonian shear-thinning rheology.

Description: The aftershock distribution of the 1933 Sanriku-oki outer trench earthquake is estimated by using modern relocation methods and a newly developed velocity structure to examine the spatial extent of the source-fault and the possibility of a triggered interplate seismicity. In this study, we first examined the regional data quality of the 1933 earthquake based on smoked-paper records and then relocated the earthquakes by using the 3-D velocity structure and double-difference method. The improvements of hypocentre locations using these methods were confirmed by the examination of recent earthquakes that are accurately located based on ocean bottom seismometer data. The results show that the 1933 aftershocks occurred under both the outer- and inner-trench-slope regions. In the outer-trench-slope region, aftershocks are distributed in a ~280-km-long area and their depths are shallower than 50 km. Although we could not constrain the fault geometry from the hypocentre distribution, the depth distribution suggests the whole lithosphere is probably not under deviatoric tension at the time of the 1933 earthquake. The occurrence of aftershocks under the inner trench slope was also confirmed by an investigation of waveform frequency difference between outer and inner trench earthquakes as recorded at Mizusawa. The earthquakes under the inner trench slope were shallow (depth less double equals30 km) and the waveforms show a low-frequency character similar to the waveforms of recent, precisely located earthquakes in the same area. They are also located where recent activity of interplate thrust earthquakes is high. These suggest that the 1933 outer-trench-slope main shock triggered interplate earthquakes, which is an unusual case in the order of occurrence in contrast with the more common pairing of a large initial interplate shock with subsequent outer-slope earthquakes. The off-trench earthquakes are distributed about 80 km width in the trench perpendicular direction. This wide width cannot be explained from a single high-angle fault confined at a shallow depth (depth less double equals50 km). The upward motion of the 1933 tsunami waveform records observed at Sanriku coast also cannot be explained from a single high-angle west-dipping normal fault. If we consider additional fault, involvement of high-angle, east-dipping normal faults can better explain the tsunami first motion and triggering of the aftershock in a wide area under the outer trench slope. Therefore multiple off-trench normal faults may have activated during the 1933 earthquake. We also relocated recent (2001–2012) seismicity by the same method. The results show that the present seismicity in the outer-trench-slope region can be divided into several groups along the trench. Comparison of the 1933 rupture dimensions based on our aftershock relocations with the morphologies of fault scarps in the outer trench slope suggest that the rupture was limited to the region where fault scarps are largely trench parallel and cross cut the seafloor spreading fabric. These findings imply that bending geometry and structural segmentation of the incoming plate largely controls the spatial extent of the 1933 seismogenic faulting. In this shallow rupture model for this largest outer trench earthquake, triggered seismicity in the forearc and structural control of faulting represent an important deformation styles for off-trench and shallow megathrust zones.

Description: Geodetic, geologic and palaeomagnetic data reveal that Oregon (western USA) rotates clockwise at 0.3 to 1.0° Ma –1 (relative to North America) about an axis near the Idaho–Oregon–Washington border, while northeast Washington is relatively fixed. This rotation has been going on for at least 15 Ma. The Yakima fold and thrust belt (YFTB) forms the boundary between northern Oregon and central Washington where convergence of the clockwise-rotating Oregon block is apparently accommodated. North–south shortening across the YFTB has been thought to occur in a fan-like manner, increasing in rate to the west. We obtained high-accuracy, high-density geodetic GPS measurements in 2012–2014 that are used with earlier GPS measurements from the 1990s to characterize YFTB kinematics. The new results show that the deformation associated with the YFTB starts at the Blue Mountains Anticline in northern Oregon and extends north beyond the Frenchman Hills in Washington, past the epicentre of the 1872 M w 7.0 Entiat earthquake to 49°N. The north–south strain rate across the region is 2 to 3 x 10 –9 yr –1 between the volcanic arc and the eastern edge of the YFTB (241.0°E); east of there it drops to about 10 –9 yr –1 . At the eastern boundary of the YFTB, faults and earthquake activity are truncated by a north-trending, narrow zone of deformation that runs along the Pasco Basin and Moses Lake regions near 240.9°E. This zone, abutting the Department of Energy Hanford Nuclear Reservation, accommodates about 0.5 mm yr –1 of east to northeast shortening. A similar zone of N-trending transpression is seen along 239.9°E where there is a change in the strike of the Yakima folds. The modern deformation of the YFTB is about 600 km wide from south to north and internally may be controlled by pre-existing crustal structure.

Description: The Central Anatolian orogenic plateau is represented by young volcanism, rapid plateau uplift and distinctive (past and active) tectonic deformation. In this study, we consider observational data in terms of regional present-day geodynamics in the region. The residual topography of Central Anatolia was derived to define the regional isostatic conditions according to Airy isostasy and infer the potential role of ‘dynamic topography’. 2-D thermomechanical forward models for coupled mantle-lithosphere flow/deformation were conducted along an N–S directional profile through the region (e.g. northern/Pontides, interior and southern/Taurides). These models were based on seismic tomography data that provide estimates about the present-day mantle thermal structure beneath the Anatolian plate. We compare the modelling results with calculated residual topography and independent data sets of geological deformation, gravity and high surface heat flow/widespread geothermal activity. Model results suggest that there is ~1 km of mantle flow induced dynamic topography associated with the sublithospheric flow driven by the seismically inferred mantle structure. The uprising mantle may have also driven the asthenospheric source of volcanism in the north (e.g. Galatia volcanic province) and the Cappadocia volcanic province in the south while elevating the surface in the last 10 Myr. Our dynamic topography calculations emphasize the role of vertical forcing under other orogenic plateaux underlain by relatively thin crust and low-density asthenospheric mantle.

Description: In many geological systems, inversion of density stratification sets in Rayleigh–Taylor (RT) instabilities, leading to an ascent of relatively low-density materials through the high-density overburden in the form of diapirs. These diapirs often originate from dipping low-density layers. This study aims to show how the initial tilt of such source layers can control the ascent behaviour of diapirs initiated by RT instabilities. Using two-layer viscous models we produced RT instabilities in physical experiments, and investigated the effects of source-layer tilts ( β ). Our experiments suggest that these diapirs ascend with contrasting lateral spreading rates in the up and down slope directions, resulting in their axi-asymmetric geometry. However, their heads retain a circular outline on the horizontal top surface, where the upwelling axis is located away from their geometric centre in the upslope direction. In this paper, we present a series of experimental models to demonstrate the spectrum of axi-symmetric to -asymmetric geometrical transitions with increasing β . Our experiments also reveal that when β is large (〉4°) the diapirs become unstable, resulting in a continuous migration of their stems in the upslope direction. Using the volume of fluid method we ran computational fluid dynamic (CFD) simulations to study the underlying hydrodynamics of axi-asymmetric diapiric growth. The CFD simulations show that β 〉 0° conditions develop stronger flow vortices on the downslope side of an ascending diapir, leading to a pressure difference between the up- and downslope flanks. Such a differential pressure causes the diapir head to spread at a faster rate in the tilt direction. An estimate of the asymmetric spreading rates is given as a function of β . Our present study provides a fundamental understanding of the hydrodynamic flow structure responsible for the asymmetric growth of RT instabilities on tilted source layers, as applicable to a wide range of large-scale geological settings, such as sedimentary basins and subduction zones.

Description: The geomechanical analysis of a highly compartmentalized reservoir is performed to simulate the seafloor subsidence due to gas production. The available observations over the hydrocarbon reservoir consist of bathymetric surveys carried out before and at the end of a 10-yr production life. The main goal is the calibration of the reservoir compressibility c M , that is, the main geomechanical parameter controlling the surface response. Two conceptual models are considered: in one (i) c M varies only with the depth and the vertical effective stress (heterogeneity due to lithostratigraphic variability); in another (ii) c M varies also in the horizontal plane, that is, it is spatially distributed within the reservoir stratigraphic units. The latter hypothesis accounts for a possible partitioning of the reservoir due to the presence of sealing faults and thrusts that suggests the idea of a block heterogeneous system with the number of reservoir blocks equal to the number of uncertain parameters. The method applied here relies on an ensemble-based data assimilation (DA) algorithm (i.e. the ensemble smoother, ES), which incorporates the information from the bathymetric measurements into the geomechanical model response to infer and reduce the uncertainty of the parameter c M . The outcome from conceptual model (i) indicates that DA is effective in reducing the c M uncertainty. However, the maximum settlement still remains underestimated, while the areal extent of the subsidence bowl is overestimated. We demonstrate that the selection of the heterogeneous conceptual model (ii) allows to reproduce much better the observations thus removing a clear bias of the model structure. DA allows significantly reducing the c M uncertainty in the five blocks (out of the seven) characterized by large volume and large pressure decline. Conversely, the assimilation of land displacements only partially constrains the prior c M uncertainty in the reservoir blocks marginally contributing to the cumulative seafloor subsidence, that is, blocks with low pressure.

Description: This work aims to explore the ongoing tectonic activity of structures in the outermost sector of the Northern Apennines, which represents the active leading edge of the thrust belt and is dominated by compressive deformation. We have applied the Persistent Scatterer Interferometry (PSI) technique to obtain new insights into the present-day deformation pattern of the frontal area of the Northern Apennine. PSI has proved to be effective in detecting surface deformation of wide regions involved in low tectonic movements. We used 34 Envisat images in descending geometry over the period of time between 2004 and 2010, performing about 300 interferometric pairs. The analysis of the velocity maps and of the PSI time-series has allowed to observe ground deformation over the sector of the Po Plain between Piacenza and Reggio Emilia. The time-series of permanent GPS stations located in the study area, validated the results of the PSI technique, showing a good correlation with the PS time-series. The PS analysis reveals the occurrence of a well-known subsidence area on the rear of the Ferrara arc, mostly connected to the exploitation of water resources. In some instances, the PS velocity pattern reveals ground uplift (with mean velocities ranging from 1 to 2.8 mm yr –1 ) above active thrust-related anticlines of the Emilia and Ferrara folds, and part of the Pede-Apennine margin. We hypothesize a correlation between the observed uplift deformation pattern and the growth of the thrust-related anticlines. As the uplift pattern corresponds to known geological features, it can be used to constrain the seismo-tectonic setting, and a working hypothesis may involve that the active Emilia and Ferrara thrust folds would be characterized by interseismic periods possibly dominated by aseismic creep.

Description: In Europe, common input data types for seismic hazard evaluation include earthquake catalogues, seismic zonation models and ground motion models, all with well-constrained epistemic uncertainties. In contrast, neotectonic deformation models and their related uncertainties are rarely considered in earthquake forecasting and seismic hazard studies. In this study, for the first time in Europe, we developed a seismic hazard model based exclusively on active fault and geodynamic deformation models. We applied it to the External Dinarides, a slow-deforming fold-and-thrust belt in the Central Mediterranean. The two deformation models furnish consistent long-term earthquake rates above the M w 4.7 threshold on a latitude/longitude grid with 0.2° spacing. Results suggest that the use of deformation models is a valid alternative to empirical-statistical approaches in earthquake forecasting in slow-deforming regions of Europe. Furthermore, we show that the variability of different deformation models has a comparable effect on the peak ground motion acceleration uncertainty as do the ground motion prediction equations.

Description: An important real world application of doublet flow occurs in well design of both geothermal and hydrocarbon reservoirs. A guiding principle for fluid management of injection and extraction wells is that mass balance is commonly assumed between the injected and produced fluid. Because the doublets are considered closed loops, the injection fluid is assumed to eventually reach the producer well and all the produced fluid ideally comes from stream tubes connected to the injector of the well pair making up the doublet. We show that when an aquifer background flow occurs, doublets will rarely retain closed loops of fluid recirculation. When the far-field flow rate increases relative to the doublet's strength, the area occupied by the doublet will diminish and eventually vanishes. Alternatively, rather than using a single injector (source) and single producer (sink), a linear array of multiple injectors separated by some distance from a parallel array of producers can be used in geothermal energy projects as well as in waterflooding of hydrocarbon reservoirs. Fluid flow in such an arrangement of parallel source-sink arrays is shown to be macroscopically equivalent to that of a line doublet. Again, any far-field flow that is strong enough will breach through the line doublet, which then splits into two vortices. Apart from fundamental insight into elementary flow dynamics, our new results provide practical clues that may contribute to improve the planning and design of doublets and direct line drives commonly used for flow management of groundwater, geothermal and hydrocarbon reservoirs.

Description: In November 2010, intense seismic activity including 29 events with a magnitude above 5.0, started in the western part of the Gulf of Aden, where the structure of the oceanic spreading ridge is characterized by a series of N115°-trending slow-spreading segments set within an EW-trending rift. Using signals recorded by permanent and temporary networks in Djibouti and Yemen, we located 1122 earthquakes, with a magnitude ranging from 2.1 to 5.6 from 2010 November 1 to 2011 March 31. By looking in detail at the space–time distribution of the overall seismicity, and both the frequency and the moment tensor of large earthquakes, we re-examine the chronology of this episode. In addition, we also interpret the origin of the activity using high-resolution bathymetric data, as well as from observations of seafloor cable damage caused by high temperatures and lava flows. The analysis allows us to identify distinct active areas. First, we interpret that this episode is mainly related to a diking event along a specific ridge segment, located at E044°. In light of previous diking episodes in nearby subaerial rift segments, for which field constraints and both seismic and geodetic data exist, we interpret the space–time evolution of the seismicity of the first few days. Migration of earthquakes suggests initial magma ascent below the segment centre. This is followed by a southeastward dike propagation below the rift immediately followed by a northwestward dike propagation below the rift ending below the northern ridge wall. The cumulative seismic moment associated with this sequence reaches 9.1 x 10 17 Nm, and taking into account a very low seismic versus geodetic moment, we estimate a horizontal opening of ~0.58–2.9 m. The seismic activity that followed occurred through several bursts of earthquakes aligned along the segment axis, which are interpreted as short dike intrusions implying fast replenishment of the crustal magma reservoir feeding the dikes. Over the whole period, the opening is estimated to be ~1.76–8.8 m across the segment. A striking feature of this episode is that the seismicity remained confined within one individual segment, whereas the adjacent en-echelon segments were totally quiescent, suggesting that the magma supply system of one segment is disconnected from those of the neighbouring segments. Second, we identify activity induced by the first intrusion with epicentres aligned along an N035°E-trending, ~30 km long at the northwestern end of the active opening segment. This group encompasses more than seven earthquakes with magnitude larger than 5.0, and with strike-slip focal mechanisms consistent with the faults identified in the bathymetry and the structural pattern of the area. We propose that a transform fault is currently in formation which indicates an early stage of the ridge segmentation, at the locus of the trend change of the spreading ridge, which also corresponds to the boundary between a clear oceanic lithosphere and the zone of transform between continental and oceanic crust.

Description: We challenge the perspective that seismicity could contribute to polar motion by arguing quantitatively that, in first approximation and on the average, interseismic deformations can compensate for it. This point is important because what we must simulate and observe in Earth Orientation Parameter time-series over intermediate timescales of decades or centuries is the residual polar motion resulting from the two opposing processes of coseismic and interseismic deformations. In this framework, we first simulate the polar motion caused by only coseismic deformations during the longest period available of instrumental seismicity, from 1900 to present, using both the CMT and ISC-GEM catalogues. The instrumental seismicity covering a little longer than one century does not represent yet the average seismicity that we should expect on the long term. Indeed, although the simulation shows a tendency to move the Earth rotation pole towards 133°E at the average rate of 16.5 mm yr –1 , this trend is still sensitive to individual megathrust earthquakes, particularly to the 1960 Chile and 1964 Alaska earthquakes. In order to further investigate this issue, we develop a global seismicity model (GSM) that is independent from any earthquake catalogue and that describes the average seismicity along plate boundaries on the long term by combining information about present-day plate kinematics with the Anderson theory of faulting, the seismic moment conservation principle and a few other assumptions. Within this framework, we obtain a secular polar motion of 8 mm yr –1 towards 112.5°E that is comparable with that estimated from 1900 to present using the earthquake catalogues, although smaller by a factor of 2 in amplitude and different by 20° in direction. Afterwards, in order to reconcile the idea of a secular polar motion caused by earthquakes with our simplest understanding of the seismic cycle, we adapt the GSM in order to account for interseismic deformations and we use it to quantify, for the first time ever, their contribution to polar motion. Taken together, coseismic and interseismic deformations make the rotation pole wander around the north pole with maximum polar excursions of about 1 m. In particular, the rotation pole moves towards about Newfoundland when the interseismic contribution dominates over the coseismic ones (i.e. during phases of low seismicity or, equivalently, when most of the fault system associated with plate boundaries is locked). When megathrust earthquakes occur, instead, the rotation pole is suddenly shifted in an almost opposite direction, towards about 133°E.

Description: While elasticity is a defining characteristic of the Earth's lithosphere, it is often ignored in numerical models of long-term tectonic processes in favour of a simpler viscoplastic description. Here we assess the consequences of this assumption on a well-studied geodynamic problem: the growth of normal faults at an extensional plate boundary. We conduct 2-D numerical simulations of extension in elastoplastic and viscoplastic layers using a finite difference, particle-in-cell numerical approach. Our models simulate a range of faulted layer thicknesses and extension rates, allowing us to quantify the role of elasticity on three key observables: fault-induced topography, fault rotation, and fault life span. In agreement with earlier studies, simulations carried out in elastoplastic layers produce rate-independent lithospheric flexure accompanied by rapid fault rotation and an inverse relationship between fault life span and faulted layer thickness. By contrast, models carried out with a viscoplastic lithosphere produce results that may qualitatively resemble the elastoplastic case, but depend strongly on the product of extension rate and layer viscosity U x L . When this product is high, fault growth initially generates little deformation of the footwall and hanging wall blocks, resulting in unrealistic, rigid block-offset in topography across the fault. This configuration progressively transitions into a regime where topographic decay associated with flexure is fully accommodated within the numerical domain. In addition, high U x L favours the sequential growth of multiple short-offset faults as opposed to a large-offset detachment. We interpret these results by comparing them to an analytical model for the fault-induced flexure of a thin viscous plate. The key to understanding the viscoplastic model results lies in the rate-dependence of the flexural wavelength of a viscous plate, and the strain rate dependence of the force increase associated with footwall and hanging wall bending. This behaviour produces unrealistic deformation patterns that can hinder the geological relevance of long-term rifting models that assume a viscoplastic rheology.

