ALBERT

Description: We present high-resolution tomographic images in source areas of 26 large crustal earthquakes ( M 6.0–7.2) which occurred in Northeast Japan (Tohoku) during the past 120 yr from 1894 to 2014. Prominent low-velocity (low- V ) and high Poisson's ratio (high- ) anomalies are revealed in the crust and mantle wedge under the source areas. Beneath the volcanic front and backarc areas, the low- V and high- zones reflect arc-magma related high-temperature anomalies which are produced by joint effects of corner flow in the mantle wedge and fluids from dehydration of the subducting Pacific slab. The hot anomalies cause locally thinning and weakening of the brittle seismogenic layer above them. Low-frequency micro-earthquakes are observed in the lower crust and uppermost mantle in or around the low- V zones, which reflect ascending of arc magma and fluids from the mantle wedge to the crust. No volcano and magma exist in the forearc area due to low temperature there, hence the low- V zones in the forearc reflect fluids from the slab dehydration. The ascending fluids may have produced a ‘water wall’ in the mantle wedge and crust beneath the forearc area. When the water enters active faults in the crust, the fault-zone friction is reduced and so large earthquakes can be induced. These results indicate that the nucleation of a large earthquake is not entirely a mechanical process, but is closely associated with subduction dynamics and physical and chemical properties of rocks in the crust and upper mantle. In particular, arc magma and fluids play an important role in the seismogenesis.

Description: For a period of about 1 yr between the summers of 2010 and 2011, 25 broad-band seismographs were deployed in a roughly linear array across the eastern end of the Qaidam basin and the Qilian Shan in the northeastern Tibetan plateau. This region is probably the most suitable place to study the ongoing convergence interaction between the high Tibetan plateau and the main Asian continental plate. Low-frequency P receiver function analysis of the data provides an image of the crust and mantle down to 700 km depth. In addition to the Moho at 45–65 km depth beneath the profile, the 410 and 660 km discontinuities bounding the mantle transition zone can be identified at 400–410 and 650–660 km depths, respectively. A possible increase in temperature in the upper mantle thought to exist beneath the northern part of the high Tibetan plateau is thus confined to this part of the plateau and lower upper-mantle temperatures similar to those beneath southern Tibet occur beneath the Qaidam basin and Qilian Shan. When higher frequencies are included in the P receiver function analysis, a positive Ps converter dipping down to the south from 70–75 km depth at 37.9°N to about 110 km depth at 36°N is imaged. As this feature is only seen in high-frequency images and not in the low-frequency image, it is modelled as the positive Ps conversion from the base of an approximately 5-km-thick anisotropic layer at the top of the Asian mantle lithosphere which is currently subducting. This south-dipping converter continues to the south on the INDEPTH IV profile. S receiver function analysis completes the image of the structure below the Qilian Shan profile with the identification of the lithosphere–asthenosphere boundary (LAB). The LAB of the Asian Plate is identified for a reference slowness of 6.4 s deg –1 at 12–14 s (105–125 km depth) between 38 and 41°N below the northern part of the S receiver function profile. To the south it increases in depth such that it is at about 19 s (170 km depth) between 34 and 35°N at the southern end of the profile. The LAB of the Asian Plate occurs at similar depths on the INDEPTH IV profile at the latitudes where the INDEPTH IV and Qilian Shan profiles overlap. As on the INDEPTH IV profile to the south, between 34 and 35°N at the southern end of the Qilian Shan profile there is evidence from the S receiver functions for the LAB of a separate Tibetan Plate.

