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The 2021 M7.4 Maduo (China) earthquake ruptured a 170 km-long left-lateral fault within the Bayan Har tectonic block in the northeast Tibetan Plateau. We use Sentinel-1 and ALOS-2 Interferometric Synthetic Aperture Radar, and Global Navigation Satellite System data to investigate the mechanisms of coseismic and postseismic deformation due to the Maduo earthquake. We present a refined coseismic slip model that features variations in both strike and dip angles, constrained by the rupture trace and precisely located aftershocks. The postseismic displacements are discontinuous along the fault trace, indicating shallow afterslip and velocity-strengthening friction in the top 2-3 km of the upper crust. Postseismic displacements that have the same sense as the coseismic ones are also observed at larger (> 50 km) distances away from the fault trace. The observed surface deformation is qualitatively consistent with either deep afterslip or viscoelastic relaxation, but does not exhibit obvious features that could be attributed to poroelastic effects. We developed a fully coupled model that accounts for both stress-driven creep on a deep localized shear zone and viscoelastic relaxation in the bulk of the lower crust. The mid- to near-field data can be reasonably well explained by either deep afterslip or non-Maxwellian visco-elasticity. However, a good fit to both the near and far-field (> 150 km) GNSS data cannot be achieved assuming the bulk viscoelastic relaxation alone, and requires a contribution of deep afterlip and/or a localized shear zone extending through much of the lower crust.

We use new geologic and paleoseismic data to demonstrate that the past 6 major earthquakes on the Southern San Andreas Fault (SSAF) correlate with high-stands of the ancient Lake Cahuilla, a ~236 km^3 body of water adjacent to the SSAF. In order to investigate possible causal relationships, we computed time-dependent Coulomb stress changes due to variations in the lake level over the last ~1100 years. Simulations were performed using a fully coupled 3-D finite element model incorporating a poroelastic crust overlying a viscoelastic mantle. We find that the Coulomb stress perturbations on the SSAF are positive (i.e., promoting failure) throughout the lake loading history. For a plausible range of lake ages and material properties of the Earth's crust, the estimated stress perturbations are of the order of 0.5 MPa, likely sufficient for triggering. Stress perturbations are dominated by pore pressure changes, but are enhanced by the poroelastic "memory" effect whereby increases in pore pressure due to previous lake high stands do not completely vanish by diffusion and constructively interfere with the undrained response in subsequent high stands. Our preferred model suggests that the lake loading complemented the interseismic stress accumulation on average by as much as 16-44%. The destabilizing effects of lake inundation are enhanced by a non-vertical fault dip, presence of a fault damage zone, and lateral pore pressure diffusion. Our model may be applicable to other regions where hydrologic loading, either natural or anthropogenic, was associated with significant seismicity.

The July 2019 Ridgecrest, California, earthquake sequence involved two large events—the M 6.4 foreshock and the M 7.1 mainshock that ruptured a system of intersecting strike-slip faults. We present analysis of space geodetic observations including Synthetic Aperture Radar and Global Navigation Satellite System data, geological field mapping, and seismicity to constrain the subsurface rupture geometry and slip distribution. The data render a complex pattern of faulting with a number of subparallel as well as cross-cutting fault strands that exhibit variations in both strike and dip angles, including a “flower structure” formed by shallow splay faults. Slip inversions are performed using both homogeneous and layered elastic half-space models informed by the local seismic tomography data. The inferred slip distribution suggests a moderate amount of the shallow coseismic slip deficit. The peak moment release occurred in the depth interval of 3–4 km, consistent with results from previous studies of major strike-slip earthquakes, and the depth distribution of seismicity in California. We use the derived slip models to investigate stress transfer and possible triggering relationships between the M 7.1 mainshock and the M 6.4 foreshock, as well as other moderate events that occurred in the vicinity of the M 7.1 hypocenter. Triggering is discouraged for the average strike of the M 7.1 rupture (320°) but encouraged for the initial orientation of the mainshock rupture suggested by the first-motion data (340°). This lends support to a scenario according to which the earthquake rupture nucleated on a small fault that was more optimally oriented with respect to the regional stress and subsequently propagated along the less-favorably oriented pre-existing faults, possibly facilitated by dynamic weakening. The nucleation site of the mainshock experienced positive dynamic Coulomb stress changes that are much larger than the static stress changes, yet the former failed to initiate rupture.

Author(s): Floyd, Michael; Funning, Gareth; Fialko, Yuri; Terry, Rachel; Herring, Thomas | Abstract: Abstract The Mwn6.4 and Mwn7.1 Ridgecrest, California, earthquakes of July 2019 occurred within 34nhr of each other on conjugate strike-slip faults in the Mojave Desert, just north of the central Garlock fault. Here, we present the results of a survey of 18 Global Navigation Satellite Systems (GNSS) sites conducted in the immediate aftermath of the earthquakes, including five sites that recorded the motion of the second earthquake after having been set up immediately following the first, as well as processed results from continuous GNSS sites throughout the region. Our field work in response to the earthquakes provides additional constraints on the ground displacement due to both earthquakes, complementing data from a spatially sparser network of continuously recording GNSS sites in the area, as well as temporally sparser Interferometric Synthetic Aperture Radar data that were able to capture a combined deformation signal from the two earthquakes.

Key Points: • We use Sentinel-1 Synthetic Aperture Radar (SAR) data to derive a finite fault model for the 2021 M7.4 Maduo (Qinghai, China) earthquake • The along-strike averaged coseismic slip has...

The 2015 M7.2 Sarez (Pamir) earthquake occurred at the north-west margin of the Tibetan Plateau. We use Sentinel-1 and ALOS-2 Synthetic Aperture Radar and Global Navigation Satellite System data to investigate coseismic and postseismic deformation due to the Sarez earthquake. Kinematic inversions show that the earthquake ruptured a ∼80 km long, sub-vertical fault producing the maximum surface offset of 3–4 m on the south-west and central fault segments. In contrast, the largest postseismic displacements are observed at the north-east end of the earthquake rupture, predominantly on the west (hanging wall) side of the fault with an average rate of 20–30 mm/yr in the satellite line of sight. We use the derived coseismic and postseismic slip models to investigate mechanisms of time-dependent relaxation, stress transfer and possible triggering relationships between the Sarez earthquake and a sequence of strong M6+ events that occurred within ∼100 km of the 2015 earthquake. We find that the near-field postseismic displacements are best explained by shallow afterslip driven by the coseismic stress changes. The data also allow some contribution from poroelastic rebound, but do not show a clear signature of viscoelastic relaxation in the lower crust and upper mantle during the observation period, suggesting a lower bound on the effective viscosity of ∼1019 Pa s. A pair of M6+ events that occurred within 100 km and several months of the 2015 mainshock have experienced near-zero and in some cases negative static Coulomb stress changes, suggesting either delayed dynamic triggering, or no relation to the mainshock.

Strength of the upper brittle part of the Earth's lithosphere controls deformation styles in tectonically active regions, surface topography, seismicity, and the occurrence of plate tectonics, yet it remains one of the least constrained and most debated quantities in geophysics. Seismic data (in particular, earthquake focal mechanisms) have been used to infer orientation of the principal stress axes. Here I show that the focal mechanism data can be combined with information from precise earthquake locations to place robust constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses machine learning to identify quasi-linear clusters of seismicity associated with active faults. A distribution of the relative attitudes of conjugate faults carries information about the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. The observed diversity of dihedral angles between conjugate faults in the Ridgecrest (California, USA) area that hosted a recent sequence of strong earthquakes suggests the effective coefficient of friction of 0.4-0.6, and depth-averaged shear stresses on the order of 25-40 MPa, intermediate between predictions of the "strong" and "weak" fault theories.

We present new Interferometric Synthetic Aperture Radar (InSAR) observations of surface deformation in the Altiplano–Puna region (South America) where previous studies documented a broad uplift at an average rate of ∼10 mm/yr. We use data from the Sentinel-1 satellite mission to produce high-resolution velocity maps and time series of surface displacements between years 2014–2017. The data reveal that the uplift has slowed down substantially compared to the 1992–2010 epoch and is characterized by short-term fluctuations on time scales of months to years. The observed variations in uplift rate may indicate a non-steady supply of melt and/or volatiles from the partially molten Altiplano–Puna Magma Body (APMB) into an incipient diapir forming in the roof of the APMB.