Description: In the context of the 2014 realization of the International Terrestrial Reference Frame, the International DORIS (Doppler Orbitography Radiopositioning Integrated by Satellite) Service (IDS) has delivered to the IERS a set of 1140 weekly SINEX files including station coordinates and Earth orientation parameters, covering the time period from 1993.0 to 2015.0. From this set of weekly SINEX files, the IDS combination centre estimated a cumulative DORIS position and velocity solution to obtain mean horizontal and vertical motion of 160 stations at 71 DORIS sites. The main objective of this study is to validate the velocities of the DORIS sites by comparison with external models or time-series. Horizontal velocities are compared with two recent global plate models (GEODVEL 2010 and NNR-MORVEL56). Prior to the comparisons, DORIS horizontal velocities were corrected for Global Isostatic Adjustment from the ICE-6G (VM5a) model. For more than half of the sites, the DORIS horizontal velocities differ from the global plate models by less than 2–3 mm yr –1 . For five of the sites (Arequipa, Dionysos/Gavdos, Manila and Santiago) with horizontal velocity differences with respect to these models larger than 10 mm yr –1 , comparisons with GNSS estimates show the veracity of the DORIS motions. Vertical motions from the DORIS cumulative solution are compared with the vertical velocities derived from the latest GPS cumulative solution over the time span 1995.0–2014.0 from the University of La Rochelle solution at 31 co-located DORIS-GPS sites. These two sets of vertical velocities show a correlation coefficient of 0.83. Vertical differences are larger than 2 mm yr –1 at 23 percent of the sites. At Thule, the disagreement is explained by fine-tuned DORIS discontinuities in line with the mass variations of outlet glaciers. Furthermore, the time evolution of the vertical time-series from the DORIS station in Thule show similar trends to the GRACE equivalent water height.

Description: We have used ensemble averages of satellite-derived free-air gravity anomaly data, together with inverse modelling techniques, to determine the effective elastic thickness, T e , of circum-Pacific subducting oceanic lithosphere and its relationship to plate age. Synthetic modelling tests show that T e can be recovered best using gravity anomaly, rather than bathymetry, data and profiles that are at least 750 km long. Inverse modelling based on a uniform T e elastic plate suggests that T e increases with age of the subducting oceanic lithosphere and is given approximately by the depth to the 390 ± 10 °C oceanic isotherm based on a cooling plate model. Misfits between the observed and calculated gravity anomalies are significantly improved if a mechanically weak zone is included between the trench axis and the outer rise. This weak zone is coincident with observations of bend-faulting and seismicity. Inverse modelling shows that T e landward of the outer rise is generally 40–65 per cent less than the T e seaward of the outer rise. Both landward and seaward T e increases with age of the lithosphere and are given by the depth to the 342–349 °C and 671–714 °C oceanic isotherm, respectively. A dependence of T e on age is consistent with models for the cooling of oceanic lithosphere as it moves away from a mid-ocean ridge and the temperature-dependent ductile creep of oceanic lithospheric minerals such as olivine. By comparing the observed T e to the predicted T e based on laboratory-derived yield strength envelopes and an assumption of elastic-perfectly plastic deformation, we have attempted to constrain the rheology of oceanic lithosphere. Regardless of the assumed friction coefficient, the dry-olivine low-temperature plasticity flow laws of Goetze, Evans & Goetze, Raterron et al . and Mei et al . all provide quite a good fit to the observed T e at circum-Pacific subduction zones. This result contrasts with the Hawaiian Islands, where these flow laws are generally too strong to fit the observations. The discrepancy in rheology within Pacific plate may be caused by differences in the timescale of loading and therefore the amount of viscoelastic stress relaxation that has occurred. Other possibilities include thermal rejuvenation and magma-assisted flexure at the Hawaiian Islands.

Description: Large gaps and inconsistencies remain in published estimates of Nubia–Somalia plate motion based on reconstructions of seafloor spreading data around Africa. Herein, we use newly available reconstructions of the Southwest Indian Ridge at ~1-Myr intervals since 20 Ma to estimate Nubia–Somalia plate motion farther back in time than previously achieved and with an unprecedented degree of temporal resolution. At the northern end of the East African rift, our new estimates of Nubia–Somalia motion for six times from 0.78 Ma to 5.2 Ma differ by only 2 per cent from the rift-normal component of motion that is extrapolated from a recently estimated GPS angular velocity. The rate of rift-normal extension thus appears to have remained steady since at least 5.2 Ma. Our new rotations indicate that the two plates have moved relative to each other since at least 16 Ma and possibly longer. Motion has either been steady since at least 16 Ma or accelerated modestly between 6 and 5.2 Ma. Our Nubia–Somalia rotations predict 42.5 ± 3.8 km of rift-normal extension since 10.6 Ma across the well-studied, northern segment of the Main Ethiopian Rift, consistent with 40–50 km estimates for extension since 10.6 Myr based on seismological surveys of this narrow part of the plate boundary. Nubia–Somalia rotations are also derived by combining newly estimated Somalia–Arabia rotations that reconstruct the post-20-Ma opening of the Gulf of Aden with Nubia–Arabia rotations estimated via a probabilistic analysis of plausible opening scenarios for the Red Sea. These rotations predict Nubia–Somalia motion since 5.2 Myr that is consistent with that determined from Southwest Indian Ridge data and also predict 40 ± 3 km of rift-normal extension since 10.6 Ma across the Main Ethiopian Rift, consistent with our 42.5 ± 3.8 km Southwest Indian Ridge estimate. Our new rotations exclude at high confidence level previous estimates of 12 ± 13 and 123 ± 14 km for rift-normal extensions across the Main Ethiopian Rift since 10.6 Ma based on reconstructions of Chron 5n.2 along the Southwest Indian Ridge. Sparse coverage of magnetic reversals older than 16 Ma along the western third of the Southwest Indian Ridge precludes reliable determinations of Nubia–Somalia plate motion before 16 Ma, leaving unanswered the key question of when the motion between the two plates began.

Description: The TAMMAR segment of the Mid-Atlantic Ridge forms a classic propagating system centred about two degrees south of the Kane Fracture Zone. The segment is propagating to the south at a rate of 14 mm yr –1 , 15 per cent faster than the half-spreading rate. Here, we use seismic refraction data across the propagating rift, sheared zone and failed rift to investigate the crustal structure of the system. Inversion of the seismic data agrees remarkably well with crustal thicknesses determined from gravity modelling. We show that the crust is thickened beneath the highly magmatic propagating rift, reaching a maximum thickness of almost 8 km along the seismic line and an inferred (from gravity) thickness of about 9 km at its centre. In contrast, the crust in the sheared zone is mostly 4.5–6.5 km thick, averaging over 1 km thinner than normal oceanic crust, and reaching a minimum thickness of only 3.5 km in its NW corner. Along the seismic line, it reaches a minimum thickness of under 5 km. The PmP reflection beneath the sheared zone and failed rift is very weak or absent, suggesting serpentinisation beneath the Moho, and thus effective transport of water through the sheared zone crust. We ascribe this increased porosity in the sheared zone to extensive fracturing and faulting during deformation. We show that a bookshelf-faulting kinematic model predicts significantly more crustal thinning than is observed, suggesting that an additional mechanism of deformation is required. We therefore propose that deformation is partitioned between bookshelf faulting and simple shear, with no more than 60 per cent taken up by bookshelf faulting.

Description: A Neogene magmatic reactivation of the Manihiki Plateau, a large igneous province (LIP) in the central Pacific, is studied using seismic reflection data. Igneous diapirs have been identified exclusively within a narrow WNW–ESE striking corridor in the southern High Plateau (HP), which is parallel to the Neogene Pacific Plate motion and overlaps with an extrapolation of the Society Islands Hotspot (SIH) path. The igneous diapirs are characterized by a narrow width (〉5 km), penetration of the Neogene sediments, and they become progressively younger towards the East (23–10 Ma). The magmatic source appears to be of small lateral extent, which leads to the conclusion that the diapirs represent Neogene hotspot volcanism within a LIP, and thus may be an older, previously unknown extension of the SIH track (〉4.5 Ma). Comparing hotspot volcanism within oceanic and continental lithosphere, we further conclude that hotspot volcanism within LIP crust has similarities to tectonically faulted continental crust.

Description: Many mechanisms control the state of stress within Earth plates. First-order well-known mechanisms include stresses induced by lateral variations of lithospheric density structure, sublithospheric tractions, ridge push and subduction pull. In this study, we attempt to quantify the influence of these mechanisms to understand the origin of stresses in the lithosphere, choosing the African plate (TAP) as an example. A finite-element based suite, Proshell, was developed to combine several data sets, to estimate the gravitational potential energy (GPE) of the lithosphere and to calculate stresses acting on the real (non-planar) geometry of TAP. We introduce several quantitative parameters to measure the degree of fit between the model and observations. Our modelling strategy involves nine series of numerical experiments. We start with the simplest possible model and then, step by step, build it up to be a more physically realistic model, all the while discussing the influence of each additional component. The starting (oversimplified) model series (1) is based on the CRUST2 data set for the crust and a half-space-cooling approximation of the lithospheric mantle. We then describe models (series 2–5) that account for lithospheric mantle density heterogeneities to build a more reliable GPE model. The consecutive series involve basal traction from the convective mantle (series A, C) and the rheological heterogeneity of the TAP via variations in its effective elastic thickness (series B, C). The model quality reflects the increase in complexity between series with an improving match to observed stress regimes and directions. The most complex model (series D) also accounts for the bending stresses in the elastic lithosphere and achieves a remarkably good fit to observations. All of our experiments were based on the iteration of controlling parameters in order to achieve the best fit between modelled and observed stresses, always considering physically feasible values. This gives us confidence that our methodology appropriately models the stress pattern of Africa, and that it may be further applied to other plates on Earth. Our modelling approach allows us to quantify several important features controlling the lithospheric stress pattern. Even though the initial oversimplified model does not fit the observations satisfactorily, it shows how ridge push may create significant compressive stresses in the lithosphere. More complex models show the importance of the density structure of the lithosphere, specifically in the subcrustal lithosphere. The stress regime within the TAP mainly results from a global balance of masses and mass moments between continental and oceanic parts of the plate. The orientation of stresses, in turn, is influenced more by local features expressed by topographic and crustal density variations, whereas existent subcrustal density variations appear to be smoothed by the crust above. The models show that accounting separately for either basal tractions or rheological heterogeneities brings moderate improvement, but the combination of these two mechanisms results in a substantially better match between model and observations. The bending stresses caused by isostatical re-adjustment improve the model match, but they have to be analysed with caution because of their depth-dependent nature.

Description: The Red Sea and Gulf of Aden represent two young basins that formed between Africa and Arabia since the early Oligocene, floored by oceanic crust or by transitional and thinned continental crust. While in the easternmost Gulf of Aden, the rift–drift transition can be dated chron C6 (~20.1 Ma), here we show that in the Red Sea the first pulse of seafloor spreading occurred during chron C3n.2n (~4.6 Ma) around ~17.1°N (present-day coordinates) and propagated southwards from this location, separating the Danakil microplate from Arabia. It is also shown that seafloor spreading between Arabia and Nubia started later, around chron 2A (~2.58 Ma), and propagated northwards. At present, there is no magnetic evidence for the existence of a linear spreading centre in the northern Red Sea at latitudes higher than ~24°N and in the southern Red Sea below ~14.8°N. The present-day plate kinematics of this region can be described with high accuracy by a network of five interacting plates (Nubia, Arabia, Somalia, Sinai and Danakil) and six triple junctions. For times older than anomaly 2A (~2.58 Ma) and up to anomaly 3, the absence of marine magnetic anomalies between Arabia and Nubia prevents a rigorous kinematic description of the five-plates system. However, there is strong evidence that the unique changes in plate motions during the last 5 Myr were a dramatic slowdown at chron C2 (~1.77 Ma) in the spreading or extension rates along the ridge and rift axes, thereby a good representation of the real plate motions can be obtained anyway by backward extension of the oldest Arabia–Nubia and Arabia–Danakil stage rotations determined on the basis of marine magnetic anomalies, respectively, C2–C2A and C2A–C3. The proposed kinematic reconstructions are accompanied by a geodynamic explanation for the genesis of large continent–continent fracture zones at the rift–drift transition and by an analysis of the strain associated with plate motions in Afar, northeastern Egypt and Sinai.

Description: On 2016 February 6 the South Taiwan earthquake ( M w 6.4) occurred in the Meinong District of Kaohsiung, southern Taiwan, at a depth of 17 km. It caused 117 fatalities and widespread damage to infrastructures, especially in the Tainan city. To clarify the generating mechanism of this damaging earthquake, we determined high-resolution 3-D images of P - and S -wave velocity ( V P , V S ) and Poisson's ratio ( ) in the epicentral area. We used 105 712 P - and 61 250 S -wave arrival times of 8279 local earthquakes (1.5 ≤ M ≤ 6.4) recorded at 41 seismic stations in South Taiwan during 2000–2011. In the upper crust (depth ≤ 10 km), the most remarkable feature is low- V P , low- V S and high- anomalies in areas with known active faults in the southwestern and easternmost parts of Taiwan. In contrast, high- V P , high- V S and low- anomalies become dominant in the lower crust. The hypocentre of the 2016 South Taiwan earthquake is located in a boundary zone where seismic velocity and Poisson's ratio change drastically in both the horizontal and vertical directions. Furthermore, the hypocentre is underlain by a vertically elongated high- anomaly at depths of 23–40 km, which may reflect ascending fluids from the upper (or uppermost) mantle. The low- V and high- anomalies in the upper crust coincide with areas of low heat flow, negative Bouguer gravity anomaly, and low magnetotelluric resistivity, which may reflect crustal fluids contained in the young fold-and-thrust belt. These results suggest that the 2016 South Taiwan earthquake was triggered by ascending fluids from dehydration of the subducting Eurasian slab, invading into active faults with a high loading rate.

Description: Crystallization experiments in the dendritic regime have been carried out in hypergravity conditions (from 1 to 1300 g) from an ammonium chloride solution (NH 4 Cl and H 2 O). A commercial centrifuge was equipped with a slip ring so that electric power (needed for a Peltier device and a heating element), temperature and ultrasonic signals could be transmitted between the experimental setup and the laboratory. Ultrasound measurements (2–6 MHz) were used to detect the position of the front of the mushy zone and to determine attenuation in the mush. Temperature measurements were used to control a Peltier element extracting heat from the bottom of the setup and to monitor the evolution of crystallization in the mush and in the liquid. A significant increase of solid fraction and attenuation in the mush is observed as gravity is increased. Kinetic undercooling is significant in our experiments and has been included in a macroscopic mush model. The other ingredients of the model are conservation of energy and chemical species, along with heat/species transfer between the mush and the liquid phase: boundary-layer exchanges at the top of the mush and bulk convection within the mush (formation of chimneys). The outputs of the model compare well with our experiments. We have then run the model in a range of parameters suitable for the Earth's inner core. This has shown the role of bulk mush convection for the inner core and the reason why a solid fraction very close to unity should be expected. We have also run melting experiments: after crystallization of a mush, the liquid has been heated from above until the mush started to melt, while the bottom cold temperature was maintained. These melting experiments were motivated by the possible local melting at the inner core boundary that has been invoked to explain the formation of the anomalously slow F-layer at the bottom of the outer core or inner core hemispherical asymmetry. Oddly, the consequences of melting are an increase in solid fraction and a decrease in attenuation. It is hence possible that surface seismic velocity and attenuation of the inner core are strongly affected by melting.

Description: The coupling that exists between surface processes and deformation within both the shallow crust and the deeper mantle-lithosphere has stimulated the development of computational geodynamic models that incorporate a free surface boundary condition. We introduce a treatment of this boundary condition that is suitable for staggered grid, finite difference schemes employing a structured Eulerian mesh. Our interface capturing treatment discretizes the free surface boundary condition via an interface that conforms with the edges of control volumes (e.g. a ‘staircase’ representation) and requires only local stencil modifications to be performed. Comparisons with analytic solutions verify that the method is first-order accurate. Additional intermodel comparisons are performed between known reference models to further validate our free surface approximation. Lastly, we demonstrate the applicability of a multigrid solver to our free surface methodology and demonstrate that the local stencil modifications do not strongly influence the convergence of the iterative solver.

Description: Ellesmere Island in Arctic Canada displays a complex geological evolution. The region was affected by two distinct orogenies, the Palaeozoic Ellesmerian orogeny (the Caledonian equivalent in Arctic Canada and Northern Greenland) and the Palaeogene Eurekan orogeny, related to the opening of Baffin Bay and the consequent convergence of the Greenland plate. The details of this complex evolution and the present-day deep structure are poorly constrained in this remote area and deep geophysical data are sparse. Receiver function analysis of seven temporary broad-band seismometers of the Ellesmere Island Lithosphere Experiment complemented by two permanent stations provides important data on the crustal velocity structure of Ellesmere Island. The crustal expression of the northernmost tectonic block of Ellesmere Island (~82°–83°N), Pearya, which was accreted during the Ellesmerian orogeny, is similar to that at the southernmost part, which is part of the Precambrian Laurentian (North America-Greenland) craton. Both segments have thick crystalline crust (~35–36 km) and comparable velocity–depth profiles. In contrast, crustal thickness in central Ellesmere Island decreases from ~24–30 km in the Eurekan fold and thrust belt (~79.7°–80.6°N) to ~16–20 km in the Hazen Stable Block (HSB; ~80.6°–81.4°N) and is covered by a thick succession of metasediments. A deep crustal root (~48 km) at ~79.6°N is interpreted as cratonic crust flexed beneath the Eurekan fold and thrust belt. The Carboniferous to Palaeogene sedimentary succession of the Sverdrup Basin is inferred to be up to 1–4 km thick, comparable to geologically-based estimates, near the western margin of the HSB.

