ALBERT

Description: In this study, rock friction ‘stick-slip’ experiments are used to develop constraints on models of earthquake recurrence. Constant-rate loading of bare rock surfaces in high quality experiments produces stick-slip recurrence that is periodic at least to second order. When the loading rate is varied, recurrence is approximately inversely proportional to loading rate. These laboratory events initiate due to a slip rate-dependent process that also determines the size of the stress drop and as a consequence, stress drop varies weakly but systematically with loading rate. This is especially evident in experiments where the loading rate is changed by orders of magnitude, as is thought to be the loading condition of naturally occurring, small repeating earthquakes driven by afterslip, or low-frequency earthquakes loaded by episodic slip. As follows from the previous studies referred to above, experimentally observed stress drops are well described by a logarithmic dependence on recurrence interval that can be cast as a non-linear slip-predictable model. The fault's rate dependence of strength is the key physical parameter. Additionally, even at constant loading rate the most reproducible laboratory recurrence is not exactly periodic, unlike existing friction recurrence models. We present example laboratory catalogs that document the variance and show that in large catalogs, even at constant loading rate, stress drop and recurrence co-vary systematically. The origin of this covariance is largely consistent with variability of the dependence of fault strength on slip rate. Laboratory catalogs show aspects of both slip and time predictability and successive stress drops are strongly correlated indicating a ‘memory’ of prior slip history that extends over at least one recurrence cycle.

Description: We derive equations for HTI and orthorhombic symmetries to analyze fluid substitution effects in porous fractured media. The derivations are based on the anisotropic Gassmann equation and linear slip theory. We assess the influence of fluid substitution (gas, brine, and oil), on elastic moduli, velocities, anisotropy, and azimuthal amplitude variations. We find that in the direction normal to fractures, P-wave moduli increase as much as 56% and P-wave velocity increases up to 19% for gas-to-brine substitution. For the direction parallel to fractures, P-wave velocity remains almost constant when porosity is low (5%), but can increase up to 4% if porosity is high (25%). Since P-waves in two different directions have different sensitivities to fluids and fractures, the Thomsen's parameters (defined for HTI and orthorhombic symmetries), ε and δ , are sensitive to fluid types and fractures. We also found that δ is sensitive to porosity for liquid saturation, but insensitive to porosity for the case of gas saturation. Gassmann assumes (and as has been observed) that shear modulus does not depend on fluids. And we observe no changes in shear-wave splitting ( γ ) for different fluids. The azimuthal amplitude variation is dependent on fluid types, fractures and porosity. We observe up to 12% increase in azimuthal amplitude variation for low porosity gas sands after brine saturation, and 6% decrease for high porosity gas sands. We find that the percentage changes in gas-to-oil substitution are about half that of the gas-to-brine case. The equations we have derived provide a useful tool to quantitatively evaluate the effects of fluid substitution on seismic anisotropy.

Description: In the central part of Fennoscandia the crust is currently rising, because of the delayed response of the viscous mantle to melting of the Late Pleistocene ice sheet. This process, called Glacial Isostatic Adjustment (GIA), causes a negative anomaly in the present-day static gravity field as isostatic equilibrium has not been reached yet. Several studies have tried to use this anomaly as a constraint on models of GIA, but the uncertainty in crustal and upper mantle structures has not been fully taken intoaccount. Therefore, our aim is to revisit this using improved crustal models and compensation techniques. We find that, in contrast with other studies, the effect of crustal anomalies on the gravity field cannot be effectively removed, because of uncertainties in the crustal and upper mantle density models. Our second aim is to estimate the effects on geophysical models, which assume isostatic equilibrium, after correcting the observed gravity field with numerical models for GIA. We show that correcting for GIA in geophysical modelling can give changes of several km in the thickness of structural layers of modeled lithosphere, which is a small but significant correction. Correcting the gravity field for GIA prior to assuming isostatic equilibrium and inferring density anomalies might be relevant in other areas with ongoing post-glacial rebound such as North America and the polar regions.

Description: Large variability of earthquake stress drops and scaled energy has been commonly reported in the literature, but it is difficult to assess how much of this variability is caused by underlying physical source processes rather than simply observational uncertainties. Here, we examine a variety of dynamically realistic rupture scenarios for circular and elliptical faults and investigate to what extent the variability in seismically estimated stress drops and scaled energy comes from differences in source geometry, rupture directivity, and rupture speeds. We numerically simulate earthquake source scenarios using a cohesive-zone model with the small-scale yielding limit, where the solution approaches a singular crack model with spontaneous healing of slip. Compared to symmetrical circular source models, asymmetrical models result in larger variability of estimated corner frequencies and scaled energy over the focal sphere. The general behavior of the spherical averages of corner frequencies and scaled energy in the subshear regime extends to the supershear regime, although shear Mach waves generated by the propagation of supershear rupture lead to much higher corner-frequency and scaled-energy estimates locally. Our results suggest that at least a factor of two difference in the spherical average of corner frequencies is expected in observational studies simply from variability in source characteristics almost independent of the actual stress drops, translating into a factor ofeight difference in estimated stress drops. Furthermore, radiation efficiency estimates derived from observed seismic spectra should not be directly interpreted as describing rupture properties unless there are independent constraints on rupture speed and geometry.

Description: Petrophysical properties of rocks and their applicability at larger scale are a challenging topic in Earth sciences. Petrophysical properties of rocks are severely affected by: boundary conditions, rock fabric/microstructure, and tectonics that require a multi-scale approach to be properly defined. Here we: (1) report laboratory measurements of density, porosity, permeability and P-wave velocities at increasing confining pressure conducted on Miocene foredeep sandstones (Frosinone Fm.); (2) compare the laboratory results with larger-scale geophysical investigations; (3) discuss the effect of thrusting on the properties of sandstones. At ambient pressure, laboratory porosity varied from 2.2% to 13.8% and P-wave velocities (Vp) from 1.5 km/s to 2.7 km/s. The P-wave velocity increased with confining pressure, reaching between 3.3 km/s to 4.7 km/s at 100 MPa. In situ Vp profiles, measured using sonic logs, matched the ultrasonic laboratory measurement well. The permeability varied between 1.4 × 10 -15  m 2 to 3.9 × 10 -15  m 2 and was positively correlated with porosity. The porosity and permeability of samples taken at various distances to the Olevano-Antrodoco fault plane progressively decreased with distance while P-wave velocity increased. At about 1 km from the fault plane, the relative variations reached 43%, 65% and 20% for porosity, permeability and P-wave velocity, respectively. This suggests that tectonic loading changed the petrophysical properties inherited from sedimentation and diagenesis. Using field constraints and assuming overburden-related inelastic compaction in the proximity of the fault plane, we conclude that the fault reached the mechanical condition for rupture in compression at differential stress of 64.8 MPa at a depth of 1500 m.

Description: In a subduction system the force and the energy required to deform the overriding plate are generally thought to come from the negative buoyancy of the subducted slab and its potential energy, respectively. Such deformation might involve extension and backarc basin formation, or shortening and mountain building. How much of the slab's potential energy is consumed during overriding plate deformation remains unknown. In this work, we present dynamic three-dimensional laboratory experiments of progressive subduction with an overriding plate to quantify the force ( F OPD ) that drives overriding plate deformation and the associated energy dissipation rate (Φ OPD ), and we compare them with the negative buoyancy ( F BU ) of the subducted slab and its total potential energy release rate (Φ BU ), respectively. We varied the viscosity ratio between the plates and the sub-lithospheric upper mantle with η SP /η UM  = 157-560, and the thickness of the overriding plate with T OP  = 0.5-2.5 cm (scaling to 25-125 km in nature). The results show that F OPD / F BU has average values of 0.5-2.0%, with a maximum of 5.3% and Φ OPD /Φ BU has average values of 0.05-0.30%, with a maximum of 0.41%. The results indicate that only a small portion of the negative buoyancy of the slab and its potential energy are used to deform the overriding plate. Our models also suggest that the force required to deform the overriding plate is of comparable magnitude to the ridge push force. Furthermore, we show that in subduction models with an overriding plate bending dissipation at the subduction zone hinge remains low (3-15% during steady-state subduction).

Description: We investigated the influence of earthquake source complexity on the extent of inundation caused by the resulting tsunami. We simulated 100 scenarios with collocated sources of variable slip on the Hikurangi subduction-interface in the vicinity of Hawke's Bay and Poverty Bay in New Zealand and investigated the tsunami effects on the cities of Napier and Gisborne. Rupture complexity was found to have a first order effect on flow depth and inundation extent for local tsunami sources. The position of individual asperities in the slip distribution on the rupture interface control to some extent how severe inundation will be. However, predicting inundation extent in detail from investigating the distribution of slip on the rupture interface proves difficult. Assuming uniform slip on the rupture interface in tsunami models can underestimate the potential impact and extent of inundation. For example, simulation of an M W 8.7 to M W 8.8 earthquake with uniform slip reproduced the area that could potentially be inundated by equivalent non-uniform slip events of M W 8.4. De-aggregation, to establish the contribution of different sources with different slip distributions to the probabilistic hazard, cannot be performed based on magnitude considerations alone. We propose two predictors for inundation severity based on the offshore tsunami wave field using the linear wave equations in an attempt to keep costly simulations of full inundation to a minimum.

Description: Teleseismic transfer function analysis is used to investigate crust and upper mantle velocity structure in the vicinity of the active eastern Tennessee seismic zone (ETSZ). The ETSZ is associated with the New York – Alabama (NY-AL) magnetic lineament, a prominent aeromagnetic anomaly indicative of Grenville-age, basement structure. Radial component, P-wave transfer functions for ten short-period stations operated by the Center for Earthquake Research and Information (CERI) are inverted for velocity structure. Velocity profiles are also determined for three broadband stations by converting the instrument response to that of an S-13 short-period seismometer. Distinct differences in the velocity profiles are found for stations located on either side of the NY-AL magnetic lineament; velocities west of the lineament are lower than velocities to the east of the lineament in the upper 10 km and in the depth range 30 to 50 km. A gradational Moho boundary is found beneath several stations located in the Valley and Ridge province. A Moho boundary is absent at four Valley and Ridge stations located east of the magnetic lineament and south of 35.5°N.

Description: This integrated study provides significant insight into parameters controlling the acoustic and reservoir properties of microporous limestones, improving the knowledge of the relationships among petrophysic and microstructural content. Petrophysical properties measured from laboratory and logging tools (porosity, permeability, electrical conductivity and acoustic properties) have been coupled with thin section and SEM observations on the EST205 borehole from the Oxfordian limestone aquifer of the Eastern part of the Paris Basin. A major achievement is the establishment of the link between micrite microtexture types (particle morphology and nature of inter-crystal contacts) and the physical response, introducing a new effective and interesting rock-typing approach for microporous reservoirs. Fluid-flow properties are enhanced by the progressive augmentation of intercrystalline microporosity and associated pore throat diameter, as the coalescence of micrite particles decreases. Concerning acoustic properties, the slow increase of P-wave velocity can be seen as a reflection of crystal size and growing contact cementation leading to a more cohesive and stiffer micrite microtexture. By applying poroelasticity theory on our samples, we show that velocity dispersion can be a very useful tool for data discrimination in carbonates. This dispersion analysis highlights the presence of microcracks in the rocks, and their overall effect on acoustic and transport properties. The presence of microcracks is also confirmed with observations and permeability measurements under high confining pressure. Finally, a possible origin of high porous levels in neritic limestones is a mineralogical transformation of carbonates through freshwater-related diagenesis during subaerial exposure time. Finally, by applying poroelasticity theory on our samples, we show that velocity dispersion can be a very useful tool for data discrimination in carbonates.

Description: The “local porosity theory” proposed by Hilfer was revisited to develop a “local clay theory” (LCT) that establishes a quantitative relationship between the effective electrical conductivity and clay distribution in clay-rocks. This theory is primarily based on a “local simplicity” assumption; under this assumption, the complexity of spatial clay distribution can be captured by two local functions, namely, the local clay distribution and the local percolation probability, which are calculated from a partitioning of a mineral map. The local clay distribution provides information about spatial clay fluctuations and the local percolation probability describes the spatial fluctuations in the clay connectivity. This LCT was applied to (a) a mineral map made from a Callovo-Oxfordian mudstone sample and (b) (macroscopic) electrical conductivity measurements performed on the same sample. The direct and inverse modeling show two results. First, the textural and classical model assuming that the electrical anisotropy of clay-rock is mainly controlled by the anisotropy of the sole clay matrix provides inconsistent inverted values. An other textural effect, the anisotropy induced by elongated and oriented non-clayey grains, should be considered. Second, the effective conductivity values depend primarily on the choice of the inclusion-based models used in the LCT. The impact of local fluctuations of clay content and connectivity on the calculated effective conductivity is lower.

Description: Volcanic ash is often deposited in a hot state. Volcanic ash containing glass, deposited above the glass transition interval, has the potential to sinter viscously both to itself (particle-particle) and to exposed surfaces. Here, we constrain the kinetics of this process experimentally under non-isothermal conditions using standard glasses. In the absence of external load, this process is dominantly driven by surface relaxation. In such cases the sintering process is rate-limited by the melt viscosity, the size of the particles and the melt-vapour interfacial tension. We propose a polydisperse continuum model that describes the transition from a packing of particles to a dense pore-free melt and evaluate its efficacy in describing the kinetics of volcanic viscous sintering. We apply our model to viscous sintering scenarios for cooling crystal-poor rhyolitic ash using the 2008 eruption of Chaitén volcano as a case example. We predict that moderate linear cooling rates of 〉10 -1  °C.min -1 can result in the common observation of incomplete sintering and the preservation of pore networks.

Description: [1]  We analyse 1980-2010 ground displacements, to discern similarities or differences between Campi Flegrei (CF) inflations and deflations and highlight possible anomalies in particular areas. We show that the deformation pattern can be decomposed into two stationary (constant over time, except for a mere scaling factor) parts; both of them are satisfied by simple deformation sources. A quasi-horizontal elongated crack (oriented NW to SE, and embedded in an elastic layered half-space at a depth of about 3600 m) satisfies large-scale deformation. All source parameters but potency (volume change) are constant over time. Residual deformation is confined to the area of the Solfatara fumarolic field and satisfied by a small spheroid located at about 1900 m in depth. Again, all source parameters but potency are constant over time. The histories of the two sources are somewhat similar but not equal, supporting the existence of a genuine local deformation source at Solfatara against the emergence of a mere distortion of large-scale deformation. Although reality is probably much more complex, our simple model explains 1980-2010 CF deformation within ground-displacement data errors, and is consistent with Solfatara geochemical conceptual models, fumarolic geochemical data, and seismic attenuation imaging of CF. The observation that the CF deformation pattern can be decomposed into two stationary parts is hardly compatible with several recent works which proposed multiple sources with different features acting in different periods, fluid injections implying ample changes of large-scale deformation pattern over time, complex spatial and temporal patterns of distributions of volumetric sources.

Description: [1]  Detailed three-dimensional (3-D) P - and S -wave attenuation ( Qp and Qs ) models of the crust and upper mantle under the entire Northeast Japan (Tohoku) arc from the Japan Trench to the Japan Sea coast are determined, for the first time, using a large number of high-quality t * data measured precisely from P - and S -wave spectra of local earthquakes. The suboceanic earthquakes used in this work are relocated precisely using sP depth phases. Our results reveal a prominent landward dipping high- Q zone representing the subducting Pacific slab, a landward dipping intermediate to high Q zone in the mantle wedge between the Pacific coast and the volcanic front, and significant low- Q anomalies in the crust and mantle wedge between the volcanic front and the Japan Sea coast. Prominent high- Q patches surrounded by low- Q anomalies are revealed in the interplate megathrust zone under the Tohoku forearc where the great 2011 Tohoku-oki earthquake ( Mw 9.0) occurred. The high- Q patches in the megathrust zone generally exhibit large coseismic slips of megathrust earthquakes and large slip deficit on the plate interface. We think that these high- Q patches represent asperities in the megathrust zone, whereas the low- Q anomalies reflect weakly coupled areas. We also find that the hypocenters of the 2011 Tohoku-oki earthquake and its large foreshock ( Mw 7.3) and two large megathrust aftershocks ( Mw 7.4, 7.7) are located in areas where Qp , Qs and Qp / Qs change abruptly. These results suggest that structural heterogeneities in the megathrust zone control the interplate seismic coupling and the nucleation of megathrust earthquakes.