We use space geodetic data to investigate coseismic and postseismic deformation due to the 2015 Mw 7.8 Gorkha earthquake that occurred along the central Himalayan arc. Because the earthquake area is characterized by strong variations in surface relief and material properties, we developed finite element models that explicitly account for topography and 3-D elastic structure. We computed the line-of-sight displacement histories from three tracks of the Sentinel-1A/B Interferometric Synthetic Aperture Radar (InSAR) satellites, using persistent scatter method. InSAR observations reveal an uplift of up to ∼70 mm over ∼20 months after the main shock, concentrated primarily at the downdip edge of the ruptured asperity. GPS observations also show uplift, as well as southward movement in the epicentral area, qualitatively similar to the coseismic deformation pattern. Kinematic inversions of GPS and InSAR data and forward models of stress-driven creep suggest that the observed postseismic transient is dominated by afterslip on a downdip extension of the seismic rupture. A poroelastic rebound may have contributed to the observed uplift and southward motion, but the predicted surface displacements are small. We also tested a wide range of viscoelastic relaxation models, including 1-D and 3-D variations in the viscosity structure. Models of a low-viscosity channel previously invoked to explain the long-term uplift and variations in topography at the plateau margins predict opposite signs of horizontal and vertical displacements compared to those observed. Our results do not preclude a possibility of deep-seated viscoelastic response beneath southern Tibet with a characteristic relaxation time greater than the observation period (2 years).

We present RADARSAT-2 Differential Interferometric Synthetic Aperture Radar (DInSAR) observations of deformation due to fluid extraction at the Cerro Prieto Geothermal Field (CPGF) and afterslip on the 2010 M7.2 El Mayor-Cucapah (EMC) earthquake rupture during 2011–2016. Advanced multidimensional time series analysis reveals subsidence at the CPGF with the maximum rate greater than 100 mm/yr accompanied by horizontal motion (radial contraction) at a rate greater than 30 mm/yr. During the same time period, more than 30 mm of surface creep occurred on the Indiviso fault ruptured by the EMC earthquake. We performed inversions of DInSAR data to estimate the rate of volume changes at depth due to the geothermal production at the CPGF and the distribution of afterslip on the Indiviso fault. The maximum coseismic slip due to the EMC earthquake correlates with the Coulomb stress changes on the Indiviso fault due to fluid extraction at the CPGF. Afterslip occurs on the periphery of maximum coseismic slip areas. Time series analysis indicates that afterslip still occurs 6 years after the earthquake.

Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have made great progress over the past decades to address important questions in earthquake physics and fault mechanics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between aseismic fault slip, earthquake nucleation, and dynamic rupture; and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing capability and complexity of SEAS modeling calls for extensive efforts to verify codes and advance these simulations with rigor, reproducibility, and broadened impact. In 2018, we initiated a community code-verification exercise for SEAS simulations, supported by the Southern California Earthquake Center (SCEC). Here we report the findings from our first two benchmark problems (BP1 and BP2), designed to test the capabilities of different computational methods in correctly solving a mathematically well-defined, basic problem in crustal faulting. These benchmarks are for a 2D antiplane problem, with a 1D planar vertical strike-slip fault obeying rate-and-state friction, embedded in a 2D homogeneous, linear elastic half-space. Sequences of quasi-dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of >70 simulation results from 11 groups using different numerical methods, uploaded to our online platform, show excellent agreements in long-term and coseismic evolution of fault properties. In BP1, we found that the truncated domain boundaries influence interseismic fault stressing, earthquake recurrence, and coseismic rupture process, and that agreement between models is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of long-term fault behavior depends on how well important physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in the generation of artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard, such as earthquake size distributions, moment release, and earthquake recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

The deep limit to seismicity in continental crust is believed to be controlled by a transition from velocity-weakening to velocity-strengthening friction based on experimental measurements of the rate dependence of friction at different temperatures. Available experimental data on granite suggest a transition to stable creep at about 350 ° C (∼15 km depth). Here we present results from unconfined experiments on Westerly granite at both dry and hydrated conditions that show increasingly unstable slip (velocity-weakening behavior) at temperature up to 600 ° C. A comparison of previously published experimental results with those presented in this study suggests that the rate and state friction parameters strongly depend on normal stress and pore pressure at high (>400 ° C) temperature, which may help explain regional variations in the depth distribution of earthquakes in continental crust. Temperature dependence of the rate and state friction parameters may also contribute to strong dynamic weakening observed in high-speed friction experiments on crystalline rocks such as granite and gabbro.