Description: Joint analysis of the seismic velocities and geoid, gravity and gravity gradients are used to constrain the viscosity profile within the mantle as well as the lateral density variations. Recent ESA's Gravity field and steady-state Ocean Circulation Explorer measurements of the second-order derivatives of the Earth's gravity potential give new possibilities to determine these mantle properties. Using a simple mantle model and seismic tomography results, we investigate how the gravitational potential, the three components of the gravity vector and the gravity gradients can bring information on the radial viscosity profile and on the mantle mass anomalies. We start with lateral density variations in the Earth's mantle based either on slab history or deduced from seismic tomography. The main uncertainties are: for the latter case, the relationship between seismic velocity and density—the so-called density/velocity scaling factor—and for the former case, the variation with depth of the density contrast between the cold slabs and the surrounding mantle. We perform a Monte Carlo search for the viscosity and the density/velocity scaling factor profiles within the mantle, which allows to fit the observed geoid, gravity and gradients of gravity. We compute the posterior probability distribution of the unknown parameters, and find that the gravity gradients improve the estimate of the scaling factor within the upper mantle, because of their sensitivity to the masses within the upper mantle, whereas the geoid and the gravity better constrain the scaling factor in the lower mantle. In the upper mantle, it is less than 0.02 in the upper part and about 0.08–0.14 in the lower part, and it is significantly larger for depths greater than 1200 km (about 0.32–0.34). In any case, the density/velocity scaling factor between 670 and 1150 km depth is not well constrained. We show that the viscosity of the upper part of the mantle is strongly correlated with the viscosity of the lower part of the mantle and that the viscosity profile is characterized by a decrease in the lower part of the upper mantle (about 10 20 –2  x  10 20 Pa s) and by an increase (about 10 23 –2  x  10 23 Pa s) at the top of the lower mantle (between 670 and 1150 km). The viscosity of the mantle below 1150 km depth is well estimated in our Monte Carlo search and is about 10 22 –4  x  10 22 Pa s.

Description: Large-scale topography may be due to several causes, including (1) variations in crustal thickness and density structure, (2) oceanic lithosphere age differences, (3) subcrustal density variations in the continental lithosphere and (4) convective flow in the mantle beneath the lithosphere. The last contribution in particular may change with time and be responsible for continental inundations; distinguishing between these contributions is therefore important for linking Earth's history to its observed geological record. As a step towards this goal, this paper aims at such distinction for the present-day topography: the approach taken is deriving a ‘model’ topography due to contributions (3) and (4), along with a model geoid, using a geodynamic mantle flow model. Both lithosphere thickness and density anomalies beneath the lithosphere are inferred from seismic tomography. Density anomalies within the continental lithosphere are uncertain, because they are probably due to variations in composition and temperature, making a simple scaling from seismic to density anomalies inappropriate. Therefore, we test a number of different assumptions regarding these. As a reality check, model topography is compared, in terms of both correlation and amplitude ratio, to ‘residual’ topography, which follows from observed topography after subtracting contributions (1) and (2). The model geoid is compared to observations as well. Comparatively good agreement is found if there is either an excess density of 0.2 per cent in the lithosphere above 150 km depth, with anomalies below as inferred from tomography, or if the excess density is 0.4 per cent in the entire lithosphere. Further, a good fit is found for viscosity 10 20 Pa s in the asthenosphere, increasing to 10 23 Pa s in the lower mantle above D'. Results are quite dependent on which tomography models they are based on; for some recent ones, topography correlation is 0.6, many smaller scale features are matched, topography amplitude is less than 30 per cent too large, while geoid variance reduction exceeds 70 per cent—overall a considerable improvement compared to previous models. Correlation becomes less if smaller scale features (corresponding to spherical harmonic degrees 15 and higher), which are probably largely due to anomalies in the lithosphere, are also considered. Comparison of results with different viscosity structures, and a regional comparison of amplitude ratios, indicates that lateral viscosity variations can be quite strong, but only leading to moderate variations in model topography of a factor probably less than two.

Description: Oblique convergence across Tibet leads to slip partitioning with the coexistence of strike-slip, normal and thrust motion on major fault systems. A key point is to understand and model how faults interact and accumulate strain at depth. Here, we extract ground deformation across the Haiyuan Fault restraining bend, at the northeastern boundary of the Tibetan plateau, from Envisat radar data spanning the 2001–2011 period. We show that the complexity of the surface displacement field can be explained by the partitioning of a uniform deep-seated convergence. Mountains and sand dunes in the study area make the radar data processing challenging and require the latest developments in processing procedures for Synthetic Aperture Radar interferometry. The processing strategy is based on a small baseline approach. Before unwrapping, we correct for atmospheric phase delays from global atmospheric models and digital elevation model errors. A series of filtering steps is applied to improve the signal-to-noise ratio across high ranges of the Tibetan plateau and the phase unwrapping capability across the fault, required for reliable estimate of fault movement. We then jointly invert our InSAR time-series together with published GPS displacements to test a proposed long-term slip-partitioning model between the Haiyuan and Gulang left-lateral Faults and the Qilian Shan thrusts. We explore the geometry of the fault system at depth and associated slip rates using a Bayesian approach and test the consistency of present-day geodetic surface displacements with a long-term tectonic model. We determine a uniform convergence rate of 10 [8.6–11.5] mm yr –1 with an N89 [81–97]°E across the whole fault system, with a variable partitioning west and east of a major extensional fault-jog (the Tianzhu pull-apart basin). Our 2-D model of two profiles perpendicular to the fault system gives a quantitative understanding of how crustal deformation is accommodated by the various branches of this thrust/strike-slip fault system and demonstrates how the geometry of the Haiyuan fault system controls the partitioning of the deep secular motion.

Description: In this paper, we study stability of the rate, state and temperature friction (RSTF) model. The Segall and Rice approach is used to model heat transfer at the sliding interface with its surroundings. The effect of pore pressure is not considered in the model to avoid the complex expression for critical stiffness. Linear stability analysis of the spring-mass sliding system is carried out with the ageing law under the quasistatic conditions in order to determine the critical stiffness above which sliding behaviour changes from unstable to stable or vice versa. Our numerical simulations establish that critical stiffness of the heated surface may increase or decrease from corresponding to the critical stiffness of the unheated surface depending on the relative values of two contradictory parameters related with velocity effect and temperature effect. Parametric studies are also carried out to understand shear velocity and temperature of the sliding surface dependence of steady friction. The RSTF model is also used to study the gravity induced failure of a creeping rock slope and the results are justified.

Description: The Siple Coast region of Antarctica contains a number of fast-flowing ice streams, which control the dynamics and mass balance of the region. These ice streams are known to undergo stagnation and reactivation cycles, which lead to ice thickness changes that may be sufficient to excite a viscous solid Earth response (glacial isostatic adjustment; GIA). This study aims to quantify Siple Coast ice thickness changes during the last 2000 yr in order to determine the degree to which they might contribute to GIA and associated present-day bedrock uplift rates. This is important because accurate modelling of GIA is necessary to determine the rate of present-day ice-mass change from satellite gravimetry. Recently-published reconstructions of ice-stream variability were used to create a suite of kinematic models for the stagnation-related thickening of Kamb Ice Stream since ~1850 AD, and a GIA model was used to predict present-day deformation rates in response to this thickening. A number of longer-term loading scenarios, which include the stagnation and reactivation of ice streams across the Siple Coast over the past 2000 yr, were also constructed, and used to investigate the longer term GIA signal in the region. Uplift rates for each of the ice loading histories, based on a range of earth models, were compared with regional GPS-observed uplift rates and an empirical GIA estimate. We estimate Kamb Ice Stream to have thickened by 70–130 m since stagnation ~165 years ago. Modelled present-day vertical motion in response to this load increase peaks at –17 mm yr –1 (i.e. 17 mm yr –1 subsidence) for the weakest earth models tested here. Comparison of the solid Earth response to ice load changes throughout the last glacial cycle, including ice stream stagnation and reactivation across the Siple Coast during the last 2000 yr, with an empirical GIA estimate suggests that the upper mantle viscosity of the region is greater than 1 x 10 20 Pa s. When upper mantle viscosity values of 1 x 10 20  Pa s or smaller are combined with our suite of ice-load scenarios we predict uplift rates across Siple Coast that are at least 4 mm yr –1 smaller than those predicted by the empirical GIA estimate. GPS data are unable to further constrain model parameters due to the distance of the GPS sites from the study area. Our results demonstrate that Late Holocene ice load changes related to the stagnation and reactivation of ice streams on the Siple Coast may play a dominant role in defining the present-day uplift signal. However, both the detailed Earth structure and deglacial history of the region need to be better constrained in order to reduce uncertainties associated with the GIA signal of this region.

Description: The thickness and equivalent global sea level contribution of an improved model of the central and northern Laurentide Ice Sheet is constrained by 24 relative sea level histories and 18 present-day GPS-measured vertical land motion rates. The final model, termed Laur16, is derived from the ICE-5G model by holding the timing history constant and iteratively adjusting the thickness history, in four regions of northern Canada. In the final model, the last glacial maximum (LGM) thickness of the Laurentide Ice Sheet west of Hudson Bay was ~3.4–3.6 km. Conversely, east of Hudson Bay, peak ice thicknesses reached ~4 km. The ice model thicknesses inferred for these two regions represent, respectively, a ~30 per cent decrease and an average ~20–25 per cent increase to the load thickness relative to the ICE-5G reconstruction, which is generally consistent with other recent studies that have focussed on Laurentide Ice Sheet history. The final model also features peak ice thicknesses of 1.2–1.3 km in the Baffin Island region, a modest reduction relative to ICE-5G and unchanged thicknesses for a region in the central Canadian Arctic Archipelago west of Baffin Island. Vertical land motion predictions of the final model fit observed crustal uplift rates well, after an adjustment is made for the elastic crustal response to present-day ice mass changes of regional ice cover. The new Laur16 model provides more than a factor of two improvement of the fit to the RSL data ( 2 measure of misfit) and a factor of nine improvement to the fit of the GPS data (mean squared error measure of fit), compared to the ICE-5G starting model. Laur16 also fits the regional RSL data better by a factor of two and gives a slightly better fit to GPS uplift rates than the recent ICE-6G model. The volume history of the Laur16 reconstruction corresponds to an up to 8 m reduction in global sea level equivalent compared to ICE-5G at LGM.

Description: The 2011 October 23 M W 7.1 Van earthquake in eastern Turkey caused ~600 deaths and caused widespread damage and economic loss. The seismogenic rupture was restricted to 10–25 km in depth, but aseismic surface creep, coincident with outcrop fault exposures, was observed in the hours to months after the earthquake. We combine observations from radar interferometry, seismology, geomorphology and Quaternary dating to investigate the geological slip rate and seismotectonic context of the Van earthquake, and assess the implications for continuing seismic hazard in the region. Transient post-seismic slip on the upper Van fault started immediately following the earthquake, and decayed over a period of weeks; it may not fully account for our long-term surface slip-rate estimate of ≥0.5 mm yr –1 . Post-seismic slip on the Bostanici splay fault initiated several days to weeks after the main shock, and we infer that it may have followed the M W  5.9 aftershock on the 9th November. The Van earthquake shows that updip segmentation can be important in arresting seismic ruptures on dip-slip faults. Two large, shallow aftershocks show that the upper 10 km of crust can sustain significant earthquakes, and significant slip is observed to have reached the surface in the late Quaternary, so there may be a continuing seismic hazard from the upper Van fault and the associated splay. The wavelength of folding in the hanging wall of the Van fault is dominated by the structure in the upper 10 km of the crust, masking the effect of deeper seismogenic structures. Thus, models of subsurface faulting based solely on surface folding and faulting in regions of reverse faulting may underestimate the full depth extent of seismogenic structures in the region. In measuring the cumulative post-seismic offsets to anthropogenic structures, we show that Structure-from-Motion can be rapidly deployed to create snapshots of post-seismic displacement. We also demonstrate the utility of declassified Corona mission imagery (1960s–1970s) for geomorphic mapping in areas where recent urbanization has concealed the geomorphic markers.

Description: Within a slab experiencing present-day lateral break-off, a particular type of earthquakes is expected to cluster at the detachment horizon tip: namely, events generated by reverse faulting, with the approximately horizontal compression involved acting along the strike of the slab. Such a cluster of moderate magnitude earthquakes (4.7 ≤  m b ≤ 5.0) was identified in this study at the 160–175 km depth range of the Vrancea seismogenic body, in the Southeast Carpathians mountains collision environment. The corresponding cluster epicentres were systematically positioned at the boundary between a region being subject ( cf. published GPS records), to present-day upward movements, and another one that underwent present-day subsidence. Such an overall setting seems to suggest that a lateral break-off is currently developing at the indicated depth within the Vrancea slab, leading to topographic uplift above the already detached slab section, and to enhanced subsidence above the section to which the gravitational slab pull was being transferred. In addition, by taking into account some systematic time correspondence which we documented between moderate magnitude events of the 160–175 km depth cluster and the subsequent strong Vrancea shocks ( M w ≥ 6.9), it appears that the latter, although occurring at much shallower depths (roughly, in the 80–140 km range), were also controlled by the break-off progress.

Description: Bajada del Diablo is located in the Northern Patagonian Massif, Chubut, Argentina. The study area includes several circular structures found in Miocene olivine basalts of the Quiñelaf Eruptive Complex and in the Late Pliocene/Early Pleistocene Pampa Sastre conglomerates. An impact origin has been proposed for these circular structures. With the aim of further investigate the proposed impact origin, topographic, gravimetric, magnetic, resistivity, palaeomagnetic and electromagnetic surveys in two circular structures (‘8’ and ‘G’) located in Pampa Sastre conglomerates and in basalts of the Quiñelaf Eruptive Complex were carried out. The new geophysical results support the hypothesis of an impact origin. However, the confirmation of such an origin through the findings of shock metamorphism evidences and/or the recovery of meteorites remains elusive.

Description: Estimating in situ stress based on hydraulic fracturing data typically depends on interpretation of the breakdown, secondary breakdown (‘reopening’) and shut-in pressure. While it has been recognized that the near-wellbore stress field should be taken into account and that the compressibility of the injection system and the viscous flow of the fluid can diminish the accuracy of stress estimates, these issues have not been well quantified. A coupled numerical model that includes the compressibility of the injection system and the flow of a viscous fluid in a plane-strain hydraulic fracture extending from a wellbore, in an impermeable rock, and in the presence of a non-isotropic in situ stress field provides a basic tool for estimating the order of the error associated with hydraulic fracturing stress measurements under non-ideal conditions. The main findings of this work are model-based guidelines on the values of relevant dimensionless parameter groups to ensure sufficient accuracy of stress estimates that use idealized models. When these guidelines cannot be met under field conditions, the model can be further applied to obtain first-order corrections that account for compressibility, viscosity and near-wellbore effects.

Description: We consider the problem of seismic velocity change estimation using ambient noise recordings. Motivated by Zhan et al. , we study how the velocity change estimation is affected by seasonal fluctuations in the noise sources. More precisely, we consider a numerical model and introduce spatio-temporal seasonal fluctuations in the noise sources. We show that indeed, as pointed out by Zhan et al. , the stretching method is affected by these fluctuations and produces misleading apparent velocity variations which reduce dramatically the signal to noise ratio of the method. We also show that these apparent velocity variations can be eliminated by an adequate normalization of the cross-correlation functions. Theoretically we expect our approach to work as long as the seasonal fluctuations in the noise sources are uniform, an assumption which holds for closely located seismic stations. We illustrate with numerical simulations in homogeneous and scattering media that the proposed normalization significantly improves the accuracy of the velocity change estimation. Similar behaviour is also observed with real data recorded in the Aegean volcanic arc. We study in particular the volcano of Santorini during the seismic unrest of 2011–2012 and observe a decrease in the velocity of seismic waves which is correlated with GPS measured elevation.

Description: Passive seismic experiment was carried out at the SW contact of the Dinarides and Pannonian basin to determine the crustal structure and velocity discontinuities. The aim of the experiment was to define the relationship between the Adriatic microplate and the Pannonian segment as a part of the European plate. Most of the temporary seismic stations were deployed in Croatia along the Alp07 profile—a part of the active-source ALP 2002 project. About 300-km-long profile stretches from Istra peninsula to the Drava river, in a WSW–ESE direction. Teleseismic events recorded on 13 temporary seismic stations along the profile were analysed by P -receiver function method. Two types of characteristic receiver functions (RF) have been identified, belonging to Dinaridic and Pannonian crusts as defined on the Alp07 profile, while in transitional zone there are both types. Three major crustal discontinuities can be identified for the Dinaridic type: sedimentary basement, intracrustal discontinuity and Mohorovičić discontinuity, whereas the Pannonian type revealed only two discontinuities. The intracrustal discontinuity was not observed in the Pannonian type, thus pointing to a single-layered crust in the Pannonian basin. Two interpretation methods were applied: forward modelling of the receiver functions and H – stacking method, and the results were compared with the active-source seismic data at deep refraction profile Alp07. The receiver function modelling has given reliable results of the Moho depths that are in accordance with the seismic refraction results at the end of the Alp07 profile, that is in the area of Pannonian crust characterized by simple crustal structure and low seismic velocities ( Vp between 5.9 and 6.2 km s –1 ). In the Dinarides and its peripheral parts, receiver function modelling regularly gives greater Moho depths, up to +15 per cent, due to more complex crustal structure. The depths of the Moho calculated by the H – stacking method vary within wide limits (±13 km), due to band limited data of short-period stations. The results at five stations have to be rejected because of huge deviations in comparison with all previous results, while at the other seven stations the Moho depths vary within ±15 per cent around the Moho discontinuity of the Alp07 profile.