Description: [1]  We detected 32,078 very small, local microearthquakes (average M L  = -1) during a 9-month deployment of five ocean bottom seismometers (OBS) on the periphery of the Trans-Atlantic Geotraverse (TAG) active mound. Seismicity rates were constant without any mainshock-aftershock behavior at ~243 events per day at the beginning of the experiment, 128 events per day after an instrument failed, and 97 events per day at the end of the experiment when whale calls increased background noise levels. The microearthquake seismograms are characterized by durations of 〈1 second and most have single-phase P -wave arrivals (i.e., no S -arrivals). We accurately located 6,207 of the earthquakes, with hypocenters clustered within a narrow depth interval from ~50-125 mbsf on the south and west flanks of the deposit. We model the microearthquakes as reaction-driven fracturing events caused by anhydrite deposition in the secondary circulation system of the hydrothermal mound, and show that under reasonable modeling assumptions an average event represents a volume increase of 31-58 cm 3 , yielding an annual (seismogenic) anhydrite deposition rate of 27-51 m 3 .

Description: [1]  Acoustic velocity measurements were conducted during triaxial deformation tests of silty clay and clayey silt core samples from the Nankai subduction zone (IODP Expeditions 315, 316 and 333). We provide a new set of sonic wave velocity data, continuously measured during pressure increase and subsequent axial deformation. A new processing method for the experimental data was developed using seismic time series analysis. The results show that the identification of first arrivals by manual trace-by-trace picking alone can be erroneous. During axial deformation compressional wave velocities (Vp) range between about 1300 - 2200 m/s, and shear wave velocities (Vs) range between about 150 - 800 m/s. Vp slightly increases with rising effective confining pressure and effective axial stress. Samples from the accretionary prism toe show the highest Vp, while those from the forearc slope sediments show somewhat lower Vp. The samples from the incoming plate are slightly richer in clay minerals, and have the lowest values for Vp. Vs increases with higher effective confining pressures and effective axial stress, irrespective of composition and tectonic setting. Shear and bulk moduli, calculated from sonic velocities are in the range between 0.2 and 1.3 GPa and 3.85 and 8.41 GPa, respectively. The elastic moduli of the samples from the accretionary prism toe and the footwall of the megasplay fault ranging between 1.50 and 3.98 GPa are higher compared to those from the hanging wall samples and the incoming plate samples ranging between 0.59 and 0.88 Gpa. Thus, they allow differentiation between normal and over-consolidated sediments. The data show that in a tectono-sedimentary environment of only subtle compositional differences, the acoustic properties can be used to differentiate between stronger (accretionary prism toe) and weaker (forearc slope, incoming plate) sediments. Especially the Vp/Vs ratio may be instrumental in detecting zones of low effective stress and thus high pore fluid pressure.

Description: [1]  Studies of the petrology, mineral chemistry and rock-magnetic properties of nine pyroxenite xenoliths from Hannuoba basalts, northern North China Craton, have been made to determine the magnetization signature of the continental lower crust. These pyroxenites are weakly magnetic with low average susceptibility ( χ ) and saturation isothermal remanent magnetization ( M rs ) of 39.59 × 10 -8  m 3  kg -1 and 12.05 × 10 -3 Am 2 kg -1 , respectively. The magnetic minerals are mainly magnetite, pyrrhotite and Fe-rich spinel, which significantly contribute to χ and natural remanent magnetization. Magnetite occurs as interstitial micro-crystals together with zeolite aggregates, indicating a secondary origin in a supergene environment. In contrast, pyrrhotite and Fe-rich spinel were formed prior to the xenoliths’ ascent to the surface, as evidenced by their dominant occurrence as tiny inclusions and thin exsolution lamellae in pyroxene. The Fe-rich spinel has ~ 50% mole-fraction of Fe 3 O 4 and corresponds to the strongest magnetization, and its coexistence with Mg-rich spinel implies a reheating event due to the underplating of basaltic magma. Besides, armalcolite and ilmenite were found in the reaction rims between xenoliths and the basalt, but they contribute little to the whole rock magnetization. However, these pyroxenite xenoliths would be non-magnetic at in situ depths, as well as peridotite and mafic granulite xenoliths derived from the crust-mantle transition zone (~ 32-42 km). Therefore, we suggest the limiting depth of magnetization at the boundary between weakly magnetic deep-seated (lower crust and upper mantle) xenoliths and strongly magnetic Archean granulite facies rocks (~ 32 km) in Hannuoba, northern North China Craton.

Description: [1]  The shallow velocity structure of the Lucky Strike segment of the Mid-Atlantic Ridge is investigated using seismic refraction and reflection techniques applied to downward continued multi-channel streamer data. We present a three-dimensional velocity model beneath the Lucky Strike Volcano with unprecedented spatial resolutions of a few hundred meters. These new constraints reveal large lateral variations in P-wave velocity structure beneath this feature. Throughout the study area, uppermost crustal velocities are significantly lower than those inferred from lower-resolution OBS studies, with the lowest values (1.8-2.2 km/s) found beneath the three central volcanic cones. Within the central volcano, distinct shallow units are mapped that likely represent a systematic process such as burial of older weathered surfaces. We infer that the entire upper part of the central volcano is young relative to the underlying median valley floor and that there has been little increase in the layer 2A velocities since emplacement. Layer 2A thins significantly across the axial valley bounding faults likely as the result of footwall uplift. The upper crustal velocities increase with age, on average, at a rate of ~0.875 km/s/Myr, similar to previous measurements at fast spreading ridges, suggesting hydrothermal sealing of small scale porosity is progressing at normal to enhanced rates.

Description: [1]  High quality streaming potential coupling coefficient measurements have been carried out using a newly designed cell with both a steady-state methodology and a new pressure transient approach. The pressure transient approach has shown itself to be particularly good at providing high quality streaming potential coefficient measurements as each transient increase or decrease allows thousands of measurements to be made at different pressures to which a good linear regression can be fitted. Nevertheless, the transient method can be up to five times as fast as the conventional measurement approaches because data from all flow rates are taken in the same transient measurement rather than separately. Test measurements have been made on samples of Berea and Boise sandstone as a function of salinity (approximately 18 salinities between 10-5 mol/dm3 and 2 mol/dm3). The data have also been inverted to obtain the zeta potential. The streaming potential coefficient becomes greater (more negative) for fluids with lower salinities, which is consistent with existing measurements. Our measurements are also consistent with the high salinity streaming potential coefficient measurements made by Vinogradov et al. (2010). Both the streaming potential coefficient and the zeta potential have also been modeled using the theoretical approach of Glover et al. (2012). This modeling allows the microstructural, electrochemical and fluid properties of the saturated rock to be taken into account in order to provide a relationship that is unique to each particular rock sample. In all cases, we found that the experimental data was a good match to the theoretical model.

Description: [1]  The present study, which is a follw-up of the JGR paper by Ji et al. (2013a), provides a new calibration for both seismic and fabric properties of antigorite serpentinites. Comparisons of the laboratory velocities of antigorite serpentinites measured at high pressures with crystallographic preferred orientation (CPO) data measured using electron backscatter diffraction (EBCD) techniques demonstrate that seismic anisotropy in high T serpentinite, which is essentially controlled by the antigorite c-axis fabric, is independent on the operating slip system, but strongly dependent on the regime and magnitude of finite strain experienced by the rock. Extrapolation of the experimental data with both pressure and temperature suggests that V p anisotropy decreases but shear wave splitting (Δ V s ) and V p / V s increase with increasing pressure in either cold or hot subduction zones. For a cold, steeply subducting slab, antigorite is most likely deformed by nearly coaxial flattening or trench-parallel movements, forming trench-parallel seismic anisotropy. For a hot, shallowly subducting slab, however, antigorite is most likely deformed by simple shear or transpression. Trench-normal seismic anisotropy can be observed when the subducting dip angle is smaller than 30°. The geophysical characteristics of the Tibetan Plateau such as strong heterogeneity in V p , V s and attenuation, shear wave splitting and electric conductivity may be explained by the presence of strongly deformed serpentinites in lithospheric shear zones reactivated along former suture zones between amalgamated blocks, hydrated zones of subducting lithospheric mantle, and the crust-mantle boundary if the temperature is below 700 °C in the region of interest.

Description: [1]  We explore the concept of maximum possible earthquake magnitude, M , in a region represented by an earthquake catalog from the viewpoint of statistical testing. For this aim, we assume that earthquake magnitudes are independent events that follow a doubly-truncated Gutenberg-Richter distribution and focus on the upper truncation M . In earlier work, it has been shown that the value of M cannot be well constrained from earthquake catalogs alone. However, for two hypothesized values M and M ′, alternative statistical tests may address the question: Which value is more consistent with the data? In other words: Is it possible to reject a magnitude within reasonable errors, i.e. the error of the first and the error of the second kind? The results for realistic settings indicate that either the error of the first kind or the error of the second kind is intolerably large. We conclude that it is essentially impossible to infer M in terms of alternative testing with sufficient confidence from an earthquake catalog alone, even in regions like Japan with excellent data availability. These findings are also valid for frequency-magnitude distributions with different tail behavior, e.g. exponential tapering. Finally, we emphasize that different data may only be useful to provide additional constraints for M , if they do not correlate with the earthquake catalog, i.e. if they have not been recorded in the same observational period. In particular, longterm geological assessments might be suitable to reduce the errors, while GPS measurements provide overall the same information as the catalogs.

Description: [1]  We simulated the ascent of bubbly magma in a volcanic conduit by slow decompression experiments using syrup foam as a magma analogue. During decompression, some large voids appear in the foam. The expansion of one void deep in the foam leads to another void expansion, and the void expansion then propagates upward. The void expansion finally reaches the surface of the foam to originate outgassing. The velocity of the upward propagation of void expansions is essentially the same as the rupturing velocity of the bubble film, suggesting that the rupture of films separating each void propagates upward to create the pathway for outgassing. The calculated apparent permeability of decompressed foam can become higher than that measured for natural pumices/scoriae. The upward propagation of film ruptures thus allows for efficient outgassing. This may also appear as the mechanism for energetic gas emissions originating at a depth, such as Strombolian eruptions.

Description: [1]  Geodetic surveys now provide detailed time series maps of anthropogenic land subsidence and uplift due to injection and withdrawal of pore fluids from the subsurface. A coupled poroelastic model allows the integration of geodetic and hydraulic data in a joint inversion and has therefore the potential to improve the characterization of the subsurface and our ability to monitor pore pressure evolution. We formulate a Bayesian inverse problem to infer the lateral permeability variation in an aquifer from geodetic and hydraulic data, and from prior information. We compute the maximum a posteriori (MAP) estimate of the posterior permeability distribution, as well as a Gaussian approximation of the posterior. Computing the MAP estimate requires the solution of a large-scale minimization problem subject to the poroelastic equations, for which we propose an efficient Newton-conjugate gradient optimization algorithm. The covariance matrix of the Gaussian approximation of the posterior is given by the inverse Hessian of the log posterior, which we construct by exploiting low rank properties of the data misfit Hessian. First and second derivatives are computed using adjoints of the time dependent poroelastic equations, allowing us to fully exploit transient data. Using three increasingly complex model problems, we find the following general properties of poroelastic inversions: Augmenting standard hydraulic well data by surface deformation data improves the aquifer characterization. Surface deformation contributes the most in shallow aquifers, but provides useful information even for the characterization of aquifers down to 1 km. In general, it is more difficult to infer high permeability regions, and their characterization requires frequent measurement to resolve the associated short response time scales. In horizontal aquifers, the vertical component of the surface deformation provides a smoothed image of the pressure distribution in the aquifer. Provided that the mechanical properties are known, coupled poroelastic inversion is therefore a promising approach to detect flow barriers and to monitor pore pressure evolution.

Description: [1]  Numerical simulation experiments give insight into the evolving energy partitioning during high strain torsion experiments of calcite. Our numerical experiments are designed to derive a generic macroscopic grain size sensitive flow law capable of describing the full evolution from the transient regime to steady state. The transient regime is crucial for understanding the importance of microstructural processes that may lead to strain localization phenomena in deforming materials. This is particularly important in geological and geodynamic applications where the phenomenon of strain localization happens outside the time frame that can be observed under controlled laboratory conditions. Our method is based on an extension of the paleowattmeter approach to the transient regime. We add an empirical hardening law using the Ramberg-Osgood approximation and assess the experiments by an evolution test function of stored over dissipated energy (lambda factor). Parameter studies of, strain hardening, dislocation creep parameter, strain rates, temperature and lambda factor as well as mesh sensitivity are presented to explore the sensitivity of the newly derived transient/steady state flow law on key quantities. Our analysis can be seen as one of the first steps in a hybrid computational-laboratory-field modeling workflow. The analysis could be improved through independent verifications by thermographic analysis in physical laboratory experiments to independently assess lambda factor evolution under laboratory conditions.

Description: [1]  Despite its importance, the temporal and spatial evolution of continental dynamic topography is poorly known. Australia's isolation from active plate boundaries and its rapid northward motion within a hotspot reference frame make it a useful place to investigate the interplay between mantle convection, topography and drainage. Offshore, dynamic topography is relatively well constrained and can be accounted for by Australia's translation over the mantle's convective circulation. To build a databaseof onshore constraints, we have analyzed an inventory of longitudinal river profiles, which is sensitive to uplift rate history. Using independently constrained erosional parameters, we determine uplift rates by minimizing the misfit between observed and calculated river profiles. Resultant fits are excellent and calculated uplift histories match independent geologic constraints. We infer that western and central Australia underwent regional uplift during the last 50 Myr and that theEastern Highlands have been uplifted in two stages. The first stage from 120–80 Ma, coincided with rifting along the eastern margin and its existence is supported by thermochronological measurements. A second stage occurred at 80–10 Ma, formed the Great Escarpment, and coincided with Cenozoic volcanism. The relationship between topography, gravity anomalies, and shear wave tomographic models suggest that regional elevation is supported by temperature anomalies within the lithosphere's thermal boundary layer. Morphology and stratigraphy of the Eastern Highlands imply that these anomalies have been coupled to the base of the plate during Australia's northward motion over the last 70 Myr.

Description: We present an efficient numerical method for earthquake cycle simulations that employs a finite difference discretization of the off-fault material to accommodate spatially variable elastic properties. The method is developed for the two-dimensional antiplane shear problem of a vertical strike-slip fault with rate-and-state friction. We compare earthquake cycles in a homogeneous half-space with those in which the upper portion of the fault cuts through a sedimentary basin. In both cases, we assume velocity-weakening behavior over the full seismogenic depth, even in the basin, to isolate the influence of elastic heterogeneity. In a homogeneous half-space, events rupturing the full seismogenic depth occur periodically. Event sequences are more complex in basin models, with one or several sub-basin events confined to the lower section of the fault followed by a much larger, surface-rupturing event that breaks through the basin. This phenomenology emerges only for sufficiently compliant and deepbasins. Predicted surface velocities are essentially identical before sub-basin events and surface-rupturing events, suggesting that geodetic observations would not be useful in predicting the rupture mode. The alternating sequence of sub-basin andsurface-rupturing events would complicate interpretation of paleoseismic data. Our results also offer one potential explanation for the shallow slip deficit that has been observed in many recent earthquakes, namely, that these events, which lack appreciable surface slip, are simply one style of rupture. Subsequent events on these faults might be larger, with slip extending all the way to the surface. The 1940 M w 7.0 and 1979 M w 6.5 Imperial Valley events might be considered as examples of these two rupture styles.