Description: A new model of the deglaciation history of Antarctica over the past 25 kyr has been developed, which we refer to herein as ICE-6G_C (VM5a). This revision of its predecessor ICE-5G (VM2) has been constrained to fit all available geological and geodetic observations, consisting of: (1) the present day uplift rates at 42 sites estimated from GPS measurements, (2) ice thickness change at 62 locations estimated from exposure-age dating, (3) Holocene relative sea level histories from 12 locations estimated on the basis of radiocarbon dating and (4) age of the onset of marine sedimentation at nine locations along the Antarctic shelf also estimated on the basis of 14 C dating. Our new model fits the totality of these data well. An additional nine GPS-determined site velocities are also estimated for locations known to be influenced by modern ice loss from the Pine Island Bay and Northern Antarctic Peninsula regions. At the 42 locations not influenced by modern ice loss, the quality of the fit of postglacial rebound model ICE-6G_C (VM5A) is characterized by a weighted root mean square residual of 0.9 mm yr –1 . The Southern Antarctic Peninsula is inferred to be rising at 2 mm yr –1 , requiring there to be less Holocene ice loss there than in the prior model ICE-5G (VM2). The East Antarctica coast is rising at approximately 1 mm yr –1 , requiring ice loss from this region to have been small since Last Glacial Maximum. The Ellsworth Mountains, at the base of the Antarctic Peninsula, are inferred to be rising at 5–8 mm yr –1 , indicating large ice loss from this area during deglaciation that is poorly sampled by geological data. Horizontal deformation of the Antarctic Plate is minor with two exceptions. First, O'Higgins, at the tip of the Antarctic Peninsula, is moving southeast at a significant 2 mm yr –1 relative to the Antarctic Plate. Secondly, the margins of the Ronne and Ross Ice Shelves are moving horizontally away from the shelf centres at an approximate rate of 0.8 mm yr –1 , in viscous response to the early Holocene unloading of ice from the current locations of the ice shelf centers. ICE-6G_C (VM5A) fits the horizontal observations well (wrms residual speed of 0.7 mm yr –1 ), there being no need to invoke any influence of lateral variation in mantle viscosity. ICE-6G_C (VM5A) differs in several respects from the recently published W12A model of Whitehouse et al. First, the upper-mantle viscosity in VM5a is 5 10 20 Pa s, half that in W12A. The VM5a profile, which is identical to that inferred on the basis of the Fennoscandian relaxation spectrum, North American relative sea level histories and Earth rotation constraints, when coupled with the revised ICE-6G_C deglaciation history, fits all of the available constraints. Secondly, the net contribution of Antarctica ice loss to global sea level rise is 13.6 m, 2/3 greater than the 8 m in W12A. Thirdly, ice loss occurs quickly from 12 to 5 ka, and the contribution to global sea level rise during Meltwater Pulse 1B (11.5 ka) is large (5 m), consistent with sedimentation constraints from cores from the Antarctica ice shelf. Fourthly, in ICE-6G_C there is no ice gain in the East Antarctica interior, as there is in W12A. Finally, the new model of Antarctic deglaciation reconciles the global constraint upon the global mass loss during deglaciation provided by the Barbados record of relative sea level history when coupled with the Northern Hemisphere counterpart of this new model.

Description: Geophysical data are the main source of information about the subsurface. Geophysical techniques are, however, highly non-unique in determining specific physical parameters and boundaries of subsurface objects. To obtain actual physical information, an inversion process is often applied, in which measurements at or above the Earth surface are inverted into a 2- or 3-D subsurface spatial distribution of the physical property. Interpreting these models into structural objects, related to physical processes, requires a priori knowledge and expert analysis which is susceptible to subjective choices and is therefore often non-repeatable. In this research, we implemented a recently introduced object-based approach to interpret the 3-D inversion results of a single geophysical technique using the available a priori information and the physical and geometrical characteristics of the interpreted objects. The introduced methodology is semi-automatic and repeatable, and allows the extraction of subsurface structures using 3-D object-oriented image analysis (3-D OOA) in an objective knowledge–based classification scheme. The approach allows for a semi-objective setting of thresholds that can be tested and, if necessary, changed in a very fast and efficient way. These changes require only changing the thresholds used in a so-called ruleset, which is composed of algorithms that extract objects from a 3-D data cube. The approach is tested on a synthetic model, which is based on a priori knowledge on objects present in the study area (Tanzania). Object characteristics and thresholds were well defined in a 3-D histogram of velocity versus depth, and objects were fully retrieved. The real model results showed how 3-D OOA can deal with realistic 3-D subsurface conditions in which the boundaries become fuzzy, the object extensions become unclear and the model characteristics vary with depth due to the different physical conditions. As expected, the 3-D histogram of the real data was substantially more complex. Still, the 3-D OOA-derived objects were extracted based on their velocity and their depth location. Spatially defined boundaries, based on physical variations, can improve the modelling with spatially dependent parameter information. With 3-D OOA, the non-uniqueness on the location of objects and their physical properties can be potentially significantly reduced.