Description: Megathrust earthquakes of magnitude close to 9 are followed by large-scale (thousands of km) and long-lasting (decades), significant crustal and mantle deformation. This deformation can be observed at the surface and quantified with GPS measurements. Here we report on deformation observed during the 5 yr time span after the 2010 M w 8.8 Maule Megathrust Earthquake (2010 February 27) over the whole South American continent. With the first 2 yr of those data, we use finite element modelling (FEM) to relate this deformation to slip on the plate interface and relaxation in the mantle, using a realistic layered Earth model and Burgers rheologies. Slip alone on the interface, even up to large depths, is unable to provide a satisfactory fit simultaneously to horizontal and vertical displacements. The horizontal deformation pattern requires relaxation both in the asthenosphere and in a low-viscosity channel along the deepest part of the plate interface and no additional low-viscosity wedge is required by the data. The vertical velocity pattern (intense and quick uplift over the Cordillera) is well fitted only when the channel extends deeper than 100 km. Additionally, viscoelastic relaxation alone cannot explain the characteristics and amplitude of displacements over the first 200 km from the trench and aseismic slip on the fault plane is needed. This aseismic slip on the interface generates stresses, which induce additional relaxation in the mantle. In the final model, all three components (relaxation due to the coseismic slip, aseismic slip on the fault plane and relaxation due to aseismic slip) are taken into account. Our best-fit model uses slip at shallow depths on the subduction interface decreasing as function of time and includes (i) an asthenosphere extending down to 200 km, with a steady-state Maxwell viscosity of 4.75 x 10 18 Pa s; and (ii) a low-viscosity channel along the plate interface extending from depths of 55–135 km with viscosities below 10 18 Pa s.

Description: We provide an updated present-day stress map for the Italian territory. Following the World Stress Map (WSM) Project guidelines, we list the different stress indicators, explaining the criteria used to select data. We discuss the data, which will also be included in the 2016 release of the WSM, highlighting the areas for which we have added stress information. Our map displays the minimum horizontal stress orientations inferred from crustal stress indicators down to 40 km depth using data of A–C quality, updated for earthquakes until December 2015. We have completely reviewed all data, and the data set now contains 855 entries, in contrast to the previous 715. The number of data with A–C quality of 630 corresponds to an increase of 26 per cent relative to the previous data set. In particular, the new data set contains the results of the analysis of borehole breakouts, critically reviewed data from earthquake focal mechanisms, data concerning active faults, formal inversions of focal mechanisms of seismic sequences or of restricted areas and one stress determination from overcoring. The new data set defines the stress field in areas not well covered by the previous data: the region north to the Po Plain and the central Adriatic sea, both characterized by a thrust- and strike-faulting regime, the northern Sicilian belt with a prevailing normal-faulting regime, and the Ionian sea with a strike-slip regime.

Description: Measurements of the velocity field associated with plumes rising through a viscous fluid are performed using stereoscopic Particle-Image Velocimetry in the Rayleigh number range 4.4 x 10 5 –6.4 x 10 5 . The experimental model is analogous to a mantle plume rising from the core–mantle boundary to the base of the lithosphere. The behaviour of the plume is studied throughout its life cycle, which is broken up into four stages; the Formation Stage, when the plume forms; the Rising Stage, when the plume rises through the fluid; the Spreading Stage, when the plume reaches the surface and spreads; and finally the Declining Stage, when the heat source has been removed and the plume weakens. The latter three stages are examined in terms of the Finite-Time Lyapunov Exponent fields and the advection of passive tracers throughout the flow. The temperature at the heater and near the fluid surface are measured using thermocouples to infer how the presence of a mantle plume would produce excess temperature near the lithosphere throughout the various stages of its life cycle. In all experiments, a time lag is observed between the removal of the heat source and the decline in the excess temperature near the surface, which is proportional to the rise time. A simple analytical model is presented, which suggests that under mantle conditions (i.e. negligible thermal diffusion), the relationship between the time lag and the rise time is robust and independent of the Rayleigh number; however, the constant of proportionality is closer to unity in the absence of diffusion. Once the heat source is removed, the excess temperature near the surface declines exponentially at a rate that is inversely proportional to the rise time. The implications of this result are discussed in terms of the decline in volcanism in the Louisville hotspot chain over the past 20 Ma. The rise velocity of material in the plume is examined; the rise velocity is found to vary significantly with the plume height in a manner that is inconsistent with many of the common semi-analytical models of thermal plumes in the literature. It is also argued that this height dependency will cause estimates of the rise velocity based on the decay series of uranium isotopes to significantly underestimate the true value.

Description: The Makran subduction zone is one of the last convergent margins to be investigated using space-based geodesy. While there is a lack of historical and modern instrumentation in the region, a sparse sampling of continuous and campaign measurements over the past decade has allowed us to make the first estimates of convergence rates. We combine GPS measurements from 20 stations located in Iran, Pakistan and Oman along with hypocentral locations from the International Seismological Centre to create a preliminary 3-D estimate of the geometry of the megathrust, along with a preliminary fault-coupling model for the Makran subduction zone. Using a convergence rate which is strongly constrained by measurements from the incoming Arabia plate along with the backslip method of Savage, we find the Makran subduction zone appears to be locked to a depth of at least 38 km and accumulating strain.We also find evidence for a segmentation of plate coupling, with a 300 km long section of reduced plate coupling. The range of acceptable locking depths from our modelling and the 900 km along-strike length for the megathrust, makes the Makran subduction zone capable of earthquakes up to M w  = 8.8. In addition, we find evidence for slow-slip-like transient deformation events on two GPS stations. These observations are suggestive of transient deformation events observed in Cascadia, Japan and elsewhere.

Description: We present numerical models of mantle dynamics forced by plate velocities history in the last 450 Ma. The lower-mantle rheology and the thickness of a dense basal layer are systematically varied and several initial procedures are considered for each case. For some cases, the dependence on the mantle convection vigour is also examined. The resulting evolution of the CMB heat flux is analysed in terms of criteria to promote or inhibit reversals inferred from numerical dynamos. Most models present a rather dynamic lower mantle with the emergence of two thermochemical piles towards present-day. Only a small minority of models present two stationary piles over the last 450 Myr. At present-day, the composition field obtained in our models is found to correlate better with tomography than the temperature field. In addition, the temperature field immediately at the CMB (and thus the heat flux pattern) slightly differs from the average temperature field over the 100-km thick mantle layer above it. The evolution of the mean CMB heat flux or of the amplitude of heterogeneity seldom presents the expected correlation with the evolution of the palaeomagnetic reversal frequency suggesting these effects cannot explain the observations. In contrast, our analysis favours ‘inertial control’ on the geodynamo associated with polar cooling and in some cases break of Taylor columns in the outer core as sources of increased reversal frequency. Overall, the most likely candidates among our mantle dynamics models involve a viscosity increase in the mantle equal or smaller than 30: models with a discontinuous viscosity increase at the transition zone tend to agree better at present-day with observations of seismic tomography, but models with a gradual viscosity increase agree better with some of the criteria proposed to affect reversal frequency.

Description: Lateral viscosity variations (LVVs) in the mantle influence geodynamic processes and their surface expressions. With the observed long-wavelength geoid, free-air anomaly, gravity gradient in three directions and discrete, high-accuracy residual topography, we invert for depth- and temperature-dependent and tectonically regionalized mantle viscosity with a mantle flow model. The inversions suggest that long-wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile; the pre-Cambrian lithosphere viscosity is slightly (~ one order of magnitude) higher than that of oceanic and Phanerozoic lithosphere; plate margins are substantially weaker than plate interiors; and viscosity has only a weak apparent, dependence on temperature, suggesting either a balancing between factors or a smoothing of actual higher amplitude, but short wavelength, LVVs. The predicted large-scale lithospheric stress regime (compression or extension) is consistent with the world stress map (thrust or normal faulting). Both recent compiled high-accuracy residual topography and the predicted dynamic topography yield ~1 km amplitude long-wavelength dynamic topography, inconsistent with recent studies suggesting amplitudes of ~100 to ~500 m. Such studies use a constant, positive admittance (transfer function between topography and gravity), in contrast to the evidence which shows that the earth has a spatially and wavelength-dependent admittance, with large, negative admittances between ~4000 and ~10 4 km wavelengths.

Description: We present a new, computationally efficient numerical method to simulate global seismic wave propagation in realistic 3-D Earth models. We characterize the azimuthal dependence of 3-D wavefields in terms of Fourier series, such that the 3-D equations of motion reduce to an algebraic system of coupled 2-D meridian equations, which is then solved by a 2-D spectral element method (SEM). Computational efficiency of such a hybrid method stems from lateral smoothness of 3-D Earth models and axial singularity of seismic point sources, which jointly confine the Fourier modes of wavefields to a few lower orders. We show novel benchmarks for global wave solutions in 3-D structures between our method and an independent, fully discretized 3-D SEM with remarkable agreement. Performance comparisons are carried out on three state-of-the-art tomography models, with seismic period ranging from 34 s down to 11 s. It turns out that our method has run up to two orders of magnitude faster than the 3-D SEM, featured by a computational advantage expanding with seismic frequency.

Description: We report on a study to explore the deep electrical conductivity structure of the Dead Sea Basin (DSB) using magnetotelluric (MT) data collected along a transect across the DSB where the left lateral strike-slip Dead Sea transform (DST) fault splits into two fault strands forming one of the largest pull-apart basins of the world. A very pronounced feature of our 2-D inversion model is a deep, subvertical conductive zone beneath the DSB. The conductor extends through the entire crust and is sandwiched between highly resistive structures associated with Precambrian rocks of the basin flanks. The high electrical conductivity could be attributed to fluids released by dehydration of the uppermost mantle beneath the DSB, possibly in combination with fluids released by mid- to low-grade metamorphism in the lower crust and generation of hydrous minerals in the middle crust through retrograde metamorphism. Similar high conductivity zones associated with fluids have been reported from other large fault systems. The presence of fluids and hydrous minerals in the middle and lower crust could explain the required low friction coefficient of the DST along the eastern boundary of the DSB and the high subsidence rate of basin sediments. 3-D inversion models confirm the existence of a subvertical high conductivity structure underneath the DSB but its expression is far less pronounced. Instead, the 3-D inversion model suggests a deepening of the conductive DSB sediments off-profile towards the south, reaching a maximum depth of approximately 12 km, which is consistent with other geophysical observations. At shallower levels, the 3-D inversion model reveals salt diapirism as an upwelling of highly resistive structures, localized underneath the Al-Lisan Peninsula. The 3-D model furthermore contains an E–W elongated conductive structure to the northeast of the DSB. More MT data with better spatial coverage are required, however, to fully constrain the robustness of the above-mentioned off-profile features.

Description: Estimating how topography is maintained provides insights into the different factors responsible for surface deformations and their relative roles. Here, we develop a new and simple approach to assess the degree of isostatic compensation of continental topography at regional scale from GOCE gravity gradients. We calculate the ratio between the radial gradient observed by GOCE and that calculated from topography only. From analytical and statistical formulations, simple relationships between this ratio and the degree of compensation are obtained under the Airy–Heiskanen isostasy hypothesis. Then, a value of degree of compensation at each point of study area can be easily deduced. We apply our method to the Alaska-Canada Cordillera and validate our results by comparison with a standard isostatic gravity anomaly model and additional geophysical information for this area. Both our GOCE-based results and the isostatic anomaly show that Airy–Heiskanen isostasy prevails for the Yukon Plateau whereas additional mechanisms are required to support topography below the Northwest Territories Craton and the Yakutat collision zone.

Description: Bayesian sampling based inversions require many thousands or even millions of forward models, depending on how nonlinear or non-unique the inverse problem is, and how many unknowns are involved. The result of such a probabilistic inversion is not a single ‘best-fit’ model, but rather a probability distribution that is represented by the entire model ensemble. Often, a geophysical inverse problem is non-unique, and the corresponding posterior distribution is multimodal, meaning that the distribution consists of clusters with similar models that represent the observations equally well. In these cases, we would like to visualize the characteristic model properties within each of these clusters of models. However, even for a moderate number of inversion parameters, a manual appraisal for a large number of models is not feasible. This poses the question whether it is possible to extract end-member models that represent each of the best-fit regions including their uncertainties. Here, I show how a machine learning tool can be used to characterize end-member models, including their uncertainties, from a complete model ensemble that represents a posterior probability distribution. The model ensemble used here results from a nonlinear geodynamic inverse problem, where rheological properties of the lithosphere are constrained from multiple geophysical observations. It is demonstrated that by taking vertical cross-sections through the effective viscosity structure of each of the models, the entire model ensemble can be classified into four end-member model categories that have a similar effective viscosity structure. These classification results are helpful to explore the non-uniqueness of the inverse problem and can be used to compute representative data fits for each of the end-member models. Conversely, these insights also reveal how new observational constraints could reduce the non-uniqueness. The method is not limited to geodynamic applications and a generalized MATLAB code is provided to perform the appraisal analysis.

Description: Inference of the mantle viscosity from observations for glacial isostatic adjustment (GIA) process has usually been conducted through the analyses based on the simple three-layer viscosity model characterized by lithospheric thickness, upper- and lower-mantle viscosities. Here, we examine the viscosity structures for the simple three-layer viscosity model and also for the two-layer lower-mantle viscosity model defined by viscosities of 670, D (670- D km depth) and D ,2891 ( D -2891 km depth) with D -values of 1191, 1691 and 2191 km. The upper-mantle rheological parameters for the two-layer lower-mantle viscosity model are the same as those for the simple three-layer one. For the simple three-layer viscosity model, rate of change of degree-two zonal harmonics of geopotential due to GIA process (GIA-induced 2 ) of –(6.0–6.5)  x  10 –11 yr –1 provides two permissible viscosity solutions for the lower mantle, (7–20)  x  10 21 and (5–9)  x  10 22  Pa s, and the analyses with observational constraints of the 2 and Last Glacial Maximum (LGM) sea levels at Barbados and Bonaparte Gulf indicate (5–9)  x  10 22  Pa s for the lower mantle. However, the analyses for the 2 based on the two-layer lower-mantle viscosity model only require a viscosity layer higher than (5–10)  x  10 21  Pa s for a depth above the core–mantle boundary (CMB), in which the value of (5–10)  x  10 21  Pa s corresponds to the solution of (7–20)  x  10 21  Pa s for the simple three-layer one. Moreover, the analyses with the 2 and LGM sea level constraints for the two-layer lower-mantle viscosity model indicate two viscosity solutions: 670,1191  〉 3  x  10 21 and 1191,2891  ~ (5–10)  x  10 22  Pa s, and 670,1691  〉 10 22 and 1691,2891  ~ (5–10)  x  10 22  Pa s. The inferred upper-mantle viscosity for such solutions is (1–4)  x  10 20  Pa s similar to the estimate for the simple three-layer viscosity model. That is, these analyses require a high viscosity layer of (5–10)  x  10 22  Pa s at least in the deep mantle, and suggest that the GIA-based lower-mantle viscosity structure should be treated carefully in discussing the mantle dynamics related to the viscosity jump at ~670 km depth. We also preliminarily put additional constraints on these viscosity solutions by examining typical relative sea level (RSL) changes used to infer the lower-mantle viscosity. The viscosity solution inferred from the far-field RSL changes in the Australian region is consistent with those for the 2 and LGM sea levels, and the analyses for RSL changes at Southport and Bermuda in the intermediate region for the North American ice sheets suggest the solution of 670, D  〉 10 22 , D ,2891  ~ (5–10)  x  10 22  Pa s ( D  = 1191 or 1691 km) and upper-mantle viscosity higher than 6  x  10 20  Pa s.

Description: Mid-crustal conductors are a common phenomenon in magnetotelluric studies. In the Andean Cordillera of southern Chile, they appear to concentrate along major fault zones. A high-resolution, broad-band magnetotelluric survey including 31 stations has been carried out along two profiles perpendicular to (1) the Liquiñe-Ofqui Fault Systems (LOFS) and (2) the Villarrica-Quetrupillán-Lanín volcanic lineament running parallel to the Mocha-Villarrica Fault Zone (MVFZ). The survey aimed at tracing one of the known conductors from mid-crustal depth to near-surface along these faults. Directionality and dimensionality were analysed using tensor decomposition. Phase tensors and induction arrows reveal two major geoelectric strike directions following the strike of LOFS and MVFZ. 2-D inversion shows low resistivity zones along both fault systems down to a depth of 〉10 km, where the brittle-ductile transition is expected. Along the LOFS, the two anomalies are linked to (1) Lake Caburgua, where the LOFS broadens to about 2 km of lateral extension and seems to represent a pull-apart structure, and (2) the intersection with the Villarrica-Quetrupillán-Lanín volcanic lineament, where seismic activity was observed during the latest eruption in March 2015. A connection of the mid-crustal conductor to the ESE-WNW-striking fault zones is indicated from the presented data.

Description: We present new rotations that describe the relative positions and velocities of the Pacific and North America plates at 22 times during the past 19.7 Myr, offering 1-Myr temporal resolution for studies of the geotectonic evolution of western North America and other plate boundary locations. Derived from 18 000 magnetic reversal, fracture zone and transform fault identifications from the Pacific–Antarctic–Nubia–North America plate circuit and the velocities of 935 GPS sites on the Pacific and North America plates, the new rotations and GPS-derived angular velocity indicate that the rate of motion between the two plates increased by 70 per cent from 19.7 to 9±1 Ma, but changed by less than 2 per cent since 8 Ma and even less since 4.2 Ma. The rotations further suggest that the relative plate direction has rotated clockwise for most of the past 20 Myr, with a possible hiatus from 9 to 5 Ma. This conflicts with previously reported evidence for a significant clockwise change in the plate direction at 8–6 Ma. Our new rotations indicate that Pacific plate motion became obliquely convergent with respect to the San Andreas Fault of central California at 5.2–4.2 Ma, in agreement with geological evidence for a Pliocene onset of folding and faulting in central California. Our reconstruction of the northern Gulf of California at 6.3 Ma differs by only 15–30 km from structurally derived reconstructions after including 3–4 km Myr –1 of geodetically measured slip between the Baja California Peninsula and Pacific plate. This implies an approximate 15–30 km upper bound for plate non-rigidity integrated around the global circuit at 6.3 Ma. A much larger 200±54 km discrepancy between our reconstruction of the northern Gulf of California at 12 Ma and that estimated from structural and marine geophysical observations suggests that faults in northwestern Mexico or possibly west of the Baja California Peninsula accommodated large amounts of obliquely divergent dextral shear from 12–6.3 Ma. Pacific–North America plate motion since 16 Myr estimated with our new rotations agrees well with structurally summed deformation along two transects of western North America between the Colorado Plateau and western California, with a difference as small as 40 km out of 760 km of margin-parallel motion. A strong resemblance between a 20-Myr-to-present flow line reconstructed with our new rotations and the traces of the 700-km-long Queen Charlotte Fault and continental slope west of Canada suggests that the plate margin geometry was influenced by the passage of the Pacific plate and Yakutat block. The new rotations also suggest that (1) oblique convergence west of Canada initiated at 12–11 Ma, 5–8 Myr earlier than previously estimated, (2) no significant margin-normal shortening has occurred in areas of Canada located east of the Haida Gwaii archipelago since 20 Ma and (3) Pacific plate underthrusting of Haida Gwaii has accommodated the margin-normal component of plate motion since 12–11 Ma. Our rotations suggest an 70 per cent increase in the rate that the Pacific plate has been consumed by subduction beneath the Aleutian arc since 19.7 Ma, with still-unknown consequences for the rate of arc magmatism.