Description: The 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼ 100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr -1 , respectively. This moderate slip rate of 1.8 ± 0.7 mm yr -1 , averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M w  ∼ 7) earthquakes during the last 17.4 ± 1.3 ka and of two older earthquakes, one before ∼ 23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼ 7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data.

Description: Yellowstone National Park (YNP) displays numerous and extensive hydrothermal features. Although hydrothermal alteration in YNP has been extensively studied, the volume, geometry, and type of rock alteration at depth remain poorly constrained. In this study, we use high-resolution airborne and ground magnetic surveys and measurements of remanent and induced magnetization of field and drill-hole samples to provide constraints on the geometry of hydrothermal alteration within the subsurface of three thermal areas in YNP (Firehole River, Smoke Jumper Hot Springs, and Norris Geyser Basin). We observe that hydrothermal zones from both liquid- and vapor-dominated systems coincide with magnetic lows observed in aeromagnetic surveys and with a decrease of the amplitude of short-wavelength anomalies seen in ground magnetic surveys. This suggests strong demagnetization of both the shallow and deep substratum within these areas associated with the removal or transformation of magnetic minerals by hydrothermal alteration processes. Such demagnetization is confirmed by measurements of rock samples from hydrothermal areas which display significantly decreased total magnetization. Measurements of rock samples from drill holes also suggest that the degree of demagnetization and therefore hydrothermal alteration of the substratum varies dramatically over short distances both vertically and horizontally. A pronounced negative anomaly is observed over the Lone Star Geyser and suggests a significant demagnetization of the substratum associated with areas displaying large-scale fluid flow. We use the damped short-wavelength signal over ground magnetic profiles to map hydrothermal alteration in the shallow substratum. Maximum values of the magnetic anomaly gradient measured along ground magnetic profiles are also used to estimate the thickness of a non-magnetic layer that is assumed to cover the volcanic unaltered substratum. Finally, the magnetic lows observed over ground and airborne magnetic surveys are used to invert the distribution of magnetization using two types of simplifying assumptions. First, the substratum magnetization is inverted assuming it is constant in the vertical direction and second, the topography of the base of a superficial non-magnetic layer is inverted assuming the underlying substratum has a constant magnetization. Both the magnetic gradient analysis and the topography inversion show that significant demagnetization occurs over a thickness of at least a few hundred meters in hydrothermal areas at YNP. We also show that the maximum degree or maximum thickness of demagnetization correlates closely with the location of hydrothermal activity and mapped alteration. Our results suggest that areas of hydrothermal alteration are composed of substrata with different degrees of alteration, with significant alteration occurring within narrow zones that may represent major conduits for hydrothermal fluid flow.

Description: The inverse problem of determining the flow at the Earth's Core Mantle Boundary according to an outer core magnetic field and secular variation model has been investigated through a Bayesian formalism. To circumvent the issue arising from the truncated nature of the available fields, we combined two modeling methods. In the first step, we applied a filter on the magnetic field to isolate its large scales by reducing the energy contained in its small scales, we then derived the dynamical equation, referred as filtered frozen flux equation, describing the spatio-temporal evolution of the filtered part of the field. In the second step, we proposed a statistical parametrization of the filtered magnetic field in order to account for both its remaining unresolved scales and its large scale uncertainties. These two modeling techniques were then included in the Bayesian formulation of the inverse problem. To explore the complex posterior distribution of the velocity field resulting from this development, we numerically implemented an algorithm based on Markov Chain Monte Carlo methods. After evaluating our approach on synthetic data and comparing it to previously introduced methods, we applied it to a magnetic field model derived from satellite data for the single epoch 2005.0. We could confirm the existence of specific features already observed in previous studies. In particular we retrieved the planetary-scale eccentric gyre characteristic of flow evaluated under the compressible quasi-geostrophy assumption although this hypothesis was not considered in our study. In addition, through the sampling of the velocity field posterior distribution, we could evaluate the reliability, at any spatial location and at any scale, of the flow we calculated. The flow uncertainties we determined are nevertheless conditioned by the choice of the prior constraints we applied to the velocity field.

Description: The Longitudinal Valley Fault (LVF) in Eastern Taiwan is a high slip rate fault (about 5 cm/yr), which exhibits both seismic and aseismic slip. Deformation of anthropogenic features shows that aseismic creep accounts for a significant fraction of faultslip near the surface, whereas a fraction of the slip is also seismic, since this fault has produced large earthquakes with five M w  〉 6.8 events in 1951 and 2003. In this study, we analyze a dense set of geodetic and seismological data around the LVF, including campaign-mode Global Positionnig System(GPS) measurements, time series of daily solutions for continuous GPS stations (cGPS), leveling data, and accelerometric records of the 2003 Chenkung earthquake. To enhance the spatial resolutionprovided by these data, we complement them with Interferometric Synthetic Aperture Radar (InSAR) measurements produced from a series of Advanced Land Observing Satellite (ALOS) images processed using a persistent scatterer (PS) technique. The combined dataset covers the entire LVF and spans the period from 1992 to 2010. We invert this data to infer the temporal evolution of fault slip at depth using the Principal Component Analysis-based Inversion Method (PCAIM). This technique allows the joint inversion of diverse data, taking the advantage of the spatial resolution given by the InSAR measurements and the temporal resolution afforded by the cGPS data. We find that (1) seismic slip during the 2003 Chengkung earthquake occurred on a fault patch whichhad remained partially locked in the interseismic period; (2) the seismic rupture propagated partially into a zone of shallow aseismic interseismic creep but failed to reach the surface; (3) that aseismic afterslip occurred around the area that ruptured seismically. We find consistency between geodetic and seismological constraints on the partitioning between seismic and aseismic creep. About 80-90% of slip on the LVF in the 0-26 km, seismogenic depth range is actually aseismic. We infer that the clay-rich Lichi Mélange is the key factor promoting aseismic creep at shallow depth.

Description: We performed numerical calculations of compaction in aggregates of spherical grains, using Lehner and Leroy's (2004) constitutive model of pressure solution at grain contacts (LL). That model is founded on a local definition of the thermodynamic driving force and leads to a fully coupled formulation of elastic deformation, dissolution, and diffusive transport along the grain boundaries. The initial geometry of the aggregate was generated by random packing of spheres with a small standard deviation of the diameters. During the simulations, isostatic loading was applied. The elastic displacements at the contacts were calculated according to Digby's (1981) non-linear contact force model and deformation by dissolution was evaluated using the LL formulation. The aggregate strain and porosity were tracked as a function of time for fixed temperature, applied effective pressure, and grain size. We also monitored values of the average and standard deviation of total load at each contact, the coordination number for packing, and the statistics of the contact dimensions. Because the simulations explicitly exclude processes such as fracturing, plastic flow, and transport owing to surface curvature, they can be used to test the influence of relative changes in the kinetics of dissolution and diffusion processes caused by contact growth and packing re-arrangements. We found that the simulated strain data could be empirically fitted by two successive power laws of the form, ε x  ∝  t ξ , where ξ was equal to 1 at very early times, but dropped to as low as 0.3 at longer times. The apparent sensitivity of strain rate to stress found in the simulations was much lower than predicted from constitutive laws that assume a single dominant process driven by average macroscopic loads. Likewise, the apparent activation enthalpy obtained from the simulated data was intermediate between that assumed for dissolution and diffusion, and, further, tended to decrease with time. These results are similar to the experimental observations of Visser et al.’s (2012), who used an aggregate geometry and physical conditions closely resembling the present numerical simulations.

Description: To understand the energy release process that operates at the end of the mainshock rupture and start of the aftershock activity, we propose an inversion method that uses continuous high-frequency seismogram envelopes of the mainshock and early aftershocks (i.e., events that occur at short times after the mainshock). In our approach, the aftershock sequence is regarded as a continuous energy release process, rather than a discrete time series of events. To correct for the contribution of coda wave energy excited by multiple scattering, we use the theoretical envelope synthesized on the basis of the radiative transfer theory as a Green's function. The site amplification factors are corrected considering the conservation of energy flux and using the coda normalization method. The inverted temporal energy release rate for the 2008 M W 6.9 Iwate-Miyagi Nairiku earthquake, Japan, decays following t -1.1 , at the lapse time t of 40-900 s after the mainshock origin time. This exponent of the decay rate is similar to the p -value of the modified Omori law. The amount of estimated energy release is consistent with that calculated from the magnitude listed in the aftershock catalog. Although the uncertainty is large, the location of large energy release at the lapse times of 40-900 s approximately overlaps to that of the aftershocks, which surrounds the large energy release area during the mainshock faulting. The maxima of the energy release rate normalized by the average decay rate distributes following a power-law, similar to the Gutenberg-Richter law.

Description: Determining the melt distribution in oceanic crust at mid-ocean ridges is critical to understanding how magma is transported and emplaced in the crust. Seafloor compliance —deformation under ocean wave forcing — is primarily sensitive to regions of low shear velocity in the crust, making it a useful tool to probe melt distribution. Analysis of compliance data collected at East Pacific Rise between 9°-10ºN through 3D numerical modeling reveals strong along-axis variations in the lower crustal shear velocities, as well as temporal variation of crustal shear velocity near 9°48’N between measurements spanning 8 years. Compliance measured across the rise axis at 9°48’N and 9°33’N suggest a deep crustal low velocity zone (DLZ) beneath the ridge axis, with a low Vs/Vp ratio consistent with melt in low aspect ratio cracks or sills. Changes in compliance measured at 9°48’N between years 1999 and 2007 suggest that the melt fraction in the axial crust decreased during this interval, perhaps following the 2005-2006 seafloor eruption. This temporal variability provides direct evidence for short-term variations of the magmatic system at a fast-spreading ridge.

Description: About 150 km of high-resolution, seismic-reflection (CHIRP) profiles (ca. 20 m penetration) were collected in Mono Lake in order to define the uppermost sedimentary architecture of the basin, which has been heavily impacted by recent volcanic, tectonic, and climatic processes. The study also provides important background for ongoing efforts to obtain paleoenvironmental records from sediment cores in the lake. The history of four seismic-stratigraphic units in the upper 20 m of section are inferred from the data, and the interpretations are generally consistent with previous interpretations of lake history for the past 2000 years, including a major lowstand at 1941 m. No shorelines below the previously documented major lowstand at 1941 m were found. A relatively steep slope segment, whose toe is at about 1918 m, and which occurs on the southern and western margins of the deep basin of the lake, is interpreted as the relict foreset slope of deposition from prograding western tributaries. This topography is unconformably overlain by a unit of interbedded tephra and lake sediments of variable lithology, which contains tephra of the North Mono (600–625 cal yr BP) eruption in its upper part. The tephra-rich unit is overlain by a mostly massive mudflow deposit that is locally more than 18 m thick and that is distributed in a radial pattern around Paoha Island. The evidence suggests that, within the past few hundred years, rapid uplift of Paoha Island through thick, pre-existing lake deposits led to widespread slope failures, which created a terrain of disrupted, intact blocks near the island, and a thick, fluid mudflow beyond. As is common in mudflows, the mudflow moved up the depositional slope of the lake floor, terminating against the pre-existing slopes, likely in multiple surges. Since about 1700 CE, fine-grained, well-laminated sediments have accumulated in the deep parts of the lake at anomalously rapid rates, probably driven by continued rapid, small-scale shedding of sediment from Paoha Island.

Description: Surface measurements of shear wave splitting patterns are widely used to infer the mantle circulations around subducting slabs; however, the relation between mantle flow and seismic anisotropy is still elusive. Finite strain is a direct measurement of time-dependent deformation, and has been proposed as a proxy for the crystal preferred orientation (CPO) of mantle minerals. We have conducted a series of numerical models to systematically investigate the mantle flow, finite strain, olivine CPO and SKS wave splitting in oceanic subduction zones with variable slab width. They demonstrate that the preferred orientations of olivine a -axes generally agree with the long (extensional) axes of the finite strain ellipsoid (FSE), even in these very complex mantle flow fields; however, neither the a -axis nor the FSE axes necessarily aligns with the instantaneous mantle velocity vector. We identify two domains with distinct deformation mechanisms in the central sub-plate mantle, where simple shear induced by plate advance dominates at shallow depths and produces trench-normal fast splitting, while pure shear induced by slab rollback dominates the deeper mantle and results in trench-parallel fast splitting. The SKS splitting patterns are thus dependent on the competing effects of these two mechanisms, and also on the subduction partition ratio γ  =  X p /X t : trench-parallel when γ  〈 1, and trench-normal when γ  〉 1. In addition, variable mantle deformation mechanisms and SKS splitting patterns are observed in the mantle wedge and around the slab edges, which may aid in the general interpretation of seismic anisotropy observations in natural subduction zones.

Description: Gravity changes occurring during the initial stage of the 2011–2012 El Hierro submarine eruption are interpreted in terms of the pre-eruptive signatures during the episode of unrest. Continuous gravity measurements were made at two sites on the island using the relative spring gravimeter LCR gPhone-054. On September 15, 2011, an observed gravity decrease of 45 μGal, associated with the southward migration of seismic epicenters, is consistent with a lateral magma migration occurred beneath the volcanic edifice, an apparently clear precursor of the eruption that took place 25 days later on October 10, 2011. High-frequency gravity signals also appeared on October 6–11, 2011, point to an interaction between a magmatic intrusion and the ocean floor was occurring. These important gravity changes, with amplitudes varying from 10 to −90 μGal, during the first three days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure. An apparent correlation of gravity variations with body tide vertical strain was also noted, which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption.

Description: In this paper we study the behavior of a fluid-saturated fault under shear, based on the assumption that the material inside exhibits rate- and temperature-dependent frictional behavior. A creeping fault of this type can produce excess heat due to shearheating, reaching temperatures which are high enough to trigger endothermic chemical reactions. We focus on fluid-release reactions and incorporate excess pore pressure generation and porosity variations due to the chemical effects (a process called chemical pressurization). We provide the mathematical formulation for coupled thermo-hydro-chemo-mechanical processes and study the influence of the frictional, hydraulic and chemical properties of the material, along with the boundary conditions of the problem on the behavior of the fault. Regimes of stable-frictional sliding and pressurization emerge, and the conditions for the appearance of periodic creep-to-pressurization instabilities are then derived. The model thus extends the classical mechanical stick- slip instabilities by identifying chemical pressurization as the process governing the slip phase. The different stability regimes identified match the geological observations about subduction zones. The model presented was specifically tested in the Episodic Tremor and Slip sequence of the Cascadia megathrust, reproducing the displacement data available from the GPS network installed. Through this process we identify that the slow slip events in Cascadia could be due to the in-situ dehydration of serpentinite minerals. During this process the fluid pressures increase to sub-lithostatic values and lead to the weakening of the creeping slab.

Description: This study investigates the influence of the substrate surface roughness on the emplacement mechanisms of pyroclastic flows. We carried out laboratory experiments on gravitational flows generated from the release of initially fluidized or non-fluidized columns of fine particles (diameter d  = 0.08 mm) in a horizontal channel. The roughness of the channel base was uniform in each experiment, created by gluing particles of diameter d 0  = 0.08 to 6 mm to the base. Other things being equal, the flow runout distance increased with the channel base roughness ( d 0 ) to a maximum of about twice that of flows on a smooth substrate when d 0  = 1.5-3 mm, before decreasing moderately at higher roughness values of d 0  = 6 mm. Long runout originated mainly during the late stages of emplacement as flow deceleration was very much reduced at high substrate roughness. This was caused by (partial) autofluidization due to an upward flux of air escaping from the substrate interstices in which flow particles settled. Autofluidization was evidenced by high pore fluid pressure measurements at the base of initially non-fluidized flows, and also by reduced flow runout when the interstices were initially partially filled so that less air was available. Furthermore, the runout distance of flows of large particles ( d  = 0.35 mm), which could not be fluidized by the ascending air flux, was independent of the substrate roughness. This study suggests that autofluidization caused by air escape from the interstices of a rough substrate is one important mechanism to explain the common long runout distance of pyroclastic flows even on sub-horizontal topographies.

Description: Microstructures in serpentine samples recovered from deformation experiments performed at high pressure (1-8 GPa), high temperature (150 – 500 °C), and laboratory strain rates (4 10 -4 – 10 -6  s -1 ) were studied using transmission electron microscopy (TEM) on thin sections prepared by focused ion beam (FIB). Lizardite crystals deform by easy glide along the basal planes associated with microkink. This mechanism is associated with a plastic strength of ~100 MPa that defines the upper bound of lizardite strength in natural conditions. Antigorite crystals deform essentially by conjugated slip along (101) and planes observed in sections close to (010). This conjugate system results in an apparent global slip system akin to [100](001). In all samples, delamination and comminution produce fine-grained interconnected regions at grain boundaries because intracrystalline deformation mechanisms are insufficient to satisfy the von Mises criterion. The deformation laws of lizardite (plastic flow) and antigorite (strain rate-dependent) differ because of their differing intracrystalline deformation mechanisms. [100](001) crystal-preferred orientations (CPO) and subgrain-boundary geometry in natural samples indicate the activation of slip systems similar to those observed in experimentally deformed samples, suggesting that the strain-rate dependent rheology of antigorite derived from experiments applies to subduction zone conditions. Delamination of antigorite crystals of a few tens to hundreds of nanometers is not observed in natural samples from subduction zones, suggesting plastic deformation of serpentinites at natural low strain-rates and stresses is likely accompanied by recrystallization through dissolution-precipitation processes that act as a low-temperature equivalent of dynamic recrystallization through diffusion at high temperature.

Description: Faults that cross cut subsurface CO 2 storage systems offer potential leakage pathways, especially if fault reactivation and dilation occur. After reactivation, however, newly formed fault gouge is expected to gradually compact and seal as a function of time. To estimate the time scale on which this occurs, the processes that control compaction must be understood. We performed uniaxial compaction experiments on simulated anhydrite fault gouge to investigate the deformation mechanisms that operate under post-slip conditions in faulted anhydrite caprocks. This involved constant stress (5-12 MPa) and stress stepping experiments (5/7.5/10 MPa) performed at 80 °C, under dry and wet conditions, on fault gouge samples prepared from crushed natural anhydrite sieved into different grain size fractions in the range 20-500 µm. Dry samples showed little to no compaction creep, whereas wet samples (i.e. flooded with pre-saturated CaSO 4 solution) showed rapid compaction. Our mechanical data and microstructural observations on wet samples suggest that, for fine grain sizes (〈50 µm) and low stresses, gouge compaction is controlled by diffusion-controlled pressure solution. With increasing grain size and stress, fluid-assisted subcritical microcracking becomes dominant. Pressurizing solution-flooded samples with CO 2 (15 MPa) led to no significant effect on compaction rates in fine-grained material, but it decreased compaction rates in coarse samples. Since fine grain sizes are expected in reactivated faults, we infer that pressure solution will dominate in anhydrite (cap)rocks, with extrapolation of our lab data to reservoir conditions suggesting sealing timescales of a few decades.

Description: Sequential permeability and chemical osmosis experiments on Wakkanai mudstones were performed to explore the relationships between the semipermeability of clayey rocks and the hydraulic and diffusion parameters as well as the pore-structure characteristics. The wide ranges in osmotic efficiency (0.0004-0.046) and intrinsic permeability (8.92 × 10 -20 -1.24 × 10 -17  m 2 ) reflect the variation in the pore-size distributions of the Wakkanai mudstones. A regression analysis between osmotic efficiency and permeability shows that the osmotic efficiency is proportional to the inverse of permeability, suggesting that the permeability is indeed indicative of the degree of semipermeability. Osmotic efficiency was determined invariant with the effective diffusion coefficient for the Wakkanai mudstones (3.59-8.36 × 10 -11  m 2 /s) due to their small osmotic efficiencies ( 〈 0.046). The wide variation in osmotic efficiencies and pore-structure characteristics of Wakkanai mudstones indicates that the nano-scale pores enable semipermeability in Wakkanai mudstones. However, the pressure evolution caused by chemical osmosis is limited by the connected wide pores that are the main conduits for water, thus dissipating the osmotic pressure build-up induced by the semipermeability of nano-scale pores.

Description: A new state evolution law has recently been proposed by Nagata et al. [2012] that includes a dependence upon stressing rate through a laboratory derived proportionality constant c. It has been claimed that this law, while retaining the time dependent healing of the Dieterich (or Aging) law, can also match the symmetric response of the Ruina (or Slip) law to velocity step tests. We show through analytical approximations and numerical results that the new law transitions between the responses of the traditional Aging and Slip laws in velocity step up/down experiments when the value of c is tuned properly. Particularly, for c =0 the response is pure Aging while for finite, non-zero c one observes Slip law like behaviour for small velocity jumps but Aging law like response for larger jumps. The magnitude of the velocity jump required to see this transition between Aging and Slip behaviour increases as c increases. In the limit of c ≫1 the response becomes purely Slip law like for all geologically plausible velocity jumps. We also present results from detailed analytical and numerical studies of the mechanism of rupture nucleation on 1D faults under this new state evolution law to demonstrate that the style of nucleation can also be made to switch from Aging-type (expanding cracks) to Slip-type (slip pulses) by adjusting the value of c as indicated by the velocity step results.

Description: [1]  In this paper, 10 years of time-variable gravity data from the Gravity Recovery and Climate Experiment (GRACE) Release 05 have been used to evaluate the glacier melting rate in High Mountain Asia (HMA) using a new computing scheme, i.e., the Space Domain Inverse (SADI) method. We find that in HMA area there are three different kinds of signal sources that should be treated together. The two generally accepted sources, glacier melting and India underground water depletion, are estimated to change at the rate of -35.0 ± 5.8 Gt/yr (0.09 mm/yr sea level rising) and -30.6 ± 5.0 Gt/yr, respectively. The third source is the remarkable positive signal (+30 Gt/yr) in the inner Tibet Plateau, which is challenging to explain. Further, we have found that there is a five-year undulation in Pamir and Karakoram, which can explain the controversies of the previous studies on the glacier melting rate here. This five-year signal can be explained by the influence of Arctic Oscillation and El Niño-Southern Oscillation.

Description: [1]  The concentration and distribution of volatiles in the Earth's mantle influences properties such as melting temperature, conductivity, and viscosity. To constrain upper mantle water content, concentrations of H 2 O, P, and F were measured in olivine, orthopyroxene and clinopyroxene by secondary ion mass spectrometry. Analyzed peridotites are xenoliths (Pali Aike, Spitsbergen, Samoa), orogenic peridotites (Josephine Peridotite), and abyssal peridotites (Gakkel Ridge, Southwest Indian Ridge, Tonga Trench). The comparison of fresh and altered peridotites demonstrates that low to moderate levels of alteration do not affect H 2 O concentrations, in agreement with mineral diffusion data. Olivines have diffusively lost water during emplacement, as demonstrated by disequilibrium between olivine and co-existing pyroxenes. In contrast, clinopyroxene and orthopyroxene preserve their high temperature water contents, and their partitioning agrees with published experiments and other xenoliths. Hence, olivine water concentrations can be determined from pyroxene concentrations using mineral-mineral partition coefficients. Clinopyroxenes have 60-670 ppm H 2 O, while orthopyroxenes have 10-300 ppm, which gives calculated olivine concentrations of 8-34 ppm. The highest olivine water concentration translates to an effective viscosity of 6 × 10 19  Pa s at 1250 °C and ~15 km depth, compared to a dry effective viscosity of 2.5 × 10 21  Pa s. Bulk rock water concentrations, calculated using mineral modes, are 20-220 ppm and correlate with peridotite indices of melt depletion. However, trace element melt modeling indicates that peridotites have too much water relative to their rare earth element concentrations, which may be explained by late-stage melt addition, during which only hydrogen diffuses fast enough for re-equilibration.

Description: [1]  To clarify the formation process of the back-arc basin in the Japan Sea, which is located next to the northwestern Pacific, a seismic survey using ocean bottom seismographs (OBSs) and an air-gun array was undertaken in areas from the northern Yamato Basin to the coast of the northeastern Japan Island Arc off Awa-shima Island. The crust beneath the northern Yamato Basin off Awa-shima Island is approximately 16 km thick. The upper and lower crusts are, respectively, about 5 km thick with a steep velocity gradient and about 10 km thick with a gentle velocity gradient. In the basin, there are very few units with P-wave velocity of 5.4–6.0 km/s, corresponding to the continental upper crust. The crustal structure of the northern Yamato Basin has characteristics of thicker oceanic crust. The high-velocity lowermost crust in the northern Yamato Basin with 7.2–7.4 km/s might show melt formed by a slightly high mantle temperature during the opening of the basin. However, the crust beneath the areas from the Sado Ridge to the coast, which is approximately 25–26 km thick, is slightly thinner than that of the continental crust and island arc crust. The crustal structure beneath this area is inferred to be a rifted continental and/or a rifted island arc crust.

Description: We used the ambient noise cross-correlation and stretching methods to calculate variations in seismic velocities in the region of Volcán de Colima, Mexico. More than 15 years of continuous records were processed, producing long time series of velocity variations related to volcanic activity, meteorological effects, and earthquakes. Velocity variations associated with eruptive activity are tenuous, which probably reflects the open state of the volcano during the study period. Fifteen events among 26 regional tectonic earthquakes produced sharp, temporary decreases in seismic velocities, which then recovered progressively following a linear trend as a function of the logarithm of time. For the 15 events, the amplitude of the perturbation increased almost linearly with the logarithm of the amplitude of the seismic waves that shook the edifice. The most dramatic apparent velocity variation was a drop of up to 2.6% during the nearby M7.4 Tecomán earthquake in 2003. In order to locate the perturbation in the horizontal plane we applied an inverse method based on the radiative transfer approximation. We also used an original approach based on the frequency dependence of velocity variations to estimate the depth of the perturbation. Our results show that the velocity variation was well localized in the shallow layers (〈 800 m) of the volcano, with almost no variations occurring outside the edifice. We discuss several possible interpretations and conclude that the most plausible explanation for the velocity decreases is the nonlinear elastic behavior of the granular volcanic material and its mechanical softening induced by transient strains.

Description: The 150-km long central section of the San Andreas Fault (CSAF) in central California creeps at the surface and has not produced a large earthquake historically. However, sections of the San Andreas fault to the north and south of the SAF are known to have ruptured repeatedly in M ~ 7-8 earthquakes. It is currently unclear whether the creeping CSAF could rupture in large earthquakes, either individually or along with earthquakes on the locked sections to the north and south. We invert Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) data with elastic block models to estimate the degree of locking on the CSAF and place bounds on moment accumulation rate on the fault. We find the inferred moment accumulation rate is highly dependent on the long-term fault slip rate, which is poorly constrained along the CSAF. The inferred moment accumulation rate, normalized by shear modulus, ranges from 3.28x10 4 to 5.85x10 7 m 3 /yr which is equivalent to a M w  = 5.5 – 7.2 earthquake every 150 years for a long-term slip rate of 26 mm/yr and M w  = 7.3–7.65 for a long-term slip rate of 34 mm/yr. Comparisons of slip distributions with microseismicity and repeating earthquakes indicate a possible locked patch between 10 and 20 km depth on the CSAF that could potentially rupture with M w  = 6.5.

Description: The thermodynamics of melting relations in the system Fe-FeO was investigated to the outer core-inner core boundary (ICB) condition from a self-consistent thermodynamic database which was evaluated from the latest static high-pressure ( P ) and -temperature ( T ) experiments. The evaluated database together with an existing non-ideal mixing model for liquids reproduces experimental data on the eutectic composition and temperature to P  = 50 GPa. On the other hand at the outer core pressures (136 to 330 GPa), employing an ideal solution model gives calculated eutectic temperatures of T  = 2990-4330 K, which are also consistent with experimental data. Hence the ideal solution model is applied to calculate the liquid property under outer core conditions, and yields the eutectic compositions of Fe-7.2-9.1 wt% O. From the Gibbs free energy for the Fe-FeO liquids, I calculated the density, sound velocity, and isentropic temperature gradient of a hypothetical oxygen-bearing outer core. Under the outer core conditions, the addition of oxygen reduces the compressional wave velocity of iron liquid, moving it away from seismologically constrained values. An overall O-rich bulk outer core model is thus excluded. Seismological observations however, suggest the presence of a low-velocity layer with a thickness of 60-70 km at the top of the outer core. The origin of such a low-velocity layer can be explained by an enrichment of oxygen which might be a consequence of chemical interactions between the core and mantle.

Description: We investigate faulting within the incoming Pacific plate at the Mariana subduction trench to understand stresses within the bending plate, regional stresses acting upon the plate interface, and the extent of possible faulting-induced mantle serpentinization. We determine accurate depths by inverting teleseismic P and SH waveforms for earthquakes occurring during 1990-2011 with Global Centroid Moment Tensor (GCMT) solutions. For earthquakes with Mw 5.0+, we determine centroid depths and source time functions and refine the fault parameters. Results from Central Mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the Moho. At the Southern Mariana trench, extensional earthquakes continue to 5 km below the Moho. One compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here. In addition, we model the stress distribution within the Pacific plate along two bathymetric profiles extending seaward from the Mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces. Results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the Pacific plate at Southern Mariana is experiencing larger, compressional stresses, possibly due to greater interplate coupling. Additionally, we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle, then the top 5-15 km of the Pacific plate mantle are partially serpentinized, and a higher percentage of serpeninization is located near the Central Mariana trench where extensional events extend deeper.

Description: No abstract is available for this article.

Description: [1]  We present the LITHO1.0 model, which is a 1° tessellated model of the crust and uppermost mantle of the earth, extending into the upper mantle to include the lithospheric lid and underlying asthenosphere. The model is parameterized laterally by tessellated nodes and vertically as a series of geophysically identified layers, such as water, ice, sediments, crystalline crust, lithospheric lid, and asthenosphere. LITHO1.0 is created by constructing an appropriate starting model and perturbing it to fit high-resolution surface wave dispersion maps (Love, Rayleigh, group, phase) over a wide frequency band (5-40 mHz). We examine and discuss the model with respect to key lithospheric parameters, such as average crustal velocity, crustal thickness, upper mantle velocity, and lithospheric thickness. We then compare the constructed model to those from a number of select studies at regional and global scales and find general consistency. It appears that LITHO1.0 represents a reasonable starting model of the earth's shallow structure (crust and uppermost mantle) for the purposes in which these models are used, such as travel time tomography or in efforts to create a 3D reference earth model. The model matches surface wave dispersion over a frequency band wider than the band used in the inversion. There are several avenues for improving the model in the future by including attenuation and anisotropy, as well as making use of surface waves at higher frequency.

Description: [1]  Triggering of earthquakes on upper plate faults during and shortly after recent great ( M 〉8.0) subduction thrust earthquakes raises concerns about earthquake triggering following Cascadia subduction zone earthquakes. Of particular regard to Cascadia was the previously noted, but only qualitatively identified, clustering of M 〉~6.5 crustal earthquakes in the Puget Sound region between about 1200–900 cal yr B.P. and the possibility that this was triggered by a great Cascadia thrust subduction thrust earthquake, and therefore portends future such clusters. We confirm quantitatively the extraordinary nature of the Puget Sound region crustal earthquake clustering between 1200–900 cal yr B.P., at least over the last 16,000. We conclude that this cluster was not triggered by the penultimate, and possibly full-margin, great Cascadia subduction thrust earthquake. However, we also show that the paleoseismic record for Cascadia is consistent with conclusions of our companion study of the global modern record outside Cascadia, that M 〉8.6 subduction thrust events have a high probability of triggering at least one or more M 〉~6.5 crustal earthquakes.

Description: [1]  The west Iberia margin has become the type example of a magma-starved margin in which extension during breakup led to the exhumation of mantle rocks apparently over a broad area. Much of our understanding of this process is based on geophysical and drilling data from the southern Iberia Abyssal Plain, where the seaward part of the region of exhumed mantle is comprised of a series of margin-parallel basement ridges. Here we analyse wide-angle and normal incidence seismic data from a series of intersecting profiles across this peridotite ridge province and develop a three-dimensional picture of its structure. Using these seismic data, we infer a location and age (magnetic anomaly M1; 125-127 Ma) of first-formed oceanic crust in this region that are consistent with Ocean Drilling Program data and data from the conjugate margin. We infer that the peridotite ridge province is about 70 km wide with strong serpentinization (〉75%) at the top of basement reducing to much lower degrees (〈 25%) around 2 km below. We map the geometry of some prominent reflectors on the landward margin of the peridotite ridge province that likely represent detachment faults and show that one of these has a domal structure similar to that of detachment faults at slow- to ultra-slow-spreading mid-ocean ridges.

Description: [1]  Understanding and forecasting earthquake occurrences is presumably linked to understanding the stress distribution in the earth's crust. This cannot be measured instrumentally with useful coverage. However, the size-distribution of earthquakes, quantified by the Gutenberg-Richter b -value, is possibly a proxy to differential stress conditions and could therewith act as a crude stress-meter wherever seismicity is observed. In this study, we improve the methodology of b -value imaging for application to a high-resolution 3-d analysis of a complex fault network. In particular, we develop a distance dependent sampling algorithm and introduce a linearity measure to restrict our output to those regions where the magnitude distribution strictly follows a power-law. We assess the catalog completeness along the fault traces using the Bayesian Magnitude of Completeness method and systematically image  b -values for 243 major fault segments in California. We identify and report b -value structures, revisiting previously published features, e.g. the Parkfield asperity, and documenting additional anomalies, e.g. along the San Andreas and Northridge faults. Combining local b -values withlocal earthquake productivity rates, we derive probability maps for the annual potential of one or more M6+ events as indicated by the microseismicity of the last three decades. We present a physical concept of how different stressing conditions along a fault surface may lead to b -value variation and explain non-linear frequency magnitude distributions. Detailed spatial b -value information and its physical interpretation can advance our understanding of earthquake occurrence and ideally lead to improved forecasting ability.

Description: [1]  Episodic GPS measurements are used to quantify the present-day velocity field in the northwestern Himalaya from the southern Pamir to the Himalayan foreland. We report large postseismic displacements following the 2005 Kashmir earthquake and several mm/year thrusting of the central segment of the Salt Ranges and Potwar plateau over the foreland, westward thrusting of Nanga Parbat above the Kohistan plateau, and ~ 12 mm/yr SSE velocities of the Karakorum Ranges and of the Deosai and Kohistan plateaus relative to the Indian plate. Numerical simulations allow to determine a first approximation of slip along active faults : (1) substantial creep of ~87 mm/year between 2006 and 2012 along the flat northeast of the Balakot-Bagh thrust affected by the 2005 earthquake; (2) ~ 5 mm/year slip of the central segment of the Salt Ranges and Potwar plateau whereas their western boundaries are clearly inactive over the time span covered by our measurements; (3) 13 mm/year ductile slip along the MHT modeled by a dislocation dipping 7° northward, locked at a depth of 15 km; and (4) ~20 mm/year slip along the shear zone forming the western boundary of Nanga Parbat, between depths of 1.6 and 6.5 km. Residuals velocities suggest the existence of left-lateral strike-slip along the Jhelum fault.

Description: [1]  Understanding the long-term earthquake recurrence pattern at subduction zones requires continuous paleoseismic records with excellent temporal and spatial resolution and stable threshold conditions. South-Central Chilean lakes are typically characterized by laminated sediments providing a quasi-annual resolution. Our sedimentary data show that lacustrine turbidite sequences accurately reflect the historical record of large interplate earthquakes (a.o. the 2010 and 1960 events). Furthermore, we found that a turbidite's spatial extent and thickness is a function of the local seismic intensity, and can be used for reconstructing paleo-intensities. Consequently, our multi-lake turbidite record aids in pinpointing magnitudes, rupture locations and extent of past subduction earthquakes in South-Central Chile. Comparison of the lacustrine turbidite records with historical reports, a paleotsunami/subsidence record and a marine megaturbidite record demonstrates that the Valdivia Segment is characterized by a variable rupture mode over the last 900 years including i) full ruptures (M w ~9.5: 1960, 1575, 1319 ±9, 1127 ±44), ii) ruptures covering half of the Valdivia Segment (M w ~9: 1837), and iii) partial ruptures of much smaller coseismic slip and extent (M w ~7.5-8: 1737, 1466 ±4). Also, distant or smaller local earthquakes can leave a specific sedimentary imprint which may resolve subtle differences in seismic intensity values. For instance, the 2010 event at the Maule Segment produced higher seismic intensities towards southeastern localities compared to previous megathrust ruptures of similar size and extent near Concepciόn.

Description: [1]  GPS data before and after the 1999 İzmit/Düzce earthquakes on the North Anatolian Fault Zone (Turkey) reveal a preseismic strain localization within about 25 km of the fault and a rapid postseismic transient. Using 3D finite element calculations of the earthquake cycle in an idealized model of the crust, comprising elastic above Maxwell viscoelastic layers, we show that spatially varying viscosity in the crust can explain these observations. Depth-dependent viscosity without lateral variations can reproduce some of the observations but cannot explain the proximity to the fault of maximum postseismic velocities. Models which include a localized weak zone beneath the faulted elastic lid satisfactorily explain the observations if the following conditions are satisfied: the weak zone extends down to mid-crustal depths, and the ratio of relaxation time to earthquake repeat time ranges from ~ 0.005 (for weak zone width of ~ 24 km) to ~ 0.01 (for weak zone width of ~ 40 km) in the weakened domain and greater than ~ 1.0 elsewhere, respectively corresponding to viscosities of ~ 10 18±0.3  Pa s and greater than ~ 10 20  Pa s. Models with sharp boundaries to the weak zone fit the data better than those with a smooth increase of viscosity away from the fault, which implies that the weak zone may be bounded by a relatively abrupt change in material properties. Such a change might result from lithological contrast, grain-size reduction, fabric development or water content, in addition to any effects from shear heating. Our models also imply that viscosities inferred from postseismic studies primarily reflect the rheology of the weak zone, and should not be used to infer the mechanical properties of normal crust.

Description: [1]  Subglacial eruptions are often associated with rapid penetration of overlying ice and release of large flowrates of water as jökulhlaups. Observations of recent subglacial eruptions indicate rapid syn-eruptive ice melting within liquid-filled subglacial cavities but quantitative descriptions of possible heat transfer processes need to be developed. Calculations of heat flux from the ice cavity fluid to the melting ice surface indicate that up to 0.6 MW m -2 may be obtained for fluids undergoing single phase free convection, similar to minimum estimates of heat flux inferred from observations of recent eruptions. Our model of boiling two-phase free convection in subglacial cavities indicates that much greater heat fluxes, in the range 3-5 MW m -2 , can be obtained in the vent region of the cavity and may be increased further by momentum transfer from the eruption jet. Rapid magma-water heat transfer from fragmented magma is needed to sustain these heat fluxes. Similar heat fluxes are anticipated for forced convection of subcooled cavity water induced by momentum transfer from an eruption jet. These heat fluxes approach those required to explain jökulhlaup flow rates and rapid ice penetration rates by melting in some, but not all recent eruptions.

Description: [1]  Unravelling the contributions of mainshock slip, aftershocks, aseismic afterslip and postseismic relaxation to the deformation observed in earthquake sequences heightens our understanding of crustal rheology, triggering phenomena and seismic hazard.Here, we revisit two recent earthquakes in the Zagros mountains (Iran) which exhibited unusual and contentious after-effects. The M w ~6 earthquakes at Qeshm (2005) and Fin (2006) are both associated with large InSAR signals, consistent with slip on steep reverse faults in carbonate rocks of the middle sedimentary cover, but small aftershocks detected with local seismic networks were concentrated at significantly greater depths. This discrepancy can be interpreted in one of two ways: either (1) there is a genuine vertical separation between mainshock and aftershocks, reflecting a complex stress state near the basement–cover interface; or (2) the aftershocks delimit the mainshock slip and the InSAR signals were caused by shallow, up-dip afterslip (“phantom earthquakes" ) with very similar magnitudes, mechanisms and geographical positions as the original earthquakes. Here, we show that mainshock centroid depths obtained from body-waveform modelling — which in this instance is the only method that can reveal for certain the depth at which seismic slip was centered — strongly support the first interpretation. At Qeshm, microseismic aftershock depths are centered at the level of the Hormuz Formation, an Infracambrian sequence of intercalated evaporitic and non-evaporitic sediments. These aftershocks may reflect the breaking up of harder Hormuz sediments and adjacent strata as the salt flows in response to mainshock strain at the base of the cover. This work bolsters recent suggestions that most large earthquakes in the Zagros are contained within carbonate rocks in the mid–lower sedimentary cover and that the crystalline basement shortens mostly aseismically.

Description: [1]  Data from six airborne magnetic surveys were compiled and analyzed to develop a structural interpretation for the Transbrasiliano Lineament in northern Paraná Basin, Brazil. Magnetic lineaments, interpreted to reflect geologic contacts and structures at different depths, were illuminated using the matched-filter technique applied to aeromagnetic anomalies. Field-based structural measurements generally support our magnetic analysis. We estimated three primary magnetic zones: (i) a zone of deep magnetic sources at 20 km depth; (ii) an intermediate basement zone at 6 km depth; and (iii) a shallow zone of near-surface geological features at 1.5 km depth. The deepest zone exhibits three major NE-trending crustal discontinuities related to the Transbrasiliano Lineament, dividing the region into four geotectonic blocks. Anomalies associated with the intermediate zone indicate directional divergence of subsidiary structures away from the main Transbrasiliano fault, which strikes N30°E. The shallow magnetic zone includes near-surface post-Brasiliano orogenic granites. Our analysis identified NE-trending sigmoidal lineaments around these intrusions, indicating intense zones of deformation associated with probable shear structures. At the shallow depth zone, magnetic anomalies caused by Cretaceous alkaline intrusive bodies and basalts of the Serra Geral Formation are enhanced by the matched-filter method. These igneous bodies are related to extensional NW-striking lineaments and seismicity aligned along these lineaments suggests that they currently are reactivated. Prior to flexural subsidence of the Paraná Basin, reactivation processes along transcurrent elements of the Transbrasiliano Lineament promoted extensional processes and formed initial Paraná Basin depocenters. Cretaceous and more recent sedimentation also correlate with reactivations of NE-striking structures.

Description: [1]  We image the subducted slab underneath a 450 km long transect of the Alaska subduction zone. Densely spaced stations in southern Alaska are set up to investigate (1) the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity, and (2) the interplate coupled zone where the great 1964 earthquake (Mw 9.2) exhibited the largest amount of rupture. The joint teleseismic migration of two array datasets based on teleseismic receiver functions (RFs) reveals a prominent, shallow-dipping low-velocity layer at ~25-30 km depth in southern Alaska. Modeling of RF amplitudes suggests the existence of a thin (3-5 km) low-velocity layer (shear wave velocity of ~2.1–2.6 km/s) that is ~ 20-40% slower than underlying oceanic crustal velocities, and is sandwiched between the subducted slab and the overriding North America plate. The observed low-velocity megathrust layer (with P-to-S velocity ratio of 1.9-2.3) may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel. The subducted crust below the low-velocity channel has gabbroic velocities with a thickness of ~15 km. Both velocities and thickness of the low-velocity channel increase downdip in central Alaska, in agreement with previously published results. Our image also includes an unusually thick low-velocity crust subducting with a ~ 20 degree dip down to 130 km depth at approximately 200 km inland beneath central Alaska. The unusual nature of this subducted segment results from the subduction of the Yakutat terrane crust. We also show a clear image of the Yakutat and Pacific crust subduction beneath the Kenai Peninsula, and the along-strike boundary between them at megathrust depths. Our imaged western edge of the Yakutat terrane, at 25-30 km depth in the central Kenai along the megathrust, aligns with the western end of a geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface.

Description: [1]  Along the San Bernardino strand of the San Andreas fault (SAF) and across the eastern California shear zone (ECSZ), geologic slip rates differ from those inverted from geodetic measurements, which may partly owe to inaccurate fault connectivity within geodetic models. We employ three-dimensional models that are mechanically compatible with long-term plate motion to simulate both fault slip rates and interseismic surface deformation. We compare results from fault networks that follow mapped geologic traces and resemble those used in block model inversions, which connect the San Jacinto fault to the SAF near Cajon Pass and connect distinct faults within the ECSZ. The connection of the SAF with the San Jacinto fault decreases strike-slip rates along the SAF by up to 10% and increases strike-slip rates along the San Jacinto fault by up to 16%; however, slip rate changes are still within the large geologic ranges along the SAF. The insensitivity of interseismic surface velocities near Cajon Pass to fault connection suggests that inverse models may utilize both an incorrect fault geometry and slip rate and still provide an excellent fit to interseismic geodetic data. Similarly, connection of faults within the ECSZ produces 36% greater cumulative strike-slip rates but less than 17% increase in interseismic velocity. When using over-connected models to invert GPS for slip rates, the reduced off-fault deformation within the models can lead to over-prediction of slip rates. While the nature of fault intersections at depth remains enigmatic, fault geometries should be chosen with caution in crustal deformation models.

Description: [1]  The onset of frictional instabilities, e.g. earthquakes nucleation, is intimately related to velocity-weakening friction, in which the frictional resistance of interfaces decreases with increasing slip velocity. While this frictional response has been studied extensively, less attention has been given to steady-state velocity-strengthening friction, in spite of its potential importance for various aspects of frictional phenomena such as the propagation speed of interfacial rupture fronts and the amount of stored energy released by them. In this note we suggest that a crossover from steady-state velocity-weakening friction at small slip velocities to steady-state velocity-strengthening friction at higher velocities might be a generic feature of dryfriction. We further argue that while thermally activated rheology naturally gives rise to logarithmic steady-state velocity-strengthening friction, a crossover to stronger-than-logarithmic strengthening might take place at higher slip velocities, possibly accompanied by a change in the dominant dissipation mechanism. We sketch a few physical mechanisms that may account for the crossover to stronger-than-logarithmic steady-state velocity-strengthening and compile a rather extensive set of experimental data available in the literature, lending support to these ideas.

Description: [1]  Micro-earthquakes have come into high public awareness due to being induced by the development and exploitation of enhanced and natural geothermal fields, hydro-fracturing, and CO 2 sequestration sites. Characterizing and understanding the faulting process of induced earthquakes, which is generally achieved through moment tensor inversion, could both help in risk prediction and in reservoir development monitoring. However, this is a challenging task because of their lower signal to noise ratio at frequencies typically used in earthquake source analyses. Therefore, higher resolution velocity models and modeling of seismic waves at higher frequencies are required. In this study, we examine both the potential to obtain moment tensor solutions for small earthquakes, and the uncertainty of those solutions. We utilize a short-period seismic network located in the Geysers geothermal field in northern California, and limit our study to that which would be achieved by industry in a typical reservoir environment. We obtain full moment tensor solutions of M ~ 3 earthquakes using waveform modeling and first-motion inversions. We find that these two data sets give complimentary, but yet different solutions. Some earthquakes correspond possibly to complex processes in which both shear and tensile failures occur simultaneously or sequentially. This illuminates the presence of fluids at depth and their role for the generation of these small magnitude earthquakes. Finally, since first motions are routinely obtained for all magnitude earthquakes, our approach could be extended to small earthquakes where noise level and complex Green's functions prohibit using waveforms in moment tensor inversions.

Description: [1]  Systematic analysis of earthquake focal solutions derived from centroid moment tensors shows well-defined orientation groups in scatter plots of fault plane normals and associated slip line vectors. Consideration of the geometry implied by these orientation groups can allow resolution of the ambiguity inherent in the choice as to which of the two conjugate fault plane solutions should apply, and in many cases the same classification can be applied to the entire orientation group. Examining scatter plots of data from normal fault earthquakes on spreading ridges typically shows orthogonal relations but there are also many cases where there is a skew with respect to the great circles defined by faults on adjacent transform faults. This can be explained by finite rock strength in the adjacent transforms, requiring resolved shear stress to allow movement, thus requiring rotation of the trajectories of the deviatoric stress axes: anticlockwise for right-lateral transforms, and clockwise for left-lateral transforms. This asymmetry also requires formation of tilt-block geometries reminiscent of Basin and Range style continental extension.

Description: We simulate gas-burst and volcanic explosions under controlled laboratory conditions inducing fragmentation of volcanic rocks by rapid depressurization. The seismic wave fields of experimental simulations of volumetric sources provide means for better understanding natural volcanic conduit dynamics. A series of experiments were performed in a shock-tube apparatus at room temperature and a pressure range of 4 to 20 MPa using Argon (Ar) gas and, particles or pumice samples of different porosities, from Popocatepetl volcano. The instrumentation of this system with high-precision piezoelectric sensors enabled us to capture elastic waves and to recognize their characteristic signatures. By relating these signals to physical processes in the wave field, we have been able to characterize the conduit mechanism and the source dynamics. We compare and discuss conspicuous features of the waveforms and frequency spectra of these experimental signals with those of volcanic origin. Despite the fact that these signals are different in amplitude (resulting from different scale conditions); our observations indicate that the physical processes that occur during simulated explosions and those that occur during volcanic eruptions yield comparable signatures in their respective records. The effects of the source-receiver configuration and resonance have also significant implications. All this suggests that the physical processes (e.g. pressurization and depressurization of a system) involve a system response that causes similar distinctive effects independent of the system size, reflecting its intrinsic dynamics. These similarities imply that powerful constraints on the source mechanisms of volcanic seismicity can emerge from seismic investigations of experimental explosions. Such constrains may yield significant advances in the interpretation of seismic unrest at volcanic centers.

Description: The deep subduction of the Pacific Plate underneath East Asia is thought to have played a key role in the destruction of the North China Craton (NCC). To test this hypothesis, this paper presents a new 2D model that includes an initial stable equilibrated craton, the formation of a big mantle wedge (BMW), and erosion by vigorous mantle convection. The model shows that subduction alone cannot thin the cold solid craton but it can form a low-viscosity BMW. The amount of convective erosion is directly proportional to viscosity within the BMW (η 0bmw ), and the rheological boundary layer thins linearly with decreasing log 10 (η 0bmw ), thereby contributing to an increase in heat flow at the lithospheric base. This model also differs from previous modeling in that the increase in heat flow decays linearly with t 1/2 , meaning that the overall thinning closely follows a natural log relationship over time. Nevertheless, convection alone can only cause a limited thinning due to a minor increase in basal heat flow. The lowering of melting temperature by peridotite–melt interaction can accelerate thinning during the early stages of this convection. The two combined actions can thin the craton significantly over tens of Myr. This modeling, combined with magmatism and heat flow data, indicates that the NCC evolution has involved four distinct stages: modification in the Jurassic by Pacific-plate subduction and BMW formation; destruction during the Early Cretaceous under combined convective erosion and peridotite−melt interaction; extension in the Late Cretaceous; and cooling since the Late Cenozoic.

Description: The evaluation of flow direction in volcanic rocks is among the most important applications of magnetic fabrics studies. A statistically significant sample set of titanomagnetite-bearing lava flows from the Malwa Plateau, the northern part of the Deccan traps in India has been investigated for a possible interference of induced and natural remanent magnetization (NRM). The NRM alters the scalar AMS parameter as well as the orientations of the AMS principal magnetic axes, which are crucial for the evaluation of the flow direction. For cleaning of the NRM component, the lava samples have been demagnetized by use of an AF tumbling demagnetizer (peak fields of 100 mT) as previous studies have shown that static AF demagnetization can bias the results. Samples with normal magnetic fabrics demonstrate a redistribution of their principal axes after the demagnetization. The evaluated flow directions show a more differentiated flow pattern of the Malwa area, which seems to fit better into the regional geological setting. In samples with inverse magnetic fabrics, carrying a higher portion of single domain particles, AMS principal axes remain unchanged after the demagnetization, indicating that these samples with high coercivity of magnetic carriers are not suitable for geological interpretations. According to these results, we propose that the AMS measurements after tumbling demagnetization give a better reflection of the intrinsic anisotropy of magnetic carriers (at least for samples with normal magnetic fabrics) and therefore a more precise and better reflection of the “actual” mineral fabric. This study underlines that special care is required when a high portion of ferromagnetic (senso lato) minerals carrying a strong remanent magnetization is present.

Description: Accurate descriptions of strength evolution are required in predictive models of fault-zone behavior during earthquakes. At low sliding rates, frictional resistance between fault rocks is much higher than the shear stress that is typically inferred to be present during earthquakes. Laboratory experiments confirm that the friction coefficient drops at high sliding rates, and it is suggested that strengthening, possibly related to an increase in the area of viscous melt patches, may occur after this initial weakening stage. Most existing weakening mechanisms do not predict such strengthening, which may exert an important control on the thickness of the shear-zone. We propose a micro-mechanical model of flash heating that describes how shear resistance evolves at the asperity scale as a result of distributed deformation over a weak layer that grows during the brief lifetime of each asperity contact. Beyond a threshold weakening-velocity, our model predicts that friction should decrease with slip rate since higher sliding speeds cause the weak layer to thicken more rapidly. A comparison with published experimental data from a range of mineral systems shows good agreement with the model predictions. The parameter choices that ensure good model fits to the laboratory friction data are consistent with a priori estimates for the onset of asperity melting at high contact normal stresses. At higher sliding rates and/or elevated temperatures, our model predicts that the frictional rate dependence should transition from velocity-weakening to become velocity-strengthening because decreases in the contact lifetime with slip rate cause the average asperity strength to increase.

Description: We present a new approach to estimate the density structure of shallow oceanic mantle by inversion of localized geoid anomalies. Our method is based on Bayesian statistics and is implemented by combining forward modeling with Markov Chain Monte Carlo sampling. The inherent nonuniqueness of such inversion is reduced by using spectral localization, reference models, and a priori bounds on the amplitude of density perturbations expected within the convecting mantle. We apply this approach to the geoid anomalies around the Mendocino Fracture Zone that have recently been revealed by wavelet analysis. The depth and vertical extent of density anomalies derived from our inversion indicate that they are intimately related to the structure of the lowermost lithosphere. The amplitude of density perturbations and their spatial organization suggest the occurrence of small-scale convection induced by a lateral temperature gradient across the fracture zone. As its applicability is not limited to the vicinity of fracture zones, the new inversion method should allow us to resolve the fine-scale density structure of shallow oceanic mantle beneath the world's oceans.

Description: We performed shear deformation experiments using quasi-Maxwell fluids. We found that, depending on the strain rates, the same material generates earthquakes associated with the elastic rebound and deforms viscously. Around the threshold, elastic rebound releases a certain fraction of the inter-seismic displacement, but the other fraction remains as a result of the viscous relaxation. We applied our experimental results to a subduction zone, in which the upper part of the hanging wall behaves as an elastic layer and generates seismicity, while the deeper part behaves as a viscous fluid and subducts with the slab. Our experimental results suggest that, around the boundary of the elastic and viscous layers, seismicity can occur, but only some partof the inter-seismic displacements is released. The experimentally obtained threshold of the seismic activity is determined by the combination of the subduction velocity v s , the viscosity of the hanging wall η , the fault length titW, and the adhesive stress σ a , v s η /( Wσ a ) 〉 1. This threshold suggests that if the viscosity of the hanging wall decreases with depth, the maximum size of the earthquakes also decreases with depth, and, finally, seismicity disappears. This hypothesis is consistent with the observed fact that slow earthquakes, characterized by their small magnitudes, are observed at the down-dip limit of the seismogenic zone.

Description: Secondary triggering of aftershocks is widely observed and often ascribed to secondary static stress transfer. However, small to moderate earthquakes are generally disregarded in estimates of Coulomb stress changes ( ΔCFS ), either because of source parameter uncertainties or a perceived lack of importance. We use recently published high-quality focal mechanisms and hypocenters to re-assess the role of small to moderate earthquakes for static stress triggering of aftershocks during the 1992 M W 7.3 Landers, California, earthquake sequence. We compare the ΔCFS imparted by aftershocks (2 ≤  M  ≤ 6) onto subsequent aftershocks with the total ΔCFS induced by the M  〉 6 main shocks. We find that incremental stress changes between aftershock pairs are potentially more often positive than expected over intermediate distances. Cumulative aftershock stress changes are not reliable for receivers with nearby aftershock stress sources because we exclude unrealistic aftershock stress shadows that result from uniform slip models. Nonetheless, 27% of aftershocks receive greater positive stress from aftershocks than from the main shocks. Overall, 85% of aftershocks are encouraged by the main shocks, while adding secondary stress encourages only 79%. We infer that source parameter uncertainties of small aftershocks remain too large to convincingly demonstrate (or rule out) that secondary stress transfer induces aftershocks. An important exception concerns aftershocks in main shock stress shadows: well-resolved secondary stress from detected aftershocks rarely compensates negative main shock stress; these aftershocks require a different triggering mechanism.

Description: ABSTRACT We present a new mantle model (YB14SHani) of azimuthal anisotropy for horizontally polarized shear-waves (SH) in parallel with our previously published vertically polarized shear-wave (SV) anisotropy model (YB13SVani). YB14SHani was obtained from higher mode Love wave phase velocity maps with sensitivity to anisotropy down to ~1200 km depth. SH anisotropy is present down to the mantle transition zone (MTZ) with an average amplitude of ~2% in the upper 250 km and ~1% in the MTZ, consistent with YB13SVani. Changes in SV and SH anisotropy were found at the top of the MTZ where olivine transforms into wadsleyite, which might indicate that MTZ anisotropy is due to the lattice preferred orientation of anisotropic material. Beneath oceanic plates, SV fast axes become sub-parallel to the absolute plate motion (APM) at a depth that marks the location of a thermally controlled lithosphere-asthenosphere boundary (LAB). In contrast, SH anisotropy does not systematically depend on ocean age. Moreover, while upper mantle SV anisotropy is anomalously high in the middle of the Pacific, as seen in radial anisotropy models, SH anisotropy amplitude remains close to the average for other oceans. Based on the depth at which SV fast axes and the APM direction begin to align, we also found that the average thickness of cratonic roots is ~ 250 km, consistent with Yuan and Romanowicz [2010] for North America. Here, we add new constraints on the nature of the cratonic LAB and show that it is characterized by changes in both SV and SH anisotropy.

Description: We use numerical modelling to investigate the proposed stress based origin for changing anisotropy at Mount Asama Volcano, Japan. Stress-induced anisotropy occurs when deviatoric stress conditions are applied to rocks which are permeated by microcracks and compliant pore space, leading to an anisotropic distribution of open crack features. Changes to the local stress field around volcanoes can thus affect the anisotropy of the region. The 2004 eruption of Mount Asama Volcano coincided with time varying shear wave splitting measurements, revealing changes in anisotropy that were attributed to stress changes associated with the eruption. To test this assertion, we create a model that incorporates knowledge of the volcanic stress, ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model. Anisotropy is calculated in two ways, by considering a basic case of having uniform crack density and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress. Our results show that this approach is sensitive to crack density, crack compliance, and the regional stress field, all of which are poorly constrained parameters. In the case of dry crack closure, results show that modelled stress conditions produce a much smaller degree of anisotropy than indicated by measurements. We propose that the source of anisotropy changes at Asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes, such as the movement of pore fluid.

Description: The Canfranc underground laboratory (LSC), excavated under the Central Pyrenees, is mainly devoted to the study of phenomena which needs “cosmic silence”, but also host a geodynamical facility, named Geodyn, which holds an accelerometer, a broad-band seismometer and two high-resolution laser strainmeters. During the routine processing of the seismic data we detected an unusual spectral signature in the 2-10 Hz frequency band, which does not correspond to the typical sources of seismic noise and which can also be recognized in the strain records. After checking against meteorological and hydrological data, we can relate those signals to variations in the discharge by the Aragon River, an Alpine-style river in the southern Pyrenees, located about 400 m from the LSC Geodyn facility. Four main episodes have been identified since early 2011, each lasting 1-2 to 6-8 days. Additionally, a limited number of shorter episodes have also been detected. Three types of river-generated seismic events have been identified, related respectively to moderate rainfall, snowmelt and flooding events associated to severe storms. Each of those types has distinctive characteristics which allow monitoring the hydrological events from the analysis of seismic and deformation data. A few previous studies have already described the seismic noise close to rivers with larger discharge or in small-scale experimental settings, and we are showing here that the so-called “fluvial seismology” can be useful to study the hydrological evolution of Alpine style streams, and may have a potential interest for the civil authorities in charge of the management of hydrological basins.

Description: This study investigates the large strain rheological behaviour of pelitic rocks undergoing melting and subsequent crystallization during deformation. Constant strain rate ( =3x10 -4  s -1 ) torsion experiments were performed to achieve large strains ( γ max  = 15) on synthetic aggregates of quartz and muscovite at 300 MPa confining pressure and temperature of 750 °C. A set of hydrostatic experiments for equivalent times of the torsion experiments was also conducted at 300 MPa and 750 °C to evaluate the reaction kinetics and microstructures under static and under deforming conditions. Microstructures of the deformed samples reveal four distinct but gradational stages of crystal-melt interactions - a) solid state deformation, b) initiation and domination of partial melting, c) simultaneous partial melting and crystallization and d) domination of crystallization. These four microstructural stages are linked to the changes of the bulk mechanical response of the deforming samples. Partial melting starts at relatively low finite shear strains ( γ  = 1-3) and is associated with strong ( ca . 60%) strain softening. With further shearing ( γ  = 4-10) the partially molten bulk material shows a constant (“steady state”) flow at low stress. Further crystallization of new crystals at the expense of melt between γ  = 10 and 15 causes weak strain hardening until the material fails by developing brittle fractures. The stress exponent ( n ) values, calculated at three different shear strains ( γ  = 1, 5 and 10), increase from ∼ 3 to ∼ 43 with increasing deformation, pointing out a transition from power to power law break-down or exponential flow of the bulk system. These new experimental data establish that partially molten rock does not flow according to a constant strain rate dependent power law (“steady state”) rheology. The rheological transition from strain rate sensitive to strain rate insensitive flow is interpreted as a function of melt-crystal ratio, their mutual interactions and the evolution of microstructures in the partially molten rock.

Description: The reliable assessment of volcanic unrest must rest on an understanding of the host rocks that form the volcanic edifice. It is in particular the microstructure of the host rocks that dictate their physical properties and mechanical behaviour, and thus the response of the edifice to the stress perturbations likely to accompany any unrest activity. Here, we evaluate the interplay between microstructure and physical and mechanical properties for a suite of representative andesitic edifice-forming rocks from Volcán de Colima (Mexico). Quantitative microstructural analyses demonstrate that these rocks exhibit (1) a pervasive, isotropic microcrack network, (2) a high, sub-spherical vesicle density and, (3) a wide vesicle size distribution. This complex microstructure severely impacts their physical and mechanical properties. In detail, porosities, the inevitable manifestation of high microcrack and vesicle densities and large vesicle sizes, are high and range from 8 to 29%. As a consequence, elastic wave velocities, Young's moduli, and uniaxial compressive strengths are low, and permeabilities are high. Further, all of the physical and mechanical properties demonstrate a wide range. For example, strength decreases by a factor of 8 and permeability increases by four orders of magnitude over the porosity range. These data highlight the heterogeneous nature of the rocks comprising the edifice at Volcán de Colima. Our permeability data reveal that, below a porosity of 11-14% the permeability-porosity trend follows a power law with a much higher exponent. Microstructurally, this “crossover porosity” is likely to represent a critical vesicle content that efficiently connects the microcrack population and permits a much more direct path through the sample, rather than restricting flow to long and tortuous microcracks. Values of tortuosity inferred from the Kozeny-Carman permeability model support this hypothesis. However, we find that the complexity of the andesite microstructure precludes a complete description of their mechanical behaviour through micromechanical modelling. Thermally stressing the andesites to 450 °C does not significantly alter their physical properties or compressive strength. We surmise that thermal stressing does not impact the most deleterious microcracks, which play an important role in compressive failure. We urge that the findings of this study be considered in volcanic hazard assessments at andesitic stratovolcanoes.

Description: We explore the issue of fault reactivation induced in Enhanced Geothermal Systems (EGS) by fluid injection. Specifically we investigate the role of late stage activation by thermal drawdown. A THM (Thermal-Hydrological-Mechanical) simulator incorporating a ubiquitous joint constitutive model is used to investigate the elastic-plastic behavior of an embedded fault.. We apply this new THM model to systematically simulate the seismic slip of a critically-stressed strike-slip fault embedded within the reservoir. We examine the effects of both pore pressure perturbation and thermal shrinkage stress on the magnitude of the resulting events and their timing. We analyze the sensitivity of event magnitude and timing to changes in the permeability of the fault and fractured host, fracture spacing, injection temperature, and fault stress obliquity. From this we determine that: (1) the fault permeability does not affect the timing of the events nor their size, since fluid transmission and cooling rate is controlled by the permeability of the host formation. (2) When the fractured medium permeability is reduced (e.g. from 10 -13 to 10 -16   m 2 ), the timing of the event is proportionately delayed (by a corresponding three orders of magnitude), although the magnitude of the seismic event is not impacted by the change in permeability. (3) Injection temperature has little influence on either the timing or size of the early hydro-mechanically induced events, but it does influence the magnitude but not the timing of the secondary thermal event. The larger the temperature differences between that of the injected fluid and the ambient rock, the larger the magnitude of the secondary slip event. (4) For equivalent permeabilities, changing the fracture spacing (10  m -50  m -100  m ) primarily influences the rate of heat energy transfer and thermal drawdown within the reservoir. Smaller spacing between fractures results in more rapid thermal recovery but does not significantly influence the timing of the secondary thermal rupture.

Description: The Mount Givens Granodiorite, a large pluton in the central Sierra Nevada batholith, California, is similar in area to zoned intrusive suites yet is comparatively chemically and texturally homogenous. New zircon U-Pb geochronology indicates the pluton was constructed over at least 7 Ma from 97.92 ± 0.06 Ma to 90.87 ± 0.05 Ma. Combining the new geochronology with the exposed volume of the pluton yields an estimated magma flux of 〈0.001 km 3 /a. The geochronologic data are at odds with the previously speculated links between plutons such as the Mount Givens Granodiorite and large-volume homogeneous ignimbrites (often termed monotonous intermediates). Existing data indicate that large plutons accumulate at rates of ≤0.001 km 3 /a, 1-2 orders of magnitude less than fluxes calculated for dated monotonous intermediates. If monotonous intermediates are remobilized, erupted plutons accumulated at rates comparable to dated examples, they should preserve a record of zircon growth of up to 10 Ma. Alternatively, the long history of zircon growth recorded in plutons may be erased during the processes of reheating and remobilization that precede supervolcano eruption. However, zircon dissolution modeling, based on hypothetical temperature-time histories for pre-eruptive monotonous intermediates, indicates rejuvenation events would not sufficiently dissolve zircon. We suggest that eruptions of monotonous intermediates occur during high magmatic flux events, leaving little behind in the intrusive rock record, whereas low fluxes favor pluton accumulation.

Description: Stress transfer between separate magma bodies is evaluated by considering how pressure changes related to magma accumulation/propagation influence the stability of a separate nearby magma body. Three-dimensional numerical models are used to evaluate the stability evolution of a magma body through the calculation of two variables: (i) the variation of the threshold pressure needed to cause failure around the magma body and (ii) the magma pressure change. A parametric study indicates that stress interactions are strongly dependent on the distance between magma bodies as well as the body's shape. Such models are then applied to evaluate stress influence of intrusive activity in 1994, 1999 and 2010 at Eyjafjallajökull volcano, that preceded two eruptions there in 2010. Two cases are considered: influence of these intrusions on i) a magma reservoir at 20 km distance under the Katla volcano, and ii) a silicic magma body under Eyjafjallajökull. The distance between the Eyjafjallajökull intrusions and the Katla reservoir is sufficiently long to reduce the stress interaction to insignificant levels, with an amplitude of the same order as Earth tides (few kPa). However, cumulative stress transfer due to the intrusions to a remnant silicic shallow body situated below the Eyjafjallajökull is much larger (0.5-2.5 MPa). This mechanical transfer could have contributed to the failure of the silicic body and promoted the chemical mixing/mingling between different magma types, which is commonly interpreted as the main cause of the 2010 explosive eruption of Eyjafjallajökull.

Description: We analyzed the P- and S-wave displacement spectra of 717 microearthquakes in the moment range 4 × 10 9 –2 × 10 14   Nm recorded at the dense networks operating in southern Apennines (Italy), and deployed along the 1980 Ms 6.9 Irpinia earthquake fault zone. Source, attenuation and site parameters are estimated by using a parametric modeling approach, which is combined with a multi-step, non-linear inversion strategy. We found that in the analyzed frequency band an attenuation model with constant-Q has to be preferred to frequency dependent Q-models. Consistent estimates of the median P and S quality factors and (114; 417) are obtained from two different techniques and relatively high values of Q S / Q P (median value 1.3, (0.8 ;2.1)) are found in the same depth range where high V P / V S and a peak in seismicity distribution are observed. This is the evidence for a highly fractured, partially or completely fluid-saturated medium embedding the Irpinia fault zone, down to crustal depths of 15–20 km. A nearly constant stress drop ( ) and apparent stress ( ) scaling of P- and S-corner frequencies and seismic energies is observed above a seismic moment value of about 10 11   Nm . The measured radiation efficiency is low , e.g. the radiated energy is only a small fraction of the whole energy spent by friction and fracture development. A large positive dynamic overshoot (high dynamic shear strength) can be the dominant mechanism controlling the microearthquake fractures along the Irpinia fault zone.

Description: Faults are usually surrounded by damaged rocks, which have lower elastic moduli and seismic wave velocities than their host rocks. Such damaged fault zones are usually several hundred meters wide and develop a complex yet interesting wave field when earthquakes happen inside them. If the interface between the fault zone and host rock is sharp enough, waves are reflected and head waves are generated. Each wave can cause either increase or drop of fault stress, depending on its polarity. These waves can interact with earthquake ruptures and modulate rupture properties, such as rupture speed, slip rate and rise time. We find through 2D dynamic rupture simulations that: 1) Reflected waves can induce multiple slip pulses. Fault zone properties have a stronger control on their dominant rise time than frictional properties. 2) Head waves can cause oscillations of rupture speed and, in a certain range of fault-zone widths, a permanent transition to supershear rupture with speeds that are unstable in homogeneous media. 3) Large attenuation smears the slip rate function and delays the initial acceleration of rupture speed, but does not affect significantly the rise time or the period of rupture speed oscillations. 4) Fault zones cause a rotation of the background stress field and can induce plastic deformations on both sides of the fault. The plastic deformations are accumulated both inside and outside the fault zone, which indicates a correlation between fault zone development and repeating ruptures. Spatially periodic patterns of plastic deformations are formed due to oscillating rupture speed, which may leave a permanent signature in the geological record. Our results indicate that damaged fault zones with sharp boundaries promote multiple slip pulses, short rise times and supershear ruptures.

Description: Higher order ionospheric effects (I2+) are one of the main limiting factors in very precise GNSS processing, for applications where millimeter accuracy is demanded. This paper summarizes a comprehensive study of the I2+ effects in range and in GNSS precise products such as receiver position and clock, tropospheric delay, geocenter offset, GNSS satellite position and clocks. All the relevant higher order contributions are considered: second and third order, geometric bending and dSTEC bending (i.e. the difference between the STEC for straight and bent paths). Using a realistic simulation with representative Solar Maximum conditions on GPS signals, both the effects and mitigation errors are analyzed. The usage of the combination of multifrequency L-band observations to cancel out the main effect, i.e. the second order ionospheric term, has to be rejected due to its increased noise level, seen in both analysis of actual data and theoretical considerations. The results of the study show that the main two effects in range come from the second order ionospheric term and the dSTEC bending: daily average values of 5 mm are obtained for second order term and up to 2 mm error for dSTEC bending (with peak values up to +20 mm and up to +14 mm respectively at south azimuth and 10 deg. of elevation). Their combined impact on the precise GNSS satellite products affects the satellite Z-coordinates (up to +1 cm) and satellite clocks (more than +/- 20 ps). Other precise products areaffected at the millimeter level (+/- 2 mm in the case of the receiver position, up to about +/- 15 ps in the receiver clock and up to more than 1 mm in the non-hydrostatic tropospheric vertical delay). After correction, with approximate affordable models, the impact on all the precise GNSS products is reduced below 5 mm. We finally quantify the corresponding impact on a Precise Point Positioning (PPP) processing, after applying consistently the precise products (satellite orbits and clocks) obtained under correction of higher order ionospheric effects: the remaining effect is shown to be lower than the current uncertainties of the PPP solutions.

Description: The Coulomb stress change has been widely employed to interpret mainshock-mainshock and mainshock-aftershock triggering as well as interactions amongst earthquake faults and volcanoes. This quantitative index is computed based on the Coulomb failure criterion and is a function of fault parameters including the source and receiver fault geometries, the friction coefficient on the receiver fault and Skempton's coefficient of the host rock. Thus, for the robust determination of the Coulomb stress change, the sensitivity of the Coulomb stress change to these model parameters should be thoroughly assessed. However, notwithstanding numerous case studies, almost no systematic investigation of the sensitivity of the Coulomb stress change has been performed. Here, we present an error estimator for the Coulomb stress change, and then quantitatively investigate the sensitivity of the Coulomb stress change to the fault model parameters for the 2008 Mw 7.9 Wenchuan earthquake. Our results indicate that for this case the Coulomb stress change is the most sensitive to the uncertainty in the dip angle of the receiver fault, while the influences of the uncertainties in the slip model of the source fault, the strike and rake angles of the receiver fault and the friction and Skempton's coefficients cannot be neglected. Accordingly, it is crucial to perform a realistic estimate of the uncertainty in the Coulomb stress change. By performing such calculation, future Coulomb stress analyses such as the stress triggering of earthquake sequence and the likelihoods of potential earthquakes could be based on more robust Coulomb stress change maps.

Description: The term “Lg-blockage” refers to the sudden disappearance of the Lg phase along a particular propagation path which is commonly seen at continental-oceanic transition zones. In this paper we present observational evidence of Lg blockage across the continental margin of Nova Scotia in eastern Canada. Regional Lg and Sn spectra from 91 events with epicentral distances between 100 and 1200 km and magnitudes between 2.5 and 4.7 are inverted simultaneously for the source spectrum, site amplification and average attenuation. The vertical displacement spectra were estimated between 0.9 and 10.75 Hz. The assumptions include a fixed frequency-independent geometric spreading rate for Lg and a frequency-dependent spreading model for the Sn. Estimates for the apparent regional attenuations are Q Lg (f) =615(±25) f 0.35(±0.04) , and Q Sn (f) =404(±23) f 0.45(±0.03) . Results from this study provide an accurate parameterization of observed amplitude spectra and are valuable for representing wave propagation in the region. Based on the observation of a strong tradeoff between Sn and Lg amplitude which have different attenuation characteristics we conclude any attenuation study based on measuring amplitude of a package of several different phases, without taking into consideration the propagation characteristics of individual waveforms at the region of study, may bias the estimation of average regional Q.

Description: Studies of low-frequency earthquakes (LFEs) have focused on detecting events within previously-identified tectonic tremor. However, the principal LFE detection tool of matched-filter searches are intrinsically incapable of detecting events that have not already been characterized previously as a template event. In this study, we therefore focus on generating the largest number possible of LFE templates by uniformly applying a recently developed LFE template detection method to a 2.5 year-long dataset in Guerrero, Mexico. Using each of the detected templates in a matched-filter search, we then form event families that each represent a single source. We finally develop simple, empirical statistics to select the event families that represent LFEs. Our resulting catalog contains 1120 unique LFE sources and a total of 1,849,486 detected LFEs over the 2.5 year-long dataset. The locations of the LFE sources are then divided into subcatalogs based on their distance from the subduction trench. Considering each LFE as a small unit of slip along the subduction interface, we observe discrete episodes of LFE activity in the region associated with large slow-slip events; this is in direct contrast to the near-continuous activity observed 35 km further downdip within the previously-identified LFE/tremor sweet-spot.

Description: Time series of soil CO 2 efflux recorded in the Azores archipelago volcanic-hydrothermal areas feature daily and seasonal variations. The recorded CO 2 efflux values were lower during summer than in the winter season. The diurnal CO 2 efflux values were higher at dawn and lower in the early afternoon, contrary to that observed in biogenic environments. CO 2 efflux cycles correlated well with the environmental variables, such as air temperature, wind speed, and barometric pressure, which also showed low and high frequency periodicities. Several simulations were performed here using the TOUGH2 geothermal simulator, to complement the study of Rinaldi et al . [2012, J . Geophys . Res ., 117, B11201]. The effects of the water-table depth, air temperature perturbation amplitude, and soil thermal gradient contributed to an explanation of the contrasts observed in the diurnal (S 1 ) and semidiurnal (S 2 ) soil CO 2 efflux peaks for the different monitoring sites and seasons. Filtering techniques (multivariate regression analysis and fast Fourier transform filters) were also applied to the recorded time series to remove effects of external variables on the soil CO 2 efflux. The resulting time series (the residuals) correspond to the best approach to the deep-seated (volcanic/hydrothermal) CO 2 emissions, and thus should be used in seismo-volcanic monitoring programs. Even if no evident correlation can be established yet between the soil CO 2 residuals and seismicity over the monitored time, a seismic swarm that occurred around the end of 2008 might have triggered some deviations from the observed daily cycles.

Description: We systematically measured oxygen self-diffusion coefficients ( D O ) in forsterite along b - crystallographic axis at a pressure of 8 GPa and temperatures of 1600 – 1800 K, over a wide range of water content ( C H2O ) from 〈1 up to ~800 wt. ppm. The experimental results suggest that D O ∝ ( C H2O ) 0.05±0.06  ≈ ( C H2O ) 0 . Thus, water has no significant effect on oxygen self-diffusion rate in forsterite. Since the C H2O dependence of silicon self-diffusion rate is also very small [ Fei et al ., 2013], the effect of water on olivine rheology is not significant by assuming the diffusion controlled creep mechanism.

Description: Studies of velocity structure in the Earth's inner core and its volumetric variations illuminate our understanding of inner core dynamics and composition. Here we use an extensive number of seismograms recorded by the Hi-net array to construct complete empirical curves of PKP BC -PKP DF differential travel-times. The nature of these curves implies that significant variations in travel-times are accumulated during the passage of PKP DF waves through the uppermost inner core, and rules out outer core structure effects. Uniform cylindrical anisotropy of a plausible strength in the uppermost inner core can also be ruled out as a cause of the observed travel-time variations because the range of sampled ray angles is too narrow. The configuration and strength of inhomogeneities from a recent tomographic model of the lowermost mantle cannot account for the observed travel-time variations. Therefore, we infer that either variations of P-wave isotropic velocity on the scale of about hundred km and less are present in the uppermost inner core or the material may be organized in distinctive anisotropic domains, and both of these features may be superimposed on long-wavelength hemispherical structure. If the former holds true, the absolute magnitude of required P-wave velocity perturbations from referent values is 0.60 ± 0.10% in the quasi-eastern and 1.55 ± 0.15% in the quasi-western hemisphere (0.85 ± 0.05% and 1.10 ± 0.10%, respectively, with the lowermost mantle correction). The existence of these variations is a plausible physical outcome given that vigorous compositional convection in the outer core and variations in heat-exchange across the inner core boundary may control the process of solidification.

Description: Two-phase fluid flow is strongly controlled by small-scale (sub-core scale) heterogeneity of porous sandstone. We monitor the heterogeneous/anisotropic two-phase flow (CO 2 and water) in porous sandstone and conduct multi-channel V p and V p -anisotropy measurements under super critical CO 2 conditions during CO 2 injection (drainage) and water re-injection (imbibition) processes. In drainage, V p shows large reduction (~10 %) in all sections of the core sample and changes from the bottom inlet side to upper outlet side. It is considered that V p reduction reflects the CO 2 movement in the specimen. The V p -anisotropy of the upper two planes indicates clear increase. The results of this experiment indicate the heterogeneous CO 2 -flow around laminae in porous sandstone and characteristic behavior of these laminae as a barrier for CO 2 . On the other hand, flow of water is not affected by this barrier. This characteristic CO 2 -water flow around laminae is observed in the numerical simulation results. This simulation study also indicates that the capillary number is not directly affected on two-phase fluid flow around small-scale heterogeneity in porous sandstone. These results suggest that the small-scale heterogeneity behaves as a CO 2 -gate and strongly controls CO 2 behavior in porous sandstone.

Description: Existing tsunami early warning systems in the world can give either one or a combination of estimated tsunami arrival times, heights, or qualitative tsunami forecasts before the tsunami hits near-field coastlines. A future tsunami early warning system should be able to provide a reliable near-field tsunami inundation forecast on high-resolution topography within a short time period. Here we describe a new methodology for near-field tsunami inundation forecasting. In this method, a pre-computed tsunami inundation and pre-computed tsunami waveform database is required. After information about a tsunami source is estimated, tsunami waveforms at near-shore points can be simulated in real-time. A scenario that gives the most similar tsunami waveforms is selected as the site-specific best scenario and the tsunami inundation from that scenario is selected as the tsunami inundation forecast. To test the algorithm, tsunami inundation along the Sanriku Coast is forecasted by using source models for the 2011 Tohoku earthquake estimated from GPS, W phase, or offshore tsunami waveform data. The forecasting algorithm is capable of providing a tsunami inundation forecast that is similar to that obtained by numerical forward modeling, but with remarkably smaller CPU time. The time required to forecast tsunami inundation in coastal sites from the Sendai Plain to Miyako City is approximately 3 minutes after information about the tsunami source is obtained. We found that the tsunami inundation forecasts from the 5-min GPS, 5-min W phase, 10-min W phase fault models, and 35-min tsunami source model are all reliable for tsunami early warning purposes and quantitatively match the observations well, although the latter model gives tsunami forecasts with highest overall accuracy. The required times to obtain tsunami forecast from the above four models are 8 min, 9 min, 14 min, and 39 min after the earthquake, respectively, or in other words 3 minutes after receiving the source model. This method can be useful in developing future tsunami forecasting systems with a capability of providing tsunami inundation forecasts for locations near the tsunami source area.

Description: We test and characterize a new method to measure 2-D radial temperature distributions in the laser-heated diamond anvil cell using comparisons against real and synthetic data. We show that this method 1) incorporates an inexpensive and robust design for laser-heated diamond cell temperature measurements, 2) yields precise measurements of radial temperature distributions of laser-heated samples, and 3) can be used in conjunction with numerical models to measure the pressure- and temperature-dependence of thermal conductivity of materials. We apply the method to determine the high-pressure, high-temperature thermal conductivity of MgO.

Description: Moment magnitudes for large earthquakes (M w ≥7.0) derived in real-time from near field seismic data can be underestimated due to instrument limitations, ground tilting, and saturation of frequency/amplitude-magnitude relationships. Real-time high-rate GPS resolves the build-up of static surface displacements with the S-Wave arrival (assuming non-supershear rupture), thus enabling the estimation of slip on a finite fault and the event's geodetic moment. Recently, a range of high-rate GPS strategies has been demonstrated on off-line data. Here, we present the first operational system for real-time GPS-enhanced earthquake early warning as implemented at the Berkeley Seismological Laboratory (BSL) and currently analyzing real-time data for Northern California. The BSL generates real-time position estimates operationally using data from 62 GPS stations in Northern California. A fully triangulated network defines 170+ station pairs processed with the software trackRT. The BSL uses G-larmS, the Geodetic Alarm System, to analyze these positioning time series, and determine static offsets and pre-event quality parameters. G-larmS derives and broadcasts finite fault and magnitude information through least-squares inversion of the static offsets for slip based on a-priori fault orientation and location information. This system tightly integrates seismic alarm systems (CISN-ShakeAlert, ElarmS-2) as it uses their P-wave detections to trigger its processing; quality control runs continuously. We use a synthetic Hayward Fault earthquake scenario on real-time streams to demonstrate recovery of slip and magnitude. Re-analysis of the M w 7.2 El Mayor-Cucapah earthquake tests the impact of dynamic motions on offset estimation. Using these test cases, we explore sensitivities to disturbances of a-priori constraints (origin time, location, fault strike/dip).

Description: The Central Asian Orogenic Belt (CAOB) is a typical accretionary orogen divided into numerous lithostratigraphic terranes. In theory, these terranes should be characterized by contrasting magnetic and gravity signatures owing to their dissimilar petrophysical properties. To test this hypothesis, the extent of tectonostratigraphic terranes in southern Mongolia was compared with the potential fields data. The analysis reveals that the terranes boundaries are not systematically defined by strong gravity and magnetic gradients. The correlation of the magnetic signal with the geology reveals that the magnetic highs coincide with late Carboniferous-early Permian volcanic-plutonic belts. The matched-filtering shows a good continuity of signal along the boundaries of these high magnetic anomalies towards the deeper crustal levels which may indicate the presence of deeply rooted tectono-magmatic zones. The axes of high density bodies in the western and central part of the study area are characterized by periodic alternations of NW-SE trending gravity anomalies corresponding to up to 20 km wide cleavage fronts of Permo-Triassic age. The matched-filtering analysis shows good continuity of signal to the depth of these gravity highs which may indicate presence of deeply rooted high-strain zones. The magnetic signal is interpreted to be as the result of a giant Permo-Triassic magmatic event associated with lithosphere scale deformation whereas the gravity pattern is related to the post-accretionary shortening of the CAOB between the North China and Siberia cratons.

Description: Extension, faulting and magmatism are the main controls on the magnitude and localization of strain at mid-ocean ridges. However, the temporal and spatial patterns of such processes are not clear since the strain distribution has not been resolved in the past at sufficient spatial resolution and over extended areas. InSAR and GPS data with unprecedented resolution are now available to us from the Afar rift of Ethiopia. Here we use a velocity field method to combine InSAR and GPS to form the first high-resolution continuous three-dimensional velocity field of Afar. We study an area 500 km wide and 700 km long, covering three branches of the Afar continental rift and their triple junctions. Our velocity field shows that plate spreading is currently achieved in Afar in contrasting modes. A transient post-diking deformation is focused at the Dabbahu rift segment while in central Afar spreading is distributed over several overlapping segments, and southern Afar exhibits an inter-diking deformation pattern focused at the Asal-Ghoubbet segment. We find that current spreading rates at Dabbahu, following the 2005–2010 intrusions, are up to 110 mm/yr, six times larger than the long-term plate divergence. A segment centered uplift of up to 80 mm/yr also occurs, indicating that magma flow is still a primary mechanism of deformation during post-diking. On the other hand, no vertical displacements are currently observed in central and southern Afar suggesting lack of significant magmatic activity at shallow levels.

Description: The mechanical behavior – and hence earthquake potential – of faults in continental interiors is an issue of critical importance for the resultant seismic hazard, but no consensus has yet been reached on this controversial topic. The debate has focused on the central and eastern United States, in particular the New Madrid Seismic Zone, struck by four magnitude 7 or greater earthquakes in 1811–1812, and to a lesser extent the Wabash Valley Seismic Zone just to the north. A key aspect of thisissue is the rate at which strain is currently accruing on those plate interior faults, a quantity that remains debated. Here we address this issue with an analysis of up to 14.6 years of continuous GPS data from a network of 200 sites in the central United States centered on the New Madrid and Wabash Valley seismic zones. We find that all the high quality sites in these regions show motions that are consistently within the 95% confidence limit of zero deformation. These results place an upper bound on strain accrual on faults of 0.2 mm/yr and 0.6 mm/yr in the New Madrid and Wabash Valley Seismic Zones, respectively. For the New Madrid region, where a paleoseismic record is available for the past ~5,000 years, we argue that strain accrual – if any – does not permit the 500–900 year repeat time of paleo-earthquakes observed in the Upper Mississippi Embayment. These results, together with increasing evidence for temporal clustering and spatial migration of earthquake sequences in continental interiors, indicate that either tectonic loading rates or fault properties vary with time in the NMSZ and possibly plate-wide.

Description: In this paper, we compare the empirical results regarding foreshocks obtained from the Japan data [ Ogata et al ., 1995] with results for synthetic catalogs in order to clarify whether or not the corresponding results are consistent with the description of the seismicity by a superposition of background activity and epidemic-type aftershock sequences (ETAS models). This question is important, because it is still controversially discussed whether the nucleation process of large earthquakes is driven by seismically cascading (ETAS-type) or by aseismic accelerating processes. To explore the foreshock characteristics, we firstly applied the same clustering algorithms to real and synthetic catalogs and analyzed the temporal, spatial and magnitude distributions of the selected foreshocks. Most properties are qualitatively the same in the real data and in synthetic catalogs. However we find some quantitative differences particularly in the temporal acceleration, spatial convergence, and magnitude dependence, which also depend on the assumed synthetic catalogs. Furthermore we calculated forecast scores based on a single-link cluster algorithm which could be appropriate for real-time applications [ Ogata et al ., 1996; Ogata and Katsura , 2012]. We find that the JMA catalog yields higher scores than all synthetic catalogs and that the ETAS models having the same magnitude sequence as the original catalog performs better (more close to the reality) than ETAS-models with randomly picked magnitudes. We also find that the ETAS model that takes account of the triggering effect by small earthquakes below threshold magnitude performs more closely to the reality.

Description: Leading theories for the presence of plate tectonics on Earth typically appeal to the role of present day conditions in promoting rheological weakening of the lithosphere. However, it is unknown whether the conditions of the early Earth were favorable for plate tectonics, or any form of subduction, and thus how subduction begins is unclear. Using physical models based on grain-damage, a grainsize-feedback mechanism capable of producing plate-like mantle convection, we demonstrate that subduction waspossible on the Hadean Earth (hereafter referred to as proto-subduction or proto-plate tectonics), that proto-subduction differed from modern day plate tectonics, and that it could initiate rapidly. Scaling laws for convection with grain-damage show that, though either higher mantle temperatures or higher surface temperatures lead to slower plates, proto-subduction, with plate speeds of ≈ 1.75 cm/yr, can still be maintained in the Hadean, even with a CO 2 rich primordial atmosphere. Furthermore, when the mantle potential temperature is high (e.g. above ≈ 2000 K), the mode of subduction switches to a “sluggish subduction" style, where downwellings are drip-like and plate boundaries are diffuse. Finally, numerical models of post-magma ocean mantle convection demonstrate that proto-plate tectonics likely initiates within ~100 Myrs of magma ocean solidification, consistent with evidence from Hadean zircons. After the initiation of proto-subduction, non-plate-tectonic“sluggish subduction" prevails, giving way to modern style plate tectonics as both the mantle interior and climate cool. Hadean proto-subduction may hasten the onset of modern plate tectonics by drawing excess CO 2 out of the atmosphere and cooling the climate.

Description: Surface processes and inherited structures are widely regarded as factors that strongly influence the evolution of mountain belts. The first-order effects of these parameters have been studied extensively throughout the last decades, but their relative importance remains notoriously difficult to assess and document. We use lithospheric scale plane-strain thermo-mechanical model experiments to study the effects of surface processes and extensional inheritance on the internal structure of contractional orogens and their foreland basins. Extensional inheritance is modeled explicitly by forward modeling the formation of a rift basin before reversing the velocity boundary conditions to model its inversion. Surface processes are modeled through the combination of a simple sedimentation algorithm, where all negative topography is filled up to a prescribed reference level, and an elevation-dependent erosion model. Our results show that (1) extensional inheritance facilitates the propagation of basement deformation in the retro-wedge and (2) increases the width of the orogen; (3) sedimentation increases the length-scale of both thin-skinned and thick-skinned thrust sheets and (4) results in a wider orogen; (5) erosion helps to localize deformation resulting in a narrower orogen and a less well developed retro-wedge. A comparison of the modeled behaviors to the High Atlas, the Pyrenees and the Central Alps, three extensively studied natural examples characterized by different degrees of inversion, is presented and confirms the predicted controls of surface processes and extensional inheritance on orogenic structure.

Description: Creep processes may relax part of the tectonic stresses in active faults, either by continuous or episodic processes. The aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space. Results are presented from microstructural studies of natural samples collected from SAFOD borehole drilled through the San Andreas Fault, that reveal the chronology of the deformation within three domain types. (i) A relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution. (ii) Shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition: abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others, (iii) Zones with mainly dissolution trended towards the present-day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite. This study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep. In parallel, the authors discuss how the creeping mechanism, mainly controlled by the very low friction of the saponite in the first 3-4 km depth, may evolve with depth.

Description: Observations of heterogeneous and complex fault slip are often attributed to the complexity of fault structure and/or spatial heterogeneity of fault frictional behavior. Such complex slip patterns have been observed for earthquakes on normal faults throughout central Italy, where many of the M w 6 to 7 earthquakes in the Apennines nucleate at depths where the lithology is dominated by carbonate rocks. To explore the relationship between fault structure and heterogeneous frictional properties, we studied the exhumed Monte Maggio Fault (MMF), located in the northern Apennines. We collected intact specimens of the fault zone, including the principal slip surface and hanging wall cataclasite, and performed experiments at a normal stress of 10 MPa under saturated conditions. Experiments designed to reactivate slip between the cemented principal slip surface and cataclasite show a 3 MPa stress drop as the fault surface fails, then velocity-neutral frictional behavior and significant frictional healing. Overall, our results suggest that 1) earthquakes may readily nucleate in areas of the fault where the slip surface separates massive limestone and are likely to propagate in areas where fault gouge is in contact with the slip surface; 2) postseismic slip is more likely to occur in areas of the fault where gouge is present; and 3) high rates of frictional healing and low creep relaxation observed between solid fault surfaces could lead to significant aftershocks in areas of low stress drop.