Description: Here we inverted the GPS data to infer the coseismic slip of the Tohoku-Oki earthquake and the time-dependent afterslip distribution in the 4 months following the main shock. The Tohoku-Oki earthquake showed an unexpected magnitude and a characteristic depth-dependent differentiation of seismic energy radiation. In this context the estimation and comparison of the distribution of the fault portions that slip coseismically and post-seismically contribute to a better understanding of the variation of frictional characteristics of the plate interface. The inferred coseismic slip extends in a relatively compact region located updip from the hypocentre and reaches its highest value (about 60 m) near the trench. Afterslip occurs mostly outside the coseismic rupture and is distributed in two main modal centres. It reaches its largest values in an area located downdip of the coseismic slip and extends to a depth of 80 km. In the depth range between 30 and 50 km afterslip overlaps the portion of the fault that experienced historical moderate earthquakes, high-frequency seismic radiation and thrust-type aftershocks. The behaviour of this area can be explained by a rheologically heterogeneous region made of a ductile fault matrix interspersed with compact brittle asperities. On the contrary, the region beneath 50–60 km depth is probably characterized by a fully velocity strengthening behaviour. Southern afterslip, located off-Chiba Prefecture, is probably related to the M w 7.9 Ibaraki-Oki aftershock. The northward extension of the afterslip stops at a latitude of about 40°N, just south of the off-Aomori region. This may be related to three large events occurred in this area during the last century and the consequent strong coupling or complete depletion of the accumulated strain that characterize this region.

Description: Thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres. Examples include flexure at seamounts, fracture zones, sedimentary basins and subduction zones. When applying this theory to real world situations, most studies assume a locally uniform plate thickness to enable simple Fourier transform solutions. However, in cases where the amplitude of the flexure is prominent, such as subduction zones, or there are rapid variations in seafloor age, such as fracture zones, these models are inadequate. Here we present a computationally efficient algorithm for solving the thin plate flexure equation for non-uniform plate thickness and arbitrary vertical load. The iterative scheme takes advantage of the 2-D fast Fourier transform to perform calculations in both the spatial and spectral domains, resulting in an accurate and computationally efficient solution. We illustrate the accuracy of the method through comparisons with known analytic solutions. Finally, we present results from three simple models demonstrating the differences in trench outer rise flexure when 2-D variations in plate rigidity and applied bending moment are taken into account. Although we focus our analysis on ocean trench flexure, the method is applicable to other 2-D flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental–oceanic crust boundary.

Description: Teleseismic data recorded at 13 broad-band seismological stations across northwest part of the Tethyan Himalaya and eastern Ladakh are analysed to determine the seismic characteristics of the crust and upper mantle beneath the northwest India–Asia collision zone. The receiver functions computed from teleseismic P- waveform for a wide range of backazimuth show strong azimuthal variation in the Indus suture zone (ISZ), the zone which marks the collision and subsequent subduction of both the Tethyan oceanic plate and Indian continental plate beneath Eurasia. The teleseismic waves piercing the ISZ do not show clear P -to- S ( Ps ) converted phase at the depth of Moho. In contrast, the waves piercing the Karakoram zone, Ladakh batholith and the Tethyan Himalayan region south of the ISZ clearly show the Moho converted Ps phase and corresponding inverted models reveal variation of crustal thickness from ~60 km beneath the Tethyan Himalaya to ~80 km beneath the Karakoram fault zone. A prominent intracrustal low velocity zone (IC-LVZ) is detected in the shear wave velocity models within the depth range ~15–40 km. The IC-LVZ identified at the stations both north and south of the ISZ can be interpreted as due to presence of fluid/partial melt. Our study provides compelling evidence that the mid-crustal low velocity zone does extend across the suture zone, in to the Tethyan Himalaya. The contact between this serpentinized ultramafic rocks and the eclogitized Indian continental crust in the suture zone is identified at ~47–50 km depth.