Description: Described are results of laboratory experiments which revealed regularities of gradual transition from stick-slip mode to aseismic creep. The behaviour of model gouge-filled fault was investigated with experimental setup of the spring-bock model. It was experimentally proven that small variations of a percentage of materials with velocity strengthening and velocity weakening properties in the fault principal slip zone may result in significant variation of the portion of seismic energy radiated during a fault slip event. The tests simulated different modes of interblock sliding whose characteristic values of scaled kinetic energy varied by several orders of magnitude, while differences in contact strength and shear stress drop remained relatively small. The obtained results led to the conclusion that the earthquake radiation efficiency and the fault slip mode are governed by the ratio of two parameters—maximum fault slip-weakening rate and shear stiffness of the enclosing massif. The ratio can be essentially changed by small variations of the material composition of the fault principal slip zone.

Description: Among the outstanding tectonic questions regarding the convergence between the Tien Shan and Tarim basin in northwestern China are the manner in which deformation is accommodated within their lithospheres, and the extent that the Tarim lithosphere underthrusts the Tien Shan. In particular, the amount and type of deformation within the Tarim basin is poorly understood. It is also uncertain if the convergence between the Tarim and the Tien Shan takes place mainly along a discrete boundary, or if the Tarim lithosphere simply indents into the Kazach shield, forming the Tien Shan through crustal thickening accommodated by a distributed series of thrust faults. In this study we use hypocentres from published earthquake catalogues and waveforms recorded by regional seismic networks to determine earthquake source parameters through regional centroid moment tensor inversion. The entire dataset consists of 160 earthquakes that occurred between 1969 and 2009 and with moment magnitudes between 3.5 and 7 distributed throughout the central Tien Shan and northwestern Tarim Basin. The estimated focal depths of these earthquakes range from the near-surface to about 44 km. Focal mechanisms throughout much of the Tien Shan indicate active deformation accommodated by thrust faults from at least the upper crust to 30 km depth. South of the Tien Shan, the Jia-shi earthquake sequence within the Tarim basin suggests that both crustal shortening and localized flexure are part of a complicated process involving rotational convergence. Inside the Tarim basin, two earthquakes with thrust faulting mechanisms near the crust–mantle boundary beneath the Bachu uplift imply a brittle rheology of the lower crust. High-angle thrust events occur broadly across the Tien Shan, suggesting that the Tarim lithosphere as a whole is strong and indents into the Kazach shield to create the mountain range.

Description: The subduction of lithospheric plates is partitioned between subducting plate motion and lateral slab migration (i.e. trench retreat and advance). We use 3-D, dynamic models of subduction to address the role of a power-law mantle viscosity on subduction dynamics and, in particular, rates of trench retreat. For all numerical models tested, we find that a power-law rheology results in reduced rates of trench retreat, and elevated slab dip angles, relative to the equivalent isoviscous mantle model. We analyse the asthenospheric pressure distribution and the style of mantle flow, which exhibits only limited variability as a function of mantle rheology, in order to compute estimates of the mantle forces associated with subduction. The inclusion of a power-law rheology reduces the mantle shear force (which resists subducting plate motion) to a greater degree than it reduces the dynamic pressure gradient across the slab (which resists trench retreat). Therefore, the inclusion of a power-law mantle rheology favours a shift towards a subduction mode with a reduced trench retreat component, typically a relative reduction of order 25 per cent in our 3-D models. We suggest that this mechanism may be of importance for reducing the high trench retreat rates observed in many previous models to levels more in line with the average subduction partitioning observed on Earth at present (i.e. trench velocity ≤ plate velocity), for most absolute plate motion reference frames.

Description: Pulverized rocks (PR) are extremely incohesive and highly fractured rocks found within the damage zones of several large strike-slip faults around the world. They maintain their crystal structure, show little evidence of shearing or chemical alteration, and are believed to be produced by strong tensile forces. Several mechanisms for pulverization have been proposed based on simple qualitative analyses or laboratory experiments under simplified loading conditions. Numerical modelling, however, can offer new insights into what is needed to produce PR and likely conditions of formation. We perform dynamic rupture simulations of different earthquakes, varying the magnitude, the slip distribution, and the rupture speed (supershear and subshear), while measuring the stresses produced away from the fault. To contextualize our results, a basic threshold of 10 MPa is set as the tensile strength of the rock mass and recordings are made of where, when, and by how much this threshold is exceeded for each earthquake type. Guided by field observations, we discern that a large (〉 M w 7.1) subshear earthquake along a bimaterial fault produces a pulverized rock distribution most consistent with observations. The damage is asymmetric with the majority on the stiffer side of the fault extending out for several hundred metres. Within this zone there is a large and sudden volumetric expansion in all directions as the rupture passes. We propose that such an extreme tensile stress state, repeated for every earthquake, eventually produces the PR seen in the field.

Description: Kinematics of divergent boundaries and Rift-Rift-Rift junctions are classically studied using long-term geodetic observations. Since significant magma-related displacements are expected, short-term deformation provides important constraints on the crustal mechanisms involved both in active rifting and in transfer of extensional deformation between spreading axes. Using InSAR and GPS data, we analyse the surface deformation in the whole Central Afar region in detail, focusing on both the extensional deformation across the Quaternary magmato-tectonic rift segments, and on the zones of deformation transfer between active segments and spreading axes. The largest deformation occurs across the two recently activated Asal-Ghoubbet (AG) and Manda Hararo-Dabbahu (MH-D) magmato-tectonic segments with very high strain rates, whereas the other Quaternary active segments do not concentrate any large strain, suggesting that these rifts are either sealed during interdyking periods or not mature enough to remain a plate boundary. Outside of these segments, the GPS horizontal velocity field shows a regular gradient following a clockwise rotation of the displacements from the Southeast to the East of Afar, with respect to Nubia. Very few shallow creeping structures can be identified as well in the InSAR data. However, using these data together with the strain rate tensor and the rotations rates deduced from GPS baselines, the present-day strain field over Central Afar is consistent with the main tectonic structures, and therefore with the long-term deformation. We investigate the current kinematics of the triple junction included in our GPS data set by building simple block models. The deformation in Central Afar can be described by adding a central microblock evolving separately from the three surrounding plates. In this model, the northern block boundary corresponds to a deep EW-trending trans-tensional dislocation, locked from the surface to 10–13 km and joining at depth the active spreading axes of the Red Sea and the Aden Ridge, from AG to MH-D rift segments. Over the long-term, this plate configuration could explain the presence of the en-échelon magmatic basins and subrifts. However, the transient behaviour of the spreading axes implies that the deformation in Central Afar evolves depending on the availability of magma supply within the well-established segments.

Description: Volcanic crises are often preceded and accompanied by volcano deformation caused by magmatic and hydrothermal processes. Fast and efficient model identification and parameter estimation techniques for various sources of deformation are crucial for process understanding, volcano hazard assessment and early warning purposes. As a simple model that can be a basis for rapid inversion techniques, we present a compound dislocation model (CDM) that is composed of three mutually orthogonal rectangular dislocations (RDs). We present new RD solutions, which are free of artefact singularities and that also possess full rotational degrees of freedom. The CDM can represent both planar intrusions in the near field and volumetric sources of inflation and deflation in the far field. Therefore, this source model can be applied to shallow dikes and sills, as well as to deep planar and equidimensional sources of any geometry, including oblate, prolate and other triaxial ellipsoidal shapes. In either case the sources may possess any arbitrary orientation in space. After systematically evaluating the CDM, we apply it to the co-eruptive displacements of the 2015 Calbuco eruption observed by the Sentinel-1A satellite in both ascending and descending orbits. The results show that the deformation source is a deflating vertical lens-shaped source at an approximate depth of 8 km centred beneath Calbuco volcano. The parameters of the optimal source model clearly show that it is significantly different from an isotropic point source or a single dislocation model. The Calbuco example reflects the convenience of using the CDM for a rapid interpretation of deformation data.

Description: Tehran, the capital city of Iran with a population of over 12 million, is one of the largest urban centres within the seismically active Alpine–Himalayan orogenic belt. Although several historic earthquakes have affected Tehran, their relation to individual faults is ambiguous for most. This ambiguity is partly due to a lack of knowledge about the locations, geometries and seismic potential of structures that have been obscured by dramatic urban growth over the past three decades, and which have covered most of the young geomorphic markers and natural exposures. Here we use aerial photographs from 1956, combined with an ~1 m DEM derived from stereo Pleiades satellite imagery to investigate the geomorphology of a growing anticline above a thrust fault—the Pardisan thrust—within central Tehran. The topography across the ridge is consistent with a steep ramp extending from close to the surface to a depth of ~2 km, where it presumably connects with a shallow-dipping detachment. No primary fault is visible at the surface, and it is possible that the faulting dissipates in the near surface as distributed shearing. We use optically stimulated luminescence to date remnants of uplifted and warped alluvial deposits that are offset vertically across the Pardisan fault, providing minimum uplift and slip-rates of at least 1 mm yr –1 . Our study shows that the faults within the Tehran urban region have relatively rapid rates of slip, are important in the regional tectonics, and have a great impact on earthquake hazard assessment of the city and surrounding region.

Description: Melt generation and migration are an important link between surface processes and the thermal and chemical evolution of the Earth's interior. However, their vastly different timescales make it difficult to study mantle convection and melt migration in a unified framework, especially for 3-D global models. And although experiments suggest an increase in melt volume of up to 20 per cent from the depth of melt generation to the surface, previous computations have neglected the individual compressibilities of the solid and the fluid phase. Here, we describe our extension of the finite element mantle convection code ASPECT that adds melt generation and migration. We use the original compressible formulation of the McKenzie equations, augmented by an equation for the conservation of energy. Applying adaptive mesh refinement to this type of problems is particularly advantageous, as the resolution can be increased in areas where melt is present and viscosity gradients are high, whereas a lower resolution is sufficient in regions without melt. Together with a high-performance, massively parallel implementation, this allows for high-resolution, 3-D, compressible, global mantle convection simulations coupled with melt migration. We evaluate the functionality and potential of this method using a series of benchmarks and model setups, compare results of the compressible and incompressible formulation, and show the effectiveness of adaptive mesh refinement when applied to melt migration. Our model of magma dynamics provides a framework for modelling processes on different scales and investigating links between processes occurring in the deep mantle and melt generation and migration. This approach could prove particularly useful applied to modelling the generation of komatiites or other melts originating in greater depths. The implementation is available in the Open Source ASPECT repository.

Description: We use recently published, high-resolution reconstructions of the Southwest Indian Ridge to test whether a previously described systematic difference between Global Positioning System (GPS) and 3.16-Myr-average estimates of seafloor spreading rates between Antarctica and Africa is evidence for a recent slowdown in Southwest Indian Ridge seafloor spreading rates. Along the Nubia-Antarctic segment of the ridge, seafloor opening rates that are estimated with the new, high-resolution reconstructions and corrected for outward displacement agree well with geodetic rate estimates and reduce previously reported, highly significant non-closure of the Nubia-Antarctic-Sur plate circuit. The observations are inconsistent with a slowdown in spreading rates and instead indicate that Nubia-Antarctic plate motion has been steady since at least 5.2 Ma. Lwandle-Antarctic seafloor spreading rates that are estimated from the new high-resolution reconstructions differ insignificantly from a GPS estimate, thereby implying steady Lwandle-Antarctic plate motion since 5.2 Ma. Between the Somalia and Antarctic plates, the new Southwest Indian Ridge reconstructions eliminate roughly half of the systematic difference between the GPS and MORVEL spreading rate estimates.We interpret the available observations as evidence that Somalia-Antarctic spreading rates have been steady since at least 5.2 Ma and postulate that the remaining difference is attributable to random and/or systematic errors in the plate kinematic estimates and the combined effects of insufficient geodetic sampling of undeforming areas of the Somalia plate, glacial isostatic adjustment in Antarctica and transient deformation triggered by the 1998 M w = 8.2 Antarctic earthquake, the 2004 M w = 9.3 Sumatra earthquake, or possibly other large historic earthquakes.

Description: A series of our linear analysis on the onset of thermal convection was applied to that of highly compressible fluids in a planar layer whose thermal conductivity and viscosity vary in space, in order to study the influences of spatial variations in physical properties expected in the mantles of massive terrestrial planets. The thermal conductivity and viscosity are assumed to exponentially depend on depth and temperature, respectively, while the variations in thermodynamic properties (thermal expansivity and reference density) with depth are taken to be relevant for the super-Earths with 10 times the Earth's. Our analysis demonstrated that the nature of incipient thermal convection is strongly affected by the interplay between the adiabatic compression and spatial variations in physical properties of fluids. Owing to the effects of adiabatic compression, a ‘stratosphere’ can occur in the deep mantles of super-Earths, where a vertical motion is insignificant. An emergence of ‘stratosphere’ is greatly enhanced by the increase in thermal conductivity with depth, while it is suppressed by the decrease in thermal expansivity with depth. In addition, by the interplay between the static stability and strong temperature dependence in viscosity, convection cells tend to be confined in narrow regions around the ‘tropopause’ at the interface between the ‘stratosphere’ of stable stratification and the ‘troposphere’ of unstable stratification. We also found that, depending on the variations in physical properties, two kinds of stagnant regions can separately develop in the fluid layer. One is well-known ‘stagnant-lids’ of cold and highly viscous fluids, and the other is ‘basal stagnant regions’ of hot and less viscous fluids. The occurrence of ‘basal stagnant regions’ may imply that convecting motions can be insignificant in the lowermost part of the mantles of massive super-Earths, even in the absence of strong increase in viscosity with pressure (or depth).

Description: We present a 2-D subsurface image of the Paganica Fault from a high-resolution refraction tomography and detailed geological investigation carried out across part of the northwestern segment of the 20-km-long Paganica–San Demetrio fault-system, and which was responsible of the 2009 April 6 M w 6.1 L'Aquila earthquake (central Italy). We acquired two seismic profiles crossing the Paganica basin with a dense-wide aperture configuration. More than 30 000 P wave first-arrival traveltimes were input to a non-linear tomographic inversion. The obtained 250–300 m deep 2-D Vp images illuminate the shallow portion of the Paganica Fault, and depict additional unreported splays defining a complex half-graben structure. We interpret local thickening of low- Vp (〈2400 m s –1 ) and intermediate- Vp (2600–3400 m s –1 ) regions as syn-tectonic clastic wedges above a high- Vp (3800–5000 m s –1 ) carbonate basement. These results are condensed in a 4.2-km-long section across the Paganica basin, clearly indicating that the Paganica Fault is a mature normal fault cutting the whole upper ~10 km of the crust. We evaluate a minimum cumulative net displacement of 650 ± 90 m and a total heave of 530 ± 65 m accomplished by the Paganica Fault, respectively. In the conservative hypothesis that the extension started during the Gelasian (1.80–2.59 Ma), we obtain a minimum long-term slip-rate of 0.30 ± 0.07 mm yr –1 and an extension-rate of 0.25 ± 0.06 mm yr –1 , respectively. Considering the regional averaged extensional field of ~1 mm yr –1 obtained from geodetic and geological analyses at 10 4  yr timescale, we infer that the Paganica Fault accounts for ~20 per cent of the NE-extension affecting this zone of the central Apennines axis due to the concurrent activity of other parallel normal fault-systems nearby (e.g. the Liri, Velino-Magnola, L'Aquila-Celano and Gran Sasso fault-systems).

Description: The Mozambique Ridge, a prominent basement high in the southwestern Indian Ocean, consists of four major geomorphological segments associated with numerous phases of volcanic activity in the Lower Cretaceous. The nature and origin of the Mozambique Ridge have been intensely debated with one hypothesis suggesting a Large Igneous Province origin. High-resolution seismic reflection data reveal a large number of extrusion centres with a random distribution throughout the southern Mozambique Ridge and the nearby Transkei Rise. Intrabasement reflections emerge from the extrusion centres and are interpreted to represent massive lava flow sequences. Such lava flow sequences are characteristic of eruptions leading to the formation of continental and oceanic flood basalt provinces, hence supporting a Large Igneous Province origin of the Mozambique Ridge. We observe evidence for widespread post-sedimentary magmatic activity that we correlate with a southward propagation of the East African Rift System. Based on our volumetric analysis of the southern Mozambique Ridge we infer a rapid sequential emplacement between ~131 and ~125 Ma, which is similar to the short formation periods of other Large Igneous Provinces like the Agulhas Plateau.

Description: Three 2-D Deep Electrical Resistivity Tomography (ERT) transects, up to 6.36 km long, were obtained across the Paganica-San Demetrio Basin, bounded by the 2009 L'Aquila M w 6.1 normal-faulting earthquake causative fault (central Italy). The investigations allowed defining for the first time the shallow subsurface basin structure. The resistivity images, and their geological interpretation, show a dissected Mesozoic-Tertiary substratum buried under continental infill of mainly Quaternary age due to the long-term activity of the Paganica-San Demetrio normal faults system (PSDFS), ruling the most recent deformational phase. Our results indicate that the basin bottom deepens up to 600 m moving to the south, with the continental infill largely exceeding the known thickness of the Quaternary sequence. The causes of this increasing thickness can be: (1) the onset of the continental deposition in the southern sector took place before the Quaternary, (2) there was an early stage of the basin development driven by different fault systems that produced a depocentre in the southern sector not related to the present-day basin shape, or (3) the fault system slip rate in the southern sector was faster than in the northern sector. We were able to gain sights into the long-term PSDFS behaviour and evolution, by comparing throw rates at different timescales and discriminating the splays that lead deformation. Some fault splays exhibit large cumulative throws (〉300 m) in coincidence with large displacement of the continental deposits sequence (〉100 m), thus testifying a general persistence in time of their activity as leading splays of the fault system. We evaluate the long-term (3–2.5 Myr) cumulative and Quaternary throw rates of most of the leading splays to be 0.08–0.17 mm yr –1 , indicating a substantial stability of the faults activity. Among them, an individual leading fault splay extends from Paganica to San Demetrio ne’ Vestini as a result of a post-Early Pleistocene linkage of two smaller splays. This 15 km long fault splay can explain the Holocene surface ruptures observed to be larger than those occurred during the 2009 L'Aquila earthquake, such as revealed by palaeoseismological investigations. Finally, the architecture of the basin at depth suggests that the PSDFS can also rupture a longer structure at the surface, allowing earthquakes larger than M 6.5, besides rupturing only small sections, as it occurred in 2009.

Description: Previous statistical studies showed that there was a correlation between the ultralow frequency (ULF) seismo-magnetic phenomena and local seismicity in the Kakioka region, Japan. In this study, utilizing Molchan's error diagram, we evaluate whether these phenomena contain precursory information and discuss how they can be used in short-term forecasting of sizable earthquakes. In practice, for given series of precursory signals and related earthquake events, each prediction strategy is characterized by the leading time of alarms ( ) and the length of alarm window ( L ). The leading time is the time length between a detected anomaly and its following alarm, and the alarm window is the duration that an alarm lasts. A modified area skill score measuring the area between actual prediction curve and random prediction line in Molchan's error diagram is used to assess the efficiency of different prediction strategies. The results indicate that predictions based on ULF magnetic data in Kakioka, Japan perform better than random prediction when is around 1 week and L is less than 4 d or is 13–14 d and L is less than 1 week. The optimal strategy of short-term forecasts has been established by setting at 8 d and L at 1 d. The methodology proposed in this study could also be useful in evaluating the prediction policy and optimizing other kinds of measurements for short-term earthquake forecasting.

Description: While the kinematics of Anatolia plate and the North Anatolian Fault System (NAFS) has been studied extensively, the slip rate and locking depth along the NAFS are usually assumed constant in the analyses due to the lack of sufficient data. This is also partly due to the reasonably good fit of Euler small circle and partly due to the lack of spatial resolution of observations to determine slip rates independently from locking depths. On the other hand, recent geodetic studies show a contrast for locking depth between Marmara and other parts of the NAFS, implying a non-uniform locking depth across the NAFS. In this study, we analyse new GPS data and homogenously combine available data sets covering the eastern part of the NAFS to form the most complete data set. In particular, we incorporate the first results of Turkish Real-Time Kinematic GPS Network (CORS-TR) into our data set. A detailed analysis of three profiles within the NAFS reveals an increase of locking depth in the middle profile to 19.1 ± 3.4 km from 11.9 ± 3.5 km in the easternmost profile while the slip rate is nearly constant (20–22 mm yr –1 ), which implies a variation of strain rate of ~100 nanostrain yr –1 . Assuming a constant locking depth throughout whole NAFS gives an average locking depth of 14.3 ± 1.7 km. Our best estimates of slip rates in block modelling which takes the variation of locking depths into account are in the range between 22.5 and 22.8 mm yr –1 over eastern part of the NAFS.

Description: The Taupo Volcanic Zone (TVZ) is one of the world's most productive regions of rhyolitic volcanism and contains the highly active Okataina Volcanic Centre (OVC). Within the TVZ, intra-arc extension is expressed as normal faulting within a zone known as the Taupo Rift. The OVC is located within a complex part of the rift, where volcanism and deformation is considered influenced by rift structure and kinematics. There has been significant research on the structural, volcanic and geophysical properties of the rift and OVC, but less focus on deformation using geodetic data. The limited studies that have utilized geodetic data do not clearly resolve the distribution of deformation and strain rates within the rift and OVC. This is essential to ensure that deformation signals from volcanic processes at the OVC are correctly identified and distinguished from those related to regional tectonic or local hydrothermal processes within the rift. In this paper, we present a picture of contemporary deformation at the OVC and within the surrounding rift in detail, using existing and new GPS campaign and continuous GPS (cGPS) data collected between 1998 and 2011. The results show a highly heterogeneous deformation and strain rate field (both extension and shortening) through the study area, partitioned into different parts of the rift. Our results agree well with earlier geodetic studies, as well as identify new features, but some deformation patterns conflict with long-term geological observations. In the OVC, we observe a locally rotated horizontal velocity field, significant vertical deformation and variable strain rates across the caldera. In the Tarawera Rift, we identify elevated extension and shear rates, which may have significant implications for volcanism there. A shortening pattern is identified through the central rift, which is unexpected in an intra-arc rifting environment. We attempt to explain the source/s of shortening and extension and discuss their implications for geodetic monitoring efforts in the OVC.

Description: The pre- and current collision of the Juan Fernández Ridge with the central Chilean margin at 31°–33°S is characterized by large-scale crustal thinning and long-term subsidence of the submarine forearc caused by subduction erosion processes. Here, we study the structure of the central Chilean margin in the ridge–trench collision zone by using wide-angle and multichannel seismic data. The transition from the upper to middle continental slope is defined by a trenchward dipping normal scarp with variable offsets of 500–2000 m height. Beneath the scarp, the 2-D velocity–depth models show a prominent lateral velocity contrast of 〉1 s –1 that propagates deep into the continental crust defining a major lateral seismic discontinuity. The discontinuity is interpreted as the lithological contact between the subsided/collapsed outermost forearc (composed of eroded and highly fractured volcanic rocks) and the seaward part of the uplifted Coastal Cordillera (made of less fractured metamorphic/igneous rocks). Extensional faults are abundant in the collapsed outermost forearc, however, landward of the continental slope scarp, both extensional and compressional structures are observed along the uplifted continental shelf that forms part of the Coastal Cordillera. Particularly, at the landward flank of the Valparaíso Forearc Basin (32°–33.5°S), shallow crustal seismicity has been recorded in 2008–2009 forming a dense cluster of thrust events of M w 4–5. The estimated hypocentres spatially correlate with the location of the fault scarp, and they highlight the upper part of the seismic crustal discontinuity.

Description: Geodetic observations of interseismic deformation in the Western United States provide constraints on microplate rotations, earthquake cycle processes, and slip partitioning across the Pacific–North America Plate boundary. These measurements may be interpreted using block models, in which the upper crust is divided into microplates bounded by faults that accumulate strain in a first-order approximation of earthquake cycle processes. The number and geometry of microplates are typically defined with boundaries representing a limited subset of the large number of potentially seismogenic faults. An alternative approach is to include a large number of potentially active faults bounding a dense array of microplates, and then algorithmically estimate the boundaries at which strain is localized. This approach is possible through the application of a total variation regularization (TVR) optimization algorithm, which simultaneously minimizes the L 2 norm of data residuals and the L 1 norm of the variation in the differential block motions. Applied to 3-D spherical block models, the TVR algorithm can be used to reduce the total variation between estimated rotation vectors, effectively grouping microplates that rotate together as larger blocks, and localizing fault slip on the boundaries of these larger block clusters. Here we develop a block model comprised of 137 microplates derived from published fault maps, and apply the TVR algorithm to identify the kinematically most important faults in the western United States. This approach reveals that of the 137 microplates considered, only 30 unique blocks are required to approximate deformation in the western United States at a residual level of 〈2 mm yr –1 .

Description: We develop and validate a systematic approach to infer plate boundary strength and rheological parameters in models of mantle flow from surface velocity observations. Based on a realistic rheological model that includes yielding and strain rate weakening from dislocation creep, we formulate the inverse problem in a Bayesian inference framework. To study the distribution of parameters that are consistent with the observations, we compute the maximum a posteriori (MAP) point, Gaussian approximations of the parameter distribution around that MAP point, and employ Markov Chain Monte Carlo (MCMC) sampling methods. The computation of the MAP point and the Gaussian approximation require first and second derivatives of an objective function subject to non-linear Stokes equations; these derivatives are computed efficiently using adjoint Stokes equations. We set up 2-D numerical experiments with many of the elements expected in a global geophysical inversion. This setup incorporates three subduction zones with slab and weak zone (interplate fault) geometry consistent with average seismic characteristics. With these experiments, we demonstrate that when the temperature field is known, we can recover the strength of plate boundaries, the yield stress and strain rate exponent in the upper mantle. When the number of uncertain parameters increases, there are trade-offs between the inferred parameters. These trade-offs depend on how well the observational data represents the surface velocities, and on the weakness of plate boundaries. As the plate boundary coupling drops below a threshold, the uncertainty of the inferred parameters increases due to insensitivity of plate motion to plate coupling. Comparing the trade-offs between inferred rheological parameters found from the Gaussian approximation of the parameter distribution and from MCMC sampling, we conclude that the Gaussian approximation—which is significantly cheaper to compute—is often a good approximation, in particular locally around the MAP point. Thus, the method can be applied to the global problem of inferring non-linear constitutive parameters and plate coupling factors for each subduction zone in a global geophysical inversion with known slab structure.

Description: We estimate fluid sources around a subducted seamount along the northern Hikurangi subduction margin of New Zealand, using thermomechanical numerical modelling informed by wedge structure and porosities from multichannel seismic data. Calculated fluid sources are input into an independent fluid-flow model to explore the key controls on overpressure generation to depths of 12 km. In the thermomechanical models, sediment transport through and beneath the wedge is calculated assuming a pressure-sensitive frictional rheology. The change in porosity, pressure and temperature with calculated rock advection is used to compute fluid release from compaction and dehydration. Our calculations yield more precise information about source locations in time and space than previous averaged estimates for the Hikurangi margin. The volume of fluid release in the wedge is smaller than previously estimated from margin-averaged calculations (~14 m 3  yr –1  m –1 ), and is exceeded by fluid release from underlying (subducting) sediment (~16 m 3  yr –1  m –1 ). Clay dehydration contributes only a small quantity of fluid by volume (~2 m 3  yr –1  m –1 from subducted sediment), but the integrated effect is still significant landward of the seamount. Fluid source terms are used to estimate fluid pressures around a subducting seamount in the fluid-flow models, using subducted sediment permeability derived from porosity, and testing two end-members for décollement permeability. Models in which the décollement acts as a fluid conduit predict only moderate fluid overpressure in the wedge and subducting sediment. However, if the subduction interface becomes impermeable with depth, significant fluid overpressure develops in subducting sediment landward of the seamount. The location of predicted fluid overpressure and associated dehydration reactions is consistent with the idea that short duration, shallow, slow slip events (SSEs) landward of the seamount are caused by anomalous fluid pressures; alternatively, it may result from frictional effects of changing clay content along the subduction interface.

Description: A method is outlined by means of which it is possible to estimate high-resolution vertical displacements due to an earthquake even in the case where high-resolution topography is lacking before the earthquake. This result can be achieved by combining a highly accurate, post-event digital elevation model (DEM), for example lidar, with archived satellite imagery. The method is illustrated by calculating vertical displacements for the 2010 El Mayor-Cucapah earthquake. For this earthquake, there are both pre- and post-event lidar DEMs from which vertical displacements may also be estimated after correcting for the lateral advection of topography due to horizontal displacements. A comparison between the two means of deriving vertical displacements shows generally good agreement, with the displacements obtained using satellite imagery performing better in high relief areas. As a result of this property, we are able to trace the vertical offsets due to the El Mayor-Cucapah earthquake as the rupture jumped from the Pescadores fault to the Borrego fault in propagating through the high relief of the Sierra Cucapah.

Description: We present observations and models of the Sulaiman Range of western Pakistan that shed new light on the evolution and deformation of fold-thrust belts. Earthquake source inversions show that the seismic deformation in the range is concentrated in the thick pile of sediments overlying the underthrusting lithosphere of the Indian subcontinent. The slip vectors of the earthquakes vary in strike around the margin of the range, in tandem with the shape of the topography, suggesting that gravitational driving forces arising from the topography play an important role in governing the deformation of the region. Numerical models suggest that the active deformation, and the extreme plan-view curvature of the range, are governed by the presence of weak sediments in a pre-existing basin on the underthrusting Indian Plate. These sediments affect the stress-state in the over-riding mountain range and allow for the rapid propagation of the nose of the range and the development of extreme curvature and laterally varying surface gradients.

Description: The mechanical damage characteristics of sandstone subjected to cyclic loading is very significant to evaluate the stability and safety of deep excavation damage zones. However to date, there are very few triaxial experimental studies of sandstone under cyclic loading. Moreover, few X-ray micro-computed tomography (micro-CT) observations have been adopted to reveal the damage mechanism of sandstone under triaxial cyclic loading. Therefore, in this research, a series of triaxial cyclic loading tests and X-ray micro-CT observations were conducted to analyse the mechanical damage characteristics of sandstone with respect to different confining pressures. The results indicated that at lower confining pressures, the triaxial strength of sandstone specimens under cyclic loading is higher than that under monotonic loading; whereas at confining pressures above 20 MPa, the triaxial strength of sandstone under cyclic loading is approximately equal to that under monotonic loading. With the increase of cycle number, the crack damage threshold of sandstone first increases, and then significantly decreases and finally remains constant. Based on the damage evolution of irreversible deformation, it appears that the axial damage value of sandstone is all higher than the radial damage value before the peak strength; whereas the radial damage value is higher than the axial damage value after the peak strength. The evolution of Young's modulus and Poisson's ratio of sandstone can be characterized as having four stages: (i) Stage I: material strengthening; (ii) Stage II: material degradation; (iii) Stage III: material failure and (iv) Stage IV: structure slippage. X-ray micro-CT observations demonstrated that the CT scanning surface images of sandstone specimens are consistent with actual surface crack photographs. The analysis of the cross-sections of sandstone supports that the system of crack planes under triaxial cyclic loading is much more complicated than that under triaxial monotonic loading. More axial and lateral tensile cracks were observed in the specimens under cyclic loading than under monotonic loading.

Description: We determine the deep structure of the eastern Algerian basin and its southern margin in the Annaba region (easternmost Algeria), to better constrain the plate kinematic reconstruction in this region. This study is based on new geophysical data collected during the SPIRAL cruise in 2009, which included a wide-angle, 240-km-long, onshore–offshore seismic profile, multichannel seismic reflection lines and gravity and magnetic data, complemented by the available geophysical data for the study area. The analysis and modelling of the wide-angle seismic data including refracted and reflected arrival travel times, and integrated with the multichannel seismic reflection lines, reveal the detailed structure of an ocean-to-continent transition. In the deep basin, there is an ~5.5-km-thick oceanic crust that is composed of two layers. The upper layer of the crust is defined by a high velocity gradient and P -wave velocities between 4.8 and 6.0 km s –1 , from the top to the bottom. The lower crust is defined by a lower velocity gradient and P -wave velocity between 6.0 and 7.1 km s –1 . The Poisson ratio in the lower crust deduced from S -wave modelling is 0.28, which indicates that the lower crust is composed mainly of gabbros. Below the continental edge, a typical continental crust with P -wave velocities between 5.2 and 7.0 km s –1 , from the top to the bottom, shows a gradual seaward thinning of ~15 km over an ~35-km distance. This thinning is regularly distributed between the upper and lower crusts, and it characterizes a rifted margin, which has resulted from backarc extension at the rear of the Kabylian block, here represented by the Edough Massif at the shoreline. Above the continental basement, an ~2-km-thick, pre-Messinian sediment layer with a complex internal structure is interpreted as allochthonous nappes of flysch backthrusted on the margin during the collision of Kabylia with the African margin. The crustal structure, moreover, provides evidence for Miocene emplacement of magmatic intrusions in both the deep basin and the continental margin. Based on the crustal structure, we propose that the eastern Algerian basin opened during the southeastward migration of the European forearc before the collision, along a NW–SE elongated spreading centre that ran perpendicular to the subduction trend. Such an atypical geometry is explained by the diverging directions of the subduction rollback during the backarc opening: eastward for the Corsica–Sardinia block, and southward for the Kabylian blocks. This geometry of the forearc can be interpreted as the surface expression of a slab tear at depth, which is responsible for atypical magmatism in the overlying backarc oceanic basin.

Description: Steady plate subduction elastically brings about permanent lithospheric deformation in island arcs, though this effect has been neglected in most studies based on elastic dislocation theory. We investigate the characteristics of the permanent lithospheric deformation using a kinematic model, in which steady slip motion is given along a plate interface in the elastic lithosphere overlying the viscoelastic asthenosphere under gravity. As a rule of thumb, long-term lithospheric deformation can be understood as a bending of an elastic plate floating on non-viscous fluid, because the asthenosphere behaves like water on the long term. The steady slip below the lithosphere–asthenosphere boundary does not contribute to long-term lithospheric deformation. Hence, the key parameters that control the lithospheric deformation are only the thickness of the lithosphere and the geometry of the plate interface. Slip on a plate interface generally causes substantial vertical displacement, and gravity always tries to retrieve the original gravitational equilibrium. For a curved plate interface gravity causes convex upward bending of the island-arc lithosphere, while for a planar plate interface gravity causes convex downward bending. Larger curvature and thicker lithosphere generally results in larger deformation. When the curvature changes along the plate interface, internal deformation is also involved intrinsically, which modifies the deformation field due to gravity. Because the plate interface generally has some curvature, at least near the trench, convex upward bending of the island-arc lithosphere, which involves uplift of island-arc and subsidence around the trench, is always realized. On the other hand, the deformation field of the island-arc lithosphere sensitively depends on lithospheric thickness and plate interface geometry. These characteristics obtained by the numerical simulation are consistent with observed topography and free-air gravity anomalies in subduction zones: a pair of topography and gravity anomalies, high in the arc and low around the trench, is observed without exceptions all over the world, while there are large variety in the amplitude and horizontal scale of the topography and gravity anomalies.

Description: We investigate the influence of the glacial isostatic adjustment (GIA) on the deformation at the surface and at seismogenic depths in Fennoscandia. The surface strain rate field, derived from geodetic data, is controlled by GIA which causes NW–SE extension of up to 4  x  10 –9  yr –1 in most of mainland Fennoscandia, surrounded by regions of radial shortening towards the centre of uplift. The seismic deformation field, derived from a new compilation of focal mechanisms, shows consistent NW–SE compression on the Norwegian continental margin and a tendency towards tension in mainland Fennoscandia. The seismic moment rate is at least two orders of magnitude smaller than the geodetic moment rate. We propose that the low level of seismicity and the tendency towards tensional focal mechanisms in mainland Fennoscandia may be explained by the destructive interference of the regional stress from ridge push with the flexural stress due to GIA.

Description: New Guinea is a region characterized by rapid oblique convergence between the Pacific and Australian tectonic plates. The detailed tectonics of the region, including the partitioning of relative block motions and fault slip rates within this complex boundary plate boundary zone are still not well understood. In this study, we quantify the distribution of the deformation throughout the central and western parts of Papua New Guinea (PNG) using 20 yr of GPS data (1993–2014). We use an elastic block model to invert the regional GPS velocities as well as earthquake slip vectors for the location and rotation rates of microplate Euler poles as well as fault slip parameters in the region. Convergence between the Pacific and the Australian plates is accommodated in northwestern PNG largely by the New Guinea Trench with rates exceeding 90 mm yr –1 , indicating that this is the major active interplate boundary. However, some convergent deformation is partitioned into a shear component with ~12 per cent accommodated by the Bewani-Torricelli fault zone and the southern Highlands Fold-and-Thrust Belt. New GPS velocities in the eastern New Guinea Highlands region have led to the identification of the New Guinea Highlands and the Papuan Peninsula being distinctly different blocks, separated by a boundary through the Aure Fold-and-Thrust Belt complex which accommodates an estimated 4–5 mm yr –1 of left-lateral and 2–3 mm yr –1 of convergent motion. This implies that the Highlands Block is rotating in a clockwise direction relative to the rigid Australian Plate, consistent with the observed transition to left-lateral strike-slip regime observed in western New Guinea Highlands. We find a better fit of our block model to the observed velocities when assigning the current active boundary between the Papuan Peninsula and the South Bismark Block to be to the north of the city of Lae on the Gain Thrust, rather than on the more southerly Ramu-Markham fault as previously thought. This may indicate a temporary shift of activity onto out of sequence thrusts like the Gain Thrust as opposed to the main frontal thrust (the Ramu-Markham fault). In addition, we show that the southern Highlands Fold-and-Thrust Belt is the major boundary between the rigid Australian Plate and the New Guinea Highlands Block, with convergence occurring at rates between ~6 and 13 mm yr –1 .

Description: We examine the geodetically derived rotational variations for the rate of change of degree-two harmonics of Earth's geopotential, $\skew5\dot J_2$ , and true polar wander, combining a recent melting model of glaciers and the Greenland and Antarctic ice sheets taken from the IPCC 2013 Report (AR5) with two representative GIA ice models describing the last deglaciation, ICE5G and the ANU model developed at the Australian National University. Geodetically derived observations of $\skew4\dot J_2$ are characterized by temporal changes of –(3.7 ± 0.1)  x 10 –11 yr –1 for the period 1976–1990 and –(0.3 ± 0.1)  x 10 –11 yr –1 after ~2000. The AR5 results make it possible to evaluate the recent melting of the major ice sheets and glaciers for three periods, 1900–1990, 1991–2001 and after 2002. The observed $\skew4\dot J_2$ and the component of $\skew4\dot J_2$ due to recent melting for different periods indicate a long-term change in $\skew4\dot J_2$ —attributed to the Earth's response to the last glacial cycle—of –(6.0–6.5)  x 10 –11 yr –1 , significantly different from the values adopted to infer the viscosity structure of the mantle in most previous studies. This is a main conclusion of this study. We next compare this estimate with the values of $\skew4\dot J_2$ predicted by GIA ice models to infer the viscosity structure of the mantle, and consequently obtain two permissible solutions for the lower mantle viscosity ( lm ), ~10 22 and (5–10)  x 10 22 Pa s, for both adopted ice models. These two solutions are largely insensitive to the lithospheric thickness and upper mantle viscosity as indicated by previous studies and relatively insensitive to the viscosity structure of the D '' layer. The ESL contributions from the Antarctic ice sheet since the last glacial maximum (LGM) for ICE5G and ANU are about 20 and 30 m, respectively, but glaciological reconstructions of the Antarctic LGM ice sheet have suggested that its ESL contribution may have been less than ~10 m. The GIA-induced $\skew4\dot J_2$ for GIA ice models with an Antarctic ESL component of ~10 m suggests two permissible lower mantle viscosity solutions of lm ~ 2  x 10 22 and ~5  x 10 22 Pa s or one solution with (2–5)  x 10 22 Pa s. These results suggest that the effective lower mantle viscosity is larger than ~10 22 Pa s regardless of the uncertainties for an Antarctic ESL component. We also examine the polar wander due to recent melting and GIA processes, suggesting that the observed polar wander may be significantly attributed to convection motions in the mantle and/or another cause, particularly for permissible lower mantle viscosity solution of (5–10)  x 10 22 Pa s.

Description: In this work, we test the analysis of anisotropy of magnetic susceptibility (AMS) for the study of diapirs in the Naval salt wall that is located in the western end of the South Pyrenean Central Unit. Two hundred eighty-six specimens from 30 sites were sampled in the mobilized Middle-Upper Triassic evaporites and shales that constitute the core of this diapir. Resulting magnetic ellipsoids are mostly oblate and scalar parameters are similar to those found in weakly deformed, sedimentary rocks. From the observation of samples and thin sections, magnetic fabrics can be related to petrofabric: magnetic foliation corresponds to flattening or shear planes and magnetic lineation to flow/shear direction, elongation of grains or tight microfolds. The magnetic foliation shows a roughly circular pattern with shallow dips and some deflections due to Pyrenean structures. Magnetic lineations show a slightly dominant NE–SW trend and shallow plunges. Magnetic lineations and foliations are consistent with the theoretical pattern expected at the roof of a vertically grown salt wall, thus supporting that magnetic fabrics registered evaporitic flow at late stages of diapiric growth. We demonstrate that AMS is an efficient and promising tool to define diapiric flow where tectonic fabric cannot be directly measured. Knowledge of the geometry and internal deformation of the diapiric body helps to minimize ambiguities in the interpretation of magnetic fabrics in terms of internal deformation.

Description: Several large earthquakes in the Hellenic subduction zone have been documented in historical records from around the eastern Mediterranean, but the relative seismic quiescence of the region over the period of instrumental observation means that the exact locations of these earthquakes and their tectonic significance are not known. We present AMS radiocarbon dates from uplifted late Holocene palæoshorelines from the island of Rhodes, showing that uplift is most consistent with a single large ( M W  ≥ 7.7) reverse-faulting earthquake between about 2000 BC and 200 BC. Analysis of the uplift treating the earthquake as a dislocation in an elastic half-space shows a predominantly reverse-faulting event with a slip vector oblique to the direction of convergence between Rhodes and Nubia. We suggest that the fault responsible for the uplift dips at an angle of 30–60° above the more gently dipping oblique subduction interface. The highly oblique convergence across the eastern Hellenic plate boundary zone appears to be partitioned into reverse slip on faults that strike parallel to the boundary and strike-parallel or oblique slip on the subduction interface. Hydrodynamical simulation of tsunami propagation from a range of tectonically plausible sources suggests that earthquakes on the fault uplifting Rhodes represent a significant tsunami hazard for Rhodes and SW Turkey, and also possibly for Cyprus and the Nile Delta.

Description: Reconstructions of the history of convergence between the Nubia and Eurasia plates constitute an important part of a broader framework for understanding deformation in the Mediterranean region and the closing of the Mediterranean Basin. Herein, we combine high-resolution reconstructions of Eurasia-North America and Nubia-North America Plate motions to determine rotations that describe Nubia-Eurasia Plate motion at ~1 Myr intervals for the past 20 Myr. We apply trans-dimensional hierarchical Bayesian inference to the Eurasia-North America and Nubia-North America rotation sequences in order to reduce noise in the newly estimated Nubia-Eurasia rotations. The noise-reduced rotation sequences for the Eurasia-North America and Nubia-North America Plate pairs describe remarkably similar kinematic histories since 20 Ma, consisting of relatively steady seafloor spreading from 20 to 8 Ma, ~20 per cent opening-rate slowdowns at 8–6.5 Ma, and steady plate motion from ~7 Ma to the present. Our newly estimated Nubia-Eurasia rotations predict that convergence across the central Mediterranean Sea slowed by ~50 per cent and rotated anticlockwise after ~25 Ma until 13 Ma. Motion since 13 Ma has remained relatively steady. An absence of evidence for a significant change in motion immediately before or during the Messinian Salinity Crisis at 6.3–5.6 Ma argues against a change in plate motion as its causative factor. The detachment of the Arabian Peninsula from Africa at 30–24 Ma may have triggered the convergence rate slowdown before 13 Ma; however, published reconstructions of Nubia-Eurasia motion for times before 20 Ma are too widely spaced to determine with confidence whether the two are correlated. A significant discrepancy between our new estimates of Nubia-Eurasia motion during the past few Myr and geodetic estimates calls for further investigation.

Description: The Yellowstone–East Snake River Plain hotspot track has been intensely studied since several decades and is widely considered to result from the interaction of a mantle plume with the North American plate. An integrated conclusive geodynamic interpretation of this extensive data set is however presently still lacking, and our knowledge of the dynamical processes beneath Yellowstone is patchy. It has been argued that the Yellowstone plume has delaminated the lower part of the thick Wyoming cratonic lithosphere. We derive an original dynamic model to quantify delamination processes related to mantle plume–lithosphere interactions. We show that fast (~300 ka) lithospheric delamination is consistent with the observed timing of formation of successive volcanic centres along the Yellowstone hotspot track and requires (i) a tensile stress regime within the whole lithosphere exceeding its failure threshold, (ii) a purely plastic rheology in the lithosphere when stresses reach this yield limit, (iii) a dense lower part of the 200 km thick Wyoming lithosphere and (iv) a decoupling melt horizon inside the median part of the lithosphere. We demonstrate that all these conditions are verified and that ~150 km large and ~100 km thick lithospheric blocks delaminate within 300 ka when the Yellowstone plume ponded below the 200 km thick Wyoming cratonic lithosphere. Furthermore, we take advantage of the available extensive regional geophysical and geological observation data sets to design a numerical 3-D upper-mantle convective model. We propose a map of the ascending convective sheets contouring the Yellowstone plume. The model further evidences the development of a counter-flow within the lower part of the lithosphere centred just above the Yellowstone mantle plume axis. This counter-flow controls the local lithospheric stress field, and as a result the trajectories of feeder dykes linking the partial melting source within the core of the mantle plume with the crust by crosscutting the lithospheric mantle. This counter-flow further explains the 50 km NE shift observed between the mantle plume axis and the present-day Yellowstone Caldera as well as the peculiar shaped crustal magma chambers.

Description: We present source models for the August 2014 Murmuri (Dehloran) earthquake sequence in the Zagros Mountains of Iran. An M w 6.2 mainshock was followed by an aftershock sequence containing five events of M w ≥ 5.4. Models of P and SH waveforms show that all events had dominantly thrust-faulting mechanisms, and had centroid depths that place them within the thick sedimentary sequence, above the crystalline basement. The combination of our estimated focal mechanisms, relative relocations of the event hypocentres and the surface displacement patterns observed using InSAR imply that the mainshock and largest aftershock ruptured different fault planes and both contributed to the surface deformation. The fault planes both slipped in horizontally elongated patches, possibly due to rheological layering limiting the updip and downdip extent of rupture. The slip vector of the Murmuri mainshock implies that the decollement beneath the Lorestan Arc is weaker than any such feature beneath the Dezful Embayment, providing an explanation for the plan-view sinuosity of the range-front of the Zagros Mountains.

Description: Displacements and stress-field changes associated with earthquakes, volcanoes, landslides and human activity are often simulated using numerical models in an attempt to understand the underlying processes and their governing physics. The application of elastic dislocation theory to these problems, however, may be biased because of numerical instabilities in the calculations. Here, we present a new method that is free of artefact singularities and numerical instabilities in analytical solutions for triangular dislocations (TDs) in both full-space and half-space. We apply the method to both the displacement and the stress fields. The entire 3-D Euclidean space $\mathbb {R}^{3}$ is divided into two complementary subspaces, in the sense that in each one, a particular analytical formulation fulfils the requirements for the ideal, artefact-free solution for a TD. The primary advantage of the presented method is that the development of our solutions involves neither numerical approximations nor series expansion methods. As a result, the final outputs are independent of the scale of the input parameters, including the size and position of the dislocation as well as its corresponding slip vector components. Our solutions are therefore well suited for application at various scales in geoscience, physics and engineering. We validate the solutions through comparison to other well-known analytical methods and provide the MATLAB codes.

Description: Using dynamic rupture models of a right-lateral fault embedded in an elastic or elastoplastic 3-D medium, we investigate elastic and inelastic responses of compliant fault zones to nearby earthquake ruptures. We particularly examine effects of fault zone depth, width, shape and rigidity reduction on the surface displacement field. Our results from elastic models show that deeper and wider fault zones generally result in larger residual displacements. However, for shallow fault zones, the vertical residual displacement is insensitive to or even decreases with fault zone width. The width of horizontal displacement anomalies across a fault zone is only indicative of the fault zone width near the Earth's surface. There are trade-off effects among fault zone depth, width, shape and rigidity reduction on the amplitude of surface residual displacements. Our elastoplastic models show that plastic strain can occur along the entire fault zone near the Earth's surface and in the extensional quadrant at depth, if fault zone rocks are close to failure before a nearby earthquake happens. Compared with results from elastic models, plastic strain near the Earth's surface generally enhances surface displacements of a fault zone and does not change the trend of effects of fault zone depth and width, while plastic strain at depth can result in reduced retrograde motion or sympathetic motion across the fault zone, and introduce complexities in effects of fault zone depth and width. Sympathetic horizontal motion more likely occurs across a narrow fault zone with inelastic response at depth. Vertical motion in the extensional quadrant may actually decrease with fault zone width in elastoplastic models. Sympathetic horizontal motion, or small retrograde horizontal motion in conjunction with large vertical motion above a fault zone is indicative of inelastic response of a fault zone at depth.

Description: Models of the glacial isostatic adjustment process, which is dominated by the influence of the Late Pleistocene cycle of glaciation and deglaciation, depend on two fundamental inputs: a history of ice-sheet loading and a model of the radial variation of mantle viscosity. These models may be tested and refined by comparing their local predictions of relative sea level history to geological inferences based upon appropriate sea level indicators. The U.S. Atlantic coast is a region of particular interest in this regard, due to the fact that data from the length of this coast provides a transect of the forebulge associated with the former Laurentide ice sheet. High-quality relative sea level histories from this region are employed herein to explore the ability of current models of mantle viscosity to explain the inferred evolution of relative sea level that have accompanied forebulge collapse following deglaciation. Existing misfits are characterized, and alternatives are explored for their reconciliation. It is demonstrated that a new model of mantle viscosity, referred to herein as VM6, when coupled with the latest model of deglaciation history ICE-6G_C, is able to eliminate the majority of these misfits, while continuing to reconcile a wide range of other important geophysical observables, as well as additional relative sea level data from the North American. West coast which also record the collapse of the forebulge but which have not been employed in tuning the viscosity profile to enable ICE-6G_C (VM6) to fit the East coast data set.

Description: Knowledge and understanding of the ocean–continent transition (OCT) structure, continent–ocean boundary (COB) location and crustal type are of critical importance in evaluating rifted continental margin formation and evolution. OCT structure, COB location and magmatic type also have important implications for the understanding of the geodynamics of continental breakup and in the evaluation of petroleum systems in deep-water frontier oil and gas exploration at rifted continental margins. Mapping the distribution of thinned continental crust and lithosphere, its distal extent and the start of unequivocal oceanic crust and hence determining the OCT structure and COB location at rifted continental margins is therefore a generic global problem. In order to assist in the determination of the OCT structure and COB location, we present methodologies using gravity anomaly inversion, residual depth anomaly (RDA) analysis and subsidence analysis, which we apply to the west Iberian rifted continental margin. The west Iberian margin has one of the most complete data sets available for deep magma-poor rifted margins, so there is abundant data to which the results can be calibrated. Gravity anomaly inversion has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning; subsidence analysis has been used to determine the distribution of continental lithosphere thinning; and RDAs have been used to investigate the OCT bathymetric anomalies with respect to expected oceanic bathymetries at rifted continental margins. These quantitative analytical techniques have been applied to the west Iberian rifted continental margin along profiles IAM9, Lusigal 12 (with the TGS-extension) and ISE-01. Our predictions of OCT structure, COB location and magmatic type (i.e. the volume of magmatic addition, whether the margin is ‘normal’ magmatic, magma-starved or magma-rich) have been tested and validated using ODP wells (Legs 103, 149 and 173), which provide observational constraints on the west Iberian margin.

Description: The large-scale geological evolution of the North Atlantic Realm during the past 450 Myr is largely understood, but crucial elements remain uncertain. These involve the Caledonian orogeny, the formation of the North Atlantic and accompanying igneous activity, and the present-day high topography surrounding the North Atlantic. Teleseismic receiver function interpretation in the Central Fjord Region of East Greenland recently suggested the presence of a fossil Caledonian subduction complex, including a slab of eclogitised mafic crust and an overlying wedge of serpentinised mantle peridotite. Here we further investigate this topic using inverse receiver functions modelling. The obtained velocity models are tested with regard to their consistency with the regional gravity field and topography. We find that the obtained receiver function model is generally consistent with gravity and isostasy. The western part of the section, with topography of 〉1000 m, is clearly supported by the 40-km-thick crust. The eastern part requires additional buoyancy as provided by the hydrated mantle wedge. The geometry, velocities and densities are consistent with interpretation of the lithospheric structure as a fossil subduction zone complex. The spatial relations with Caledonian structures suggest a Caledonian origin. The results indicate that topography is isostatically compensated by density variations within the lithosphere, and that significant dynamic topography is not required at the present-day.

Description: We present a high-resolution shear wave tomography of the Japan subduction zone down to a depth of 700 km, which is determined by inverting a large number of high-quality S -wave arrival-time data from local earthquakes and teleseismic events. The subducting Pacific and Philippine Sea (PHS) slabs are revealed clearly as high-velocity (high- V ) zones, whereas low-velocity (low- V ) anomalies are revealed in the mantle wedge above the two slabs. The PHS slab has subducted aseismically down to a depth of 480 km under the Japan Sea and to a depth of 540 km under the Tsushima Strait. A window is revealed within the aseismic PHS slab, being consistent with P -wave tomography. Prominent low- V and high-Poisson's ratio ( ) anomalies exist below the PHS slab and above the Pacific slab, which reflect hot and wet mantle upwelling caused by the joint effect of deep dehydration of the Pacific slab and convective circulation process in the mantle wedge above the Pacific slab. The hot and wet mantle upwelling has caused the complex geometry and structure of the PHS slab in SW Japan, and contributed to the Quaternary volcanism along the Japan Sea coast. In eastern Japan, low- V zones are revealed at depths of 200–700 km below the Pacific slab, which may reflect hot upwelling from the lower mantle or even the core–mantle boundary.

Description: The Piano di Pezza fault is the central section of the 35 km long L'Aquila-Celano active normal fault-system in the central Apennines of Italy. Although palaeoseismic data document high Holocene vertical slip rates (~1 mm yr –1 ) and a remarkable seismogenic potential of this fault, its subsurface setting and Pleistocene cumulative displacement are still poorly known. We investigated for the first time the shallow subsurface of a key section of the main Piano di Pezza fault splay by means of high-resolution seismic and electrical resistivity tomography coupled with time-domain electromagnetic soundings (TDEM). Our surveys cross a ~5-m-high fault scarp that was generated by repeated surface-rupturing earthquakes displacing Holocene alluvial fans. We provide 2-D Vp and resistivity images, which show significant details of the fault structure and the geometry of the shallow basin infill material down to 50 m depth. Our data indicate that the upper fault termination has a sub-vertical attitude, in agreement with palaeoseismological trench evidence, whereas it dips ~50° to the southwest in the deeper part. We recognize some low-velocity/low-resistivity regions in the fault hangingwall that we relate to packages of colluvial wedges derived from scarp degradation, which may represent the record of some Holocene palaeo-earthquakes. We estimate a ~13–15 m throw of this fault splay since the end of the Last Glacial Maximum (~18 ka), leading to a 0.7–0.8 mm yr –1 throw rate that is quite in accordance with previous palaeoseismic estimation of Holocene vertical slip rates. The 1-D resistivity models from TDEM soundings collected along the trace of the electrical profile significantly match with 2-D resistivity images. Moreover, they indicate that in the fault hangingwall, ~200 m away from the surface fault trace, the pre-Quaternary carbonate basement is at ~90–100 m depth. We therefore provide a minimal ~150–160 m estimate of the cumulative throw of the Piano di Pezza fault system in the investigated section. We further hypothesize that the onset of the Piano di Pezza fault activity may date back to the Middle Pleistocene (~0.5 Ma), so this is a quite young active normal fault if compared to other mature normal fault systems active since 2–3 Ma in this portion of the central Apennines.

Description: To investigate the coupling relationship between great earthquake and ionosphere, the GPS-derived total electron contents (TECs) by the Center for Orbit Determination in Europe and the foF2 data from the Space Weather Prediction Center were used to analyse the impending ionospheric anomalies before the Iquique Mw8.2 earthquake in Chile on 2014 April 1. Eliminating effects of the solar and geomagnetic activities on ionosphere by the sliding interquartile range with the 27-day window, the TEC analysis results represent that there were negative anomalies occurred on 15th day prior to the earthquake, and positive anomalies appeared in 5th day before the earthquake. The foF2 analysis results of ionosonde stations Jicamarca, Concepcion and Ramey show that the foF2 increased by 40, 50 and 45 per cent, respectively, on 5th day before the earthquake. The TEC anomalous distribution indicates that there was a widely TEC decrement over the epicentre with the duration of 6 hr on 15th day before the earthquake. On 5th day before the earthquake, the TEC over the epicentre increased with the amplitude of 15 TECu, and the duration exceeded 6 hr. The anomalies occurred on the side away from the equator. All TEC anomalies in these days were within the bounds of equatorial anomaly zone where should be the focal area to monitor ionospheric anomaly before strong earthquakes. The relationship between ionospheric anomalies and geomagnetic activity was detected by the cross wavelet analysis, which implied that the foF2 was not affected by the magnetic activities on 15th day and 5th day prior to the earthquake, but the TECs were partially affected by anomalous magnetic activity during some periods of 5th day prior to the earthquake.

Description: We present the first estimates of Southwest Indian Ridge (SWIR) plate motions at high temporal resolution during the Quaternary and Neogene based on nearly 5000 crossings of 21 magnetic reversals out to C6no (19.72 Ma) and the digitized traces of 17 fracture zones and transform faults. Our reconstructions of this slow-spreading mid-ocean ridge reveal several unexpected results with notable implications for regional and global plate reconstructions since 20 Ma. Extrapolations of seafloor opening distances to zero-age seafloor based on reconstructions of reversals C1n (0.78 Ma) through C3n.4 (5.2 Ma) reveal evidence for surprisingly large outward displacement of 5 ± 1 km west of 32°E, where motion between the Nubia and Antarctic plates occurs, but 2 ± 1 km east of 32°E, more typical of most mid-ocean ridges. Newly estimated SWIR seafloor spreading rates are up to 15 per cent slower everywhere along the ridge than previous estimates. Reconstructions of the numerous observations for times back to 11 Ma confirm the existence of the hypothesized Lwandle plate at high confidence level and indicate that the Lwandle plate's western and eastern boundaries respectively intersect the ridge near the Andrew Bain transform fault complex at 32°E and between ~45°E and 52°E, in accord with previous results. The Nubia–Antarctic, Lwandle–Antarctic and Somalia–Antarctic rotation sequences that best fit many magnetic reversal, fracture zone and transform fault crossings define previously unknown changes in the Neogene motions of all three plate pairs, consisting of ~20 per cent slowdowns in their spreading rates at 7.2 $^{+0.9 }_{ -1.4}$ Ma if we enforce a simultaneous change in motion everywhere along the SWIR and gradual 3°–7° anticlockwise rotations of the relative slip directions. We apply trans-dimensional Bayesian analysis to our noisy, best-fitting rotation sequences in order to estimate less-noisy rotation sequences suitable for use in future global plate reconstructions and geodynamic studies. Notably, our new Nubia–Antarctic reconstruction of C5n.2 (11.0 Ma) predicts 20 per cent less opening than do two previous estimates, with important implications for motion that is estimated between the Nubia and Somalia plates. A Nubia–Somalia rotation determined from our Nubia–Antarctic and Somalia–Antarctic plate rotations for C5n.2 (11.0 Ma) predicts cumulative opening of 45 ± 4 km (95 per cent uncertainty) across the northernmost East Africa rift since 11.0 Ma, 70 per cent less than a recent 129 ± 62 km opening estimate based on a now-superseded interpretation of Anomaly 5 along the western portion of the SWIR.

Description: Geodynamic models predict that rifting of thick, ancient continental lithosphere should not occur unless it is weakened by heating and magmatic intrusion. Therefore, the processes occurring along sections of the western branch of the East African Rift, where ~150 km thick, Palaeoproterozoic lithosphere is rifting with no surface expression of magmatism, are a significant challenge to understand. In an attempt to understand the apparently amagmatic extension we probed the regional uppermost mantle for signatures of thermal alteration using compressional ( Vp ) and shear ( Vs ) wave speeds derived from Pn and Sn tomography. Pervasive thermal alteration of the uppermost mantle and possibly the presence of melt can be inferred beneath the Rungwe volcanic centre, but no signatures on a similar scale were discerned beneath amagmatic portions of the western rift branch encompassing the southern half of the Lake Tanganyika rift and much of the Rukwa rift. In this region, Vp and Vs wave speeds indicate little, if any, heating of the uppermost mantle and no studies have reported dyking. Vp / Vs ratios are consistent with typical, melt-free, olivine-dominated upper mantle. Although our resolution limit precludes us from imaging potential localised magmatic intrusions with dimensions of tens of kilometres, the absence of surface volcanism, the amagmatic upper crustal rupture known to have occurred at disparate locations on the western branch, the presence of lower crustal seismicity and the low temperatures implied by the fast seismic wave speeds in the lower crust and uppermost mantle in this region suggests possible amagmatic extension. Most dynamic models predict that this should not happen. Indeed even with magmatic intrusion, rifting of continental lithosphere 〉100 km thick is considered improbable under conditions found on Earth. Yield strength envelopes confirm that currently modelled stresses are insufficient to produce the observed deformation along these portions of the rift system. Stresses arising from the gravitational force related to the uplift of the East African Plateau provide only one-eighth of the minimum stress necessary to produce observed lower crustal earthquakes in the western branch. We expect that some of this disparity may be accounted for by considering smaller scale bending stresses and dynamic feedbacks between brittle and elastic deformation and between faulting, topography and weathering that are not currently included in models of the East African Rift.

Description: The Zambales Ophiolite Complex (ZOC) in the western portion of Luzon Island, Philippines represents a typical exposure of an emplaced crust-upper mantle section of an ancient lithosphere. The ZOC is divided into the Acoje and Coto Blocks based on petrological, geochemical and age disparities, thus implying diverse and complex origins for the archipelago's lithospheric sources. We used gravity and magnetic data to reveal differences in the subsurface characteristics of the two ophiolite blocks for the first time. Low Bouguer gravity (〈135 mGal) and magnetic (〈69 nT) anomalies characterize the ophiolitic units in the Acoje Block whereas high Bouguer gravity (〉150 mGal) and magnetic (〉110 nT) anomalies typify the Coto Block. Such contrasting signatures further extend at greater depths which reflect the disparities in the crustal density, the basement structure, and the depth to Moho. Petrophysical characteristics such as density, magnetic susceptibility and natural remanent magnetization (NRM) intensities correlate well with the interpreted Bouguer gravity and magnetic anomalies. Densities of gabbros and peridotites from the Acoje and Coto Blocks reveal mean values ranging from 2640 to 2810 kg m –3 and 2570 to 2690 kg m –3 , respectively. Magnetic susceptibility (〉4.43 x 10 –3 SI) and NRM (〉0.69 A m –1 ) data are also generally higher over the Coto Block. Both Bouguer gravity and magnetic anomaly maps reveal a prominent steep gradient that potentially marks the structural contact between Acoje and Coto Blocks. We further infer that the steep anomaly gradient validates the presence of the Lawis Fault Zone which separates the two ophiolite blocks in the Masinloc Massif. Recent field evidence from rock exposures in Coto reveals both right lateral and vertical displacements along the fault zone.

Description: In this work, we analyse in detail how a large earthquake could cause stress changes on volcano plumbing systems and produce possible positive feedbacks in promoting new eruptions. We develop a sensitivity analysis that considers several possible parameters, providing also new constraints on the methodological approach. The work is focus on the M w 8.8 2010 earthquake that occurred along the Chile subduction zone near 24 historic/Holocene volcanoes, located in the Southern Volcanic Zone. We use six different finite fault-slip models to calculate the static stress change, induced by the coseismic slip, in a direction normal to several theoretical feeder dykes with various orientations. Results indicate different magnitudes of stress change due to the heterogeneity of magma pathway geometry and orientation. In particular, the N–S and NE–SW-striking magma pathways suffer a decrease in stress normal to the feeder dyke (unclamping, up to 0.85 MPa) in comparison to those striking NW–SE and E–W, and in some cases there is even a clamping effect depending on the magma path strike. The diverse fault-slip models have also an effect (up to 0.4 MPa) on the results. As a consequence, we reconstruct the geometry and orientation of the most reliable magma pathways below the 24 volcanoes by studying structural and morphometric data, and we resolve the stress changes on each of them. Results indicate that: (i) volcanoes where post-earthquake eruptions took place experienced earthquake-induced unclamping or very small clamping effects, (ii) several volcanoes that did not erupt yet are more prone to experience future unrest, from the point of view of the host rock stress state, because of earthquake-induced unclamping. Our findings also suggest that pathway orientation plays a more relevant role in inducing stress changes, whereas the depth of calculation (e.g. 2, 5 or 10 km) used in the analysis, is not key a parameter. Earthquake-induced magma-pathway unclamping might contribute to promote new eruptions at volcanoes as far as 450 km from the epicentre.

Description: Seismic studies indicate that the Earth's inner core has a complex structure and exhibits a strong elastic anisotropy with a cylindrical symmetry. Among the various models which have been proposed to explain this anisotropy, one class of models considers the effect of the Lorentz force associated with the magnetic field diffused within the inner core. In this paper, we extend previous studies and use analytical calculations and numerical simulations to predict the geometry and strength of the flow induced by the poloidal component of the Lorentz force in a neutrally or stably stratified growing inner core, exploring also the effect of different types of boundary conditions at the inner core boundary (ICB). Unlike previous studies, we show that the boundary condition that is most likely to produce a significant deformation and seismic anisotropy is impermeable, with negligible radial flow through the boundary. Exact analytical solutions are found in the case of a negligible effect of buoyancy forces in the inner core (neutral stratification), while numerical simulations are used to investigate the case of stable stratification. In this situation, the flow induced by the Lorentz force is found to be localized in a shear layer below the ICB, whose thickness depends on the strength of the stratification, but not on the magnetic field strength. We obtain scaling laws for the thickness of this layer, as well as for the flow velocity and strain rate in this shear layer as a function of the control parameters, which include the magnitude of the magnetic field, the strength of the density stratification, the viscosity of the inner core and the growth rate of the inner core. We find that the resulting strain rate is probably too small to produce significant texturing unless the inner core viscosity is smaller than about 10 12 Pa s.

Description: We analyse Bouguer anomaly data and previously published Moho depths estimated from receiver functions in order to determine the amount of isostatic compensation or uncompensation of the Rif topography in northern Morocco. We use Moho depth variations extracted from receiver function analyses to predict synthetic Bouguer anomalies that are then compared to observed Bouguer anomaly. We find that Moho depth variations due to isostatic compensation of topographic and/or intracrustal loads do not match Moho depth estimates obtained from receiver function analyses. The isostatic misfit map evidences excess crustal root as large as 10 km in the western part of the study area, whereas a ‘missing’ crustal root of ~5 km appears east of 4.3°E. This excess root/missing topography correlates with the presence of a dense mantle lid, the noticeable southwestward drift of the Western Rif area, and with a current surface uplift. We propose that a delaminated mantle lid progressively detaching westward or southwestward from the overlying crust is responsible for viscous flow of the ductile lower crust beneath the Rif area. This gives rise to isostatic uplift and westward drift due to viscous coupling at the upper/lower crust boundary. At the same time, the presence of this dense sinking mantle lid causes a negative dynamic topography, which explains why the observed topography is too low compared to the crustal thickness.

Description: Flow-like landslides are rapidly moving fluid–solid mixtures that can cause significant destruction along paths that run far from their original sources. Existing models for run out prediction and motion simulation of flow-like landslides have many limitations. In this paper, we develop a new method named ‘Tsunami Squares’ to simulate the generation, propagation and stoppage of flow-like landslides based on conservation of volume and momentum. Landslide materials in the new method form divisible squares that are displaced, then further fractured. The squares move under the influence of gravity-driven acceleration and suffer decelerations due to basal and dynamic frictions. Distinctively, this method takes into account solid and fluid mechanics, particle interactions and flow regime transitions. We apply this approach to simulate the 1982 El Picacho landslide in San Salvador, capital city of El Salvador. Landslide products from Tsunami Squares such as run out distance, velocities, erosion and deposition depths and impacted area agree well with field investigated and eyewitness data.

Description: Numerous regional plate reorganizations and the coeval ages of the Hawaiian Emperor bend (HEB) and Louisville bend of 50–47 Ma have been interpreted as a possible global tectonic plate reorganization at ~chron 21 (47.9 Ma). Yet for a truly global event we would expect a contemporaneous change in Africa absolute plate motion (APM) reflected by physical evidence distributed on the Africa Plate. This evidence has been postulated to take the form of the Réunion-Mascarene bend which exhibits many HEB-like features, such as a large angular change close to ~chron 21. However, the Réunion hotspot trail has recently been interpreted as a sequence of continental fragments with incidental hotspot volcanism. Here we show that the alternative Réunion-Mascarene Plateau trail can also satisfy the age progressions and geometry of other hotspot trails on the Africa Plate. The implied motion, suggesting a pivoting of Africa from 67 to 50 Ma, could explain the apparent bifurcation of the Tristan hotspot chain, the age reversals seen along the Walvis Ridge, the sharp curve of the Canary trail, and the diffuse nature of the St. Helena chain. To test this hypothesis further we made a new Africa APM model that extends back to ~80 Ma using a modified version of the Hybrid Polygonal Finite Rotation Method. This method uses seamount chains and their associated hotspots as geometric constraints for the model, and seamount age dates to determine APM through time. While this model successfully explains many of the volcanic features, it implies an unrealistically fast global lithospheric net rotation, as well as improbable APM trajectories for many other plates, including the Americas, Eurasia and Australia. We contrast this speculative model with a more conventional model in which the Mascarene Plateau is excluded in favour of the Chagos-Laccadive Ridge rotated into the Africa reference frame. This second model implies more realistic net lithospheric rotation and far-field APMs, but fails to explain key details of the Atlantic Ocean volcanic chains. Both models predict a Canary plume influence beneath the Madeiras. Neither model, when projected via the global plate circuit into the Pacific, predicts any significant change in plate motion around chron 21. Consequently, Africa APM models do not appear to provide independent support for a chron 21 global reorganization.