Author(s): Funning, Gareth J; Brooks, Benjamin; Fialko, Yuri; Floyd, Michael; Haase, Jennifer; Hammond, William; Sandwell, David T; Svarc, Jerry; Xu, Xiaohua

Geothermal energy is an important source of renewable energy, yet its production is known to induce seismicity. Here we analyze seismicity at the three largest geothermal fields in California: The Geysers, Salton Sea, and Coso. We focus on resolving the temporal evolution of seismicity rates, which provides important observational constraints on how geothermal fields respond to natural and anthropogenic loading. We develop an iterative, regularized inversion procedure to partition the observed seismicity rate into two components: (1) the interaction rate due to earthquake-earthquake triggering and (2) the smoothly varying background rate controlled by other time-dependent stresses, including anthropogenic forcing. We apply our methodology to compare long-term changes in seismicity to monthly records of fluid injection and withdrawal. At The Geysers, we find that the background seismicity rate is highly correlated with fluid injection, with the mean rate increasing by approximately 50% and exhibiting strong seasonal fluctuations following construction of the Santa Rosa pipeline in 2003. In contrast, at both Salton Sea and Coso, the background seismicity rate has remained relatively stable since 1990, though both experience short-term rate fluctuations that are not obviously modulated by geothermal plant operation. We also observe significant temporal variations in Gutenberg-Richter b value, earthquake magnitude distribution, and earthquake depth distribution, providing further evidence for the dynamic evolution of stresses within these fields. The differing field-wide responses to fluid injection and withdrawal may reflect differences in in situ reservoir conditions and local tectonics, suggesting that a complex interplay of natural and anthropogenic stressing controls seismicity within California's geothermal fields.

We conducted a series of experiments to explore the rate and state frictional properties of gabbro at conditions thought to be representative of slow slip events (SSEs) in subduction zones. The experiments were conducted using a heated direct shear apparatus. We tested both solid and simulated gouge samples at low effective normal stress (5-30 MPa) over a broad range of temperatures (20–600°C) under dry and hydrated conditions. In tests performed on dry solid samples we observed stable sliding at low temperatures (20–150°C), stick-slip at high temperatures (350–600°C) and a transitional “episodic slow slip” behavior at intermediate temperatures (200–300°C). In tests performed on dry gouge samples we observed stable sliding at all temperatures. Under hydrated conditions, the gouge samples exhibited episodic slow slip and stick-slip behavior at temperatures between 300–500°C. Our results show a decrease in the rate parameter (a– b) with temperature for both solid and gouge samples; friction transitions from velocity-strengthening to velocity-weakening at temperature of about 150°C for both solid and gouge samples. We do not observe transition to velocity-strengthening friction at the high end of the tested temperature range. Our results suggest that the occurrence of slow slip events and the downdip limit of the seismogenic zone on subduction megathrusts cannot be solely explained by the temperature dependence of frictional properties of gabbro. Further experimental studies are needed to evaluate the effects of water fugacity and compositional heterogeneity (e.g., the presence of phyllosilicates) on frictional stability of subduction megathrusts. This article is protected by copyright. All rights reserved.

We use surface deformation measurements including Interferometric Synthetic Aperture Radar data acquired by the ALOS-2 mission of the Japanese Aerospace Exploration Agency and Global Positioning System (GPS) data to invert for the fault geometry and coseismic slip distribution of the 2015 Mw 7.8 Gorkha earthquake in Nepal. Assuming that the ruptured fault connects to the surface trace of the Main Frontal Thrust (MFT) fault between 84.34°E and 86.19°E, the best fitting model suggests a dip angle of 7°. The moment calculated from the slip model is 6.08 × 1020 Nm, corresponding to the moment magnitude of 7.79. The rupture of the 2015 Gorkha earthquake was dominated by thrust motion that was primarily concentrated in a 150 km long zone 50 to 100 km northward from the surface trace of the Main Frontal Thrust (MFT), with maximum slip of ∼ 5.8 m at a depth of ∼8 km. Data thus indicate that the 2015 Gorkha earthquake ruptured a deep part of the seismogenic zone, in contrast to the 1934 Bihar-Nepal earthquake, which had ruptured a shallow part of the adjacent fault segment to the east.

We present a method for estimating radar phase delays due to propagation through the troposphere and the ionosphere based on the averaging of redundant interferograms that share a common scene. Estimated atmospheric contributions can then be subtracted from the radar interferograms to improve measurements of surface deformation. Inversions using synthetic data demonstrate that this procedure can considerably reduce scatter in the time series of the line-of-sight displacements. We demonstrate the feasibility of this method by comparing the interferometric synthetic aperture radar (InSAR) time series derived from ERS-1/2 and Envisat data to continuous Global Positioning System data from eastern California. We also present results from several sites in the eastern California shear zone where anomalous deformation has been reported by previous studies, including the Blackwater fault, the Hunter Mountain fault, and the Coso geothermal plant.

We investigate the spatial pattern of surface creep and off-fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey-mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003-2010 were used to separate the pattern of horizontal and vertical motion, providing a high-resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1-2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two-dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long-term slip rates.

We use the L-band Advanced Land Observing Satellite (ALOS) and C-band Envisat interferometric synthetic aperture data and campaign GPS observations to study the postseismic deformation due to the 2005 magnitude 7.6 Kashmir (Pakistan) earthquake that occurred in the northwestern Himalaya. Envisat data are available from both the descending and ascending orbits and span a time period of ~4.5 years immediately following the earthquake (2005–2010), with nearly monthly acquisitions. However, the Envisat data are highly decorrelated due to high topography and snow cover. ALOS data are available from the ascending orbit and span a time period of ~2.5 years between 2007 and 2009, over which they remain reasonably well correlated. We derive the mean line-of-sight (LOS) postseismic velocity maps in the epicentral area of the Kashmir earthquake using persistent scatterer method for Envisat data and selective stacking for ALOS data. LOS velocities from all data sets indicate an uplift (decrease in radar range), primarily in the hanging wall of the earthquake rupture over the entire period of synthetic aperture radar observations (2005–2010). Models of poroelastic relaxation predict uplift of both the footwall and the hanging wall, while models of viscoelastic relaxation below the brittle-ductile transition predict subsidence (increase in radar range) in both the footwall and the hanging wall. Therefore, the observed pattern of surface velocities indicates that the early several years of postseismic deformation were dominated by afterslip on the fault plane, possibly with a minor contribution from poroelastic rebound. Kinematic inversions of interferometric synthetic aperture radar and GPS data confirm that the observed deformation is consistent with afterslip, primarily downdip of the seismic asperity. To place constraints on the effective viscosity of the ductile substrate in the study area, we subtract the surface deformation predicted by stress-driven afterslip model from the mean LOS velocities and compare the residuals to models of viscoelastic relaxation for a range of assumed viscosities. We show that in order to prevent surface subsidence, the effective viscosity has to be greater than 1019 Pa s.

Author(s): Gonzalez-Ortega, A; Fialko, Y; Sandwell, D; Nava-Pichardo, FA; Fletcher, J; Gonzalez-Garcia, J; Lipovsky, B; Floyd, M; Funning, G | Abstract: El Mayor-Cucapah earthquake occurred on 4 April 2010 in northeastern Baja California just south of the U.S.-Mexico border. The earthquake ruptured several previously mapped faults, as well as some unidentified ones, including the Pescadores, Borrego, Paso Inferior and Paso Superior faults in the Sierra Cucapah, and the Indiviso fault in the Mexicali Valley and Colorado River Delta.We conducted several Global Positioning System (GPS) campaign surveys of preexisting and newly established benchmarks within 30 km of the earthquake rupture. Most of the benchmarks were occupied within days after the earthquake, allowing us to capture the very early postseismic transient motions. The GPS data show postseismic displacements in the same direction as the coseismic displacements; time series indicate a gradual decay in postseismic velocities with characteristic time scales of 66 ± 9 days and 20 ± 3 days, assuming exponential and logarithmic decay, respectively. We also analyzed interferometric synthetic aperture radar (InSAR) data from the Envisat and ALOS satellites. The main deformation features seen in the line-of-sight displacement maps indicate subsidence concentrated in the southern and northern parts of the main rupture, in particular at the Indiviso fault, at the Laguna Salada basin, and at the Paso Superior fault. We show that the near-field GPS and InSAR observations over a time period of 5 months after the earthquake can be explained by a combination of afterslip, fault zone contraction, and a possible minor contribution of poroelastic rebound. Far-field data require an additional mechanism, most likely viscoelastic relaxation in the ductile substrate. ©2014. American Geophysical Union.

We investigate interseismic deformation across the San Jacinto fault at Anza, California where previous geodetic observations have indicated an anomalously high shear strain rate. We present an updated set of secular velocities from GPS and InSAR observations that reveal a 2–3 km wide shear zone deforming at a rate that exceeds the background strain rate by more than a factor of two. GPS occupations of an alignment array installed in 1990 across the fault trace at Anza allow us to rule out shallow creep as a possible contributor to the observed strain rate. Using a dislocation model in a heterogeneous elastic half space, we show that a reduction in shear modulus within the fault zone by a factor of 1.2–1.6 as imaged tomographically by Allam and Ben-Zion (Geophys J Int 190:1181–1196, 2012) can explain about 50 % of the observed anomalous strain rate. However, the best-fitting locking depth in this case (10.4 ± 1.3 km) is significantly less than the local depth extent of seismicity (14–18 km). We show that a deep fault zone with a shear modulus reduction of at least a factor of 2.4 would be required to explain fully the geodetic strain rate, assuming the locking depth is 15 km. Two alternative possibilities include fault creep at a substantial fraction of the long-term slip rate within the region of deep microseismicity, or a reduced yield strength within the upper fault zone leading to distributed plastic failure during the interseismic period.

[1] We present three-dimensional (3-D) numerical models of postseismic deformation following repeated earthquakes on a vertical strike-slip fault. Our models use linear Maxwell, Burgers, and temperature-dependent power law rheology for the lower crust and upper mantle. We consider effects of viscous shear zones that result from thermomechanical coupling and investigate potential kinematic similarities between viscoelastic models incorporating shear zones and elastic models incorporating rate-strengthening friction on a deep aseismic fault root. We find that the thermally activated shear zones have little effect on postseismic relaxation. In particular, the presence of shear zones does not change the polarity of vertical displacements in cases of rheologies that are able to generate robust postseismic transients. Stronger rheologies can give rise to an opposite polarity of vertical displacements, but the amplitude of the predicted transient deformation is generally negligible. We conclude that additional (to thermomechanical coupling) mechanisms of strain localization are required for a viscoelastic model to produce a vertical deformation pattern similar to that due to afterslip on a deep extension of a fault. We also investigate the discriminating power of models incorporating Burgers and power law rheology. These rheologies were proposed to explain postseismic transients following large (M7) earthquakes in the Mojave desert, Eastern California. Numerical simulations indicate that it may be difficult to distinguish between these rheologies even with high-quality geodetic observations for observation periods less than a decade. Longer observations, however, may potentially allow discrimination between the competing models, as illustrated by the model comparisons with available GPS and interferometric synthetic aperture radar data.

[1] The Salton Trough represents a complex transition between the spreading center in Baja California and the strike-slip San Andreas fault system and is one of the most active zones of deformation and seismicity in California. We present a high-resolution interseismic velocity field for the Salton Trough derived from 74 continuous GPS sites and 109 benchmarks surveyed in three GPS campaigns during 2008–2009 and previous surveys between 2000 and 2005. We also investigate small-scale deformation by removing the regional velocity field predicted by an elastic block model for Southern California from the observed velocities. We find a total extension rate of 11 mm/yr from the Mesquite Basin to the southern edge of the San Andreas Fault, coupled with 15 mm/yr of left-lateral shear, the majority of which is concentrated in the southern Salton Sea and Obsidian Buttes and is equivalent to 17 mm/yr oriented in the direction of the San Andreas Fault. Differential shear strain is exclusively localized in the Brawley Seismic Zone, and dilatation rate indicates widespread extension throughout the zone. In addition, we infer clockwise rotation of 10°/Ma, consistent with northwestward propagation of the Brawley Seismic Zone over geologic time.

[1] We use high resolution interferometric synthetic aperture radar and GPS measurements of crustal motion across the southern San Andreas Fault system to investigate the effects of elastic heterogeneity and fault geometry on inferred slip rates and locking depths. Geodetically measured strain rates are asymmetric with respect to the mapped traces of both the southern San Andreas and San Jacinto faults. Two possibilities have been proposed to explain this observation: large contrasts in crustal rigidity across the faults, or an alternate fault geometry such as a dipping San Andreas fault or a blind segment of the San Jacinto Fault. We evaluate these possibilities using a two-dimensional elastic model accounting for heterogeneous structure computed from the Southern California Earthquake Center crustal velocity model CVM-H 6.3. The results demonstrate that moderate variations in elastic properties of the crust do not produce a significant strain rate asymmetry and have only a minor effect on the inferred slip rates. However, we find that small changes in the location of faults at depth can strongly impact the results. Our preferred model includes a San Andreas Fault dipping northeast at 60 , and two active branches of the San Jacinto fault zone. In this case, we infer nearly equal slip rates of 18 1 and 19 2mm/yr for the San Andreas and San Jacinto fault zones, respectively. These values are in good agreement with geologic measurements representing average slip rates over the last 10–10 years, implying steady long-term motion on these faults.

[1] We present high-resolution measurements of interseismic deformation along the central section of the North Anatolian Fault (NAF) in Turkey using interferometric synthetic aperture radar data from the Advanced Land Observing Satellite and Envisat missions. We generated maps of satellite line-of-sight velocity using five ascending Advanced Land Observing Satellite tracks and one descending Envisat track covering the NAF between 31.2°E and 34.3°E. The line-of-sight velocity reveals discontinuities of up to ∼5 mm/yr across the Ismetpasa segment of the NAF, implying surface creep at a rate of ∼9 mm/yr; this is a large fraction of the inferred slip rate of the NAF (21–25 mm/yr). The lateral extent of significant surface creep is about 75 km. We model the inferred surface velocity and shallow fault creep using numerical simulations of spontaneous earthquake sequences that incorporate laboratory-derived rate and state friction. Our results indicate that frictional behavior in the Ismetpasa segment is velocity strengthening at shallow depths and transitions to velocity weakening at a depth of 3–6 km. The inferred depth extent of shallow fault creep is 5.5–7 km, suggesting that the deeper locked portion of the partially creeping segment is characterized by a higher stressing rate, smaller events, and shorter recurrence interval. We also reproduce surface velocity in a locked segment of the NAF by fault models with velocity-weakening conditions at shallow depth. Our results imply that frictional behavior in a shallow portion of major active faults with little or no shallow creep is mostly velocity weakening.

Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (∼0.1-1 m s(-1)), and at moderate normal stresses (1-10 MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting. The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood. The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt ('melt welts'), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1-0.4 MW m(-2), while the average temperature of the shear zone is well below the solidus (250-450 °C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics.

We investigate the capabilities and limitations of the Differential Interferometric Synthetic Aperture Radar (DInSAR) techniques, in particular of the Small BAseline Subset (SBAS) approach, to measure surface deformation in active seismogenetic areas. The DInSAR analysis of low-amplitude, long-wavelength deformation, such as that due to interseismic strain accumulation, is limited by intrinsic trade-offs between deformation signals and orbital uncertainties of SAR platforms in their contributions to the interferometric phases, the latter being typically well approximated by phase ramps. Such trade-offs can be substantially reduced by employing auxiliary measurements of the long-wavelength velocity field. We use continuous Global Positioning System (GPS) measurements from a properly distributed set of stations to perform a pre-filtering operation of the available DInSAR interferograms. In particular, the GPS measurements are used to estimate the secular velocity signal, approximated by a spatial ramp within the azimuth-range radar imaging plane; the phase ramps derived from the GPS data are then subtracted from the available set of DInSAR interferograms. This pre-filtering step allows us to compensate for the major component of the long-wavelength range change that, within the SBAS procedure, might be wrongly interpreted and filtered out as orbital phase ramps. With this correction, the final results are obtained by simply adding the pre-filtered long-wavelength deformation signal to the SBAS retrieved time series. The proposed approach has been applied to a set of ERS-1/2 SAR data acquired during the 1992-2006 time interval over a 200 x 200 km area around the Coachella Valley section of the San Andreas Fault in Southern California, USA. We present results of the comparison between the SBAS and the Line Of Sight (LOS)-projected GPS time series of the USGC/PBO network, as well as the mean LOS velocity fields derived using SBAS, GPS and stacking techniques. Our analysis demonstrates the effectiveness of the presented approach and provides a quantitative assessment of the accuracy of DInSAR measurements of interseismic deformation in a tectonically active area.

SUMMARY Slip inversions of geodetic data from several large (magnitude ∼7) strike-slip earthquakes point to coseismic slip deficit at shallow depths (

We present a unified continuum mechanics representation of the mechanisms believed to be commonly involved in post-seismic transients such as viscoelasticity, fault creep and poroelasticity. The time-dependent relaxation that follows an earthquake, or any other static stress perturbation, is considered in a framework of a generalized viscoelastoplastic rheology whereby some inelastic strain relaxes a physical quantity in the material. The relaxed quantity is the deviatoric stress in case of viscoelastic relaxation, the shear stress in case of creep on a fault plane and the trace of the stress tensor in case of poroelastic rebound. In this framework, the instantaneous velocity field satisfies the linear inhomogeneous Navier's equation with sources parametrized as equivalent body forces and surface tractions. We evaluate the velocity field using the Fourier-domain Green's function for an elastic half-space with surface buoyancy boundary condition. The accuracy of the proposed method is demonstrated by comparisons with finite-element simulations of viscoelastic relaxation following strike-slip and dip-slip ruptures for linear and power-law rheologies. We also present comparisons with analytic solutions for afterslip driven by coseismic stress changes. Finally, we demonstrate that the proposed method can be used to model time-dependent poroelastic rebound by adopting a viscoelastic rheology with bulk viscosity and work hardening. The proposed method allows one to model post-seismic transients that involve multiple mechanisms (afterslip, poroelastic rebound, ductile flow) with an account for the effects of gravity, non-linear rheologies and arbitrary spatial variations in inelastic properties of rocks (e.g. the effective viscosity, rate-and-state frictional parameters and poroelastic properties).

SUMMARY We present an analytic solution in the Fourier domain for an elastic deformation in a semi-infinite solid due to an arbitrary surface traction. We generalize the so-called Boussinesq's and Cerruti's problems to include a restoring buoyancy boundary condition at the surface. Buoyancy due to a large density contrast at the Earth's surface is an approximation to the full effect of gravity that neglects the perturbation of the gravitational potential and the change in density in the interior. Using the perturbation method, and assuming that the effect of gravity is small compared to the elastic deformation, we derive an approximation in the space domain to the Boussinesq's problem that accounts for a buoyancy boundary condition at the surface. The Fourier- and space-domain solutions are shown to be in good agreement. Numerous problems of elastostatic or quasi-static time-dependent deformation relevant to faulting in the Earth's interior (including inelastic deformation) can be modelled using equivalent body forces and surface tractions. Solving the governing equations with the elastic Green's function in the space domain can be impractical as the body force can be distributed over a large volume. We present a computationally efficient method to evaluate the elastic deformation in a 3-D half space due to the presence of an arbitrary distribution of internal forces and tractions at the surface of the half space. We first evaluate the elastic deformation in a periodic Cartesian volume in the Fourier domain, then use the analytic solutions to the generalized Boussinesq's and Cerruti's problems to satisfy the prescribed mixed boundary condition at the surface. We show some applications for magmatic intrusions and faulting. This approach can be used to solve elastostatic problems involving spatially heterogeneous elastic properties (by employing a homogenization method) and time-dependent problems such as non-linear viscoelastic relaxation, poroelastic rebound and non-steady fault creep under the assumption of spatially homogeneous elastic properties.

SUMMARY We present a semi-analytic iterative procedure for evaluating the 3-D deformation due to faults in an arbitrarily heterogeneous elastic half-space. Spatially variable elastic properties are modelled with equivalent body forces and equivalent surface traction in a ‘homogenized’ elastic medium. The displacement field is obtained in the Fourier domain using a semi-analytic Green function. We apply this model to investigate the response of 3-D compliant zones (CZ) around major crustal faults to coseismic stressing by nearby earthquakes. We constrain the two elastic moduli, as well as the geometry of the fault zones by comparing the model predictions to Synthetic Aperture Radar inferferometric (InSAR) data. Our results confirm that the CZ models for the Rodman, Calico and