Description: The Pacific and Australian plates in the South Island, New Zealand (NZ) converge at a rate of about 4 cm yr –1 . Accommodation of the continental part of this convergence in the lithospheric mantle is both poorly understood and currently controversial yet it is a problem of fundamental importance for understanding lithospheric thickening. End-member possibilities range from the classical model of asymmetric subduction to symmetric viscous thickening. Seismic tomography has the potential to image this process. However, tomographic images to date are poorly constrained due to the lack of appropriate earthquakes. Improved teleseismic tomography of the region has been achieved by increasing data coverage and applying a novel scheme of correcting for crustal structure by ray tracing through a newly created model of shallow shear wave velocity derived from the inversion of noise-based dispersion measurements. Our resulting models suggest the lithospheric mantle high velocities at the continental plate boundary extend no deeper than approximately 125 km, evidence against both previous models of viscous drip and typical asymmetric subduction zones. This high velocity core extends from north to south along the axis of South Island suggesting that mantle convergence is accommodated along the older, mid-Cenozoic, plate boundary. West of South Island, a high velocity west dipping zone may define the remnant Cretaceous subduction zone that has been distorted by Cenozoic transcurrent deformation. We present our new 3-D seismic velocity models together with a compatible tectonic model and discuss their implications for the nature of lithospheric evolution at this convergent boundary.

Description: Numerical experiments of convection with grain-damage are used to develop scaling laws for convective heat flow, mantle velocity and plate velocity across the stagnant lid and plate-tectonic regimes. Three main cases are presented in order of increasing complexity: a simple case wherein viscosity is only dependent on grain size, a case where viscosity depends on temperature and grain size, and finally a case where viscosity is temperature and grain size sensitive, and the grain-growth (or healing) is also temperature sensitive. In all cases, convection with grain-damage scales differently than Newtonian convection; whereas the Nusselt number (Nu), typically scales with the reference Rayleigh number, Ra 0 , to the 1/3 power, for grain-damage this exponent is larger because increasing Ra 0 also enhances damage. In addition, Nu, mantle velocity, and plate velocity are also functions of the damage to healing ratio, ( D / H ); increasing D / H increases Nu because more damage leads to more vigorous convection. For the fully realistic case, numerical results show stagnant lid convection, fully mobilized convection that resembles the temperature-independent viscosity case, and partially mobile or transitional convection, depending on D / H , Ra 0 , and the activation energies for viscosity and healing. Applying our scaling laws for the fully realistic case to Earth and Venus we demonstrate that increasing surface temperature dramatically decreases plate speed and heat flow, essentially shutting down plate tectonics, due to increased healing in lithospheric shear zones, as proposed previously. Contrary to many previous studies, the transitional regime between the stagnant lid and fully mobilized regimes is large, and the transition from stagnant lid to mobile convection is gradual and continuous. Thus planets could exhibit a full range of surface mobility, as opposed to the bimodal distribution of fully mobile lid planets and stagnant lid planets that is typically assumed.

Description: Three magnetotelluric (MT) profiles in northwestern Canada cross the central and western segments of Great Slave Lake shear zone (GSLsz), a continental scale strike-slip structure active during the Slave-Rae collision in the Proterozoic. Dimensionality analysis indicates that (i) the resistivity structure is approximately 2-D with a geoelectric strike direction close to the dominant geological strike of N45°E and that (ii) electrical anisotropy may be present in the crust beneath the two southernmost profiles. Isotropic and anisotropic 2-D inversion and isotropic 3-D inversions show different resistivity structures on different segments of the shear zone. The GSLsz is imaged as a high resistivity zone (〉5000  m) that is at least 20 km wide and extends to a depth of at least 50 km on the northern profile. On the southern two profiles, the resistive zone is confined to the upper crust and pierces an east-dipping crustal conductor. Inversions show that this dipping conductor may be anisotropic, likely caused by conductive materials filling a network of fractures with a preferred spatial orientation. These conductive regions would have been disrupted by strike-slip, ductile deformation on the GSLsz that formed granulite to greenschist facies mylonite belts. The pre-dominantly granulite facies mylonites are resistive and explain why the GSLsz appears as a resistive structure piercing the east-dipping anisotropic layer. The absence of a dipping anisotropic/conductive layer on the northern MT profile, located on the central segment of the GSLsz, is consistent with the lack of subduction at this location as predicted by geological and tectonic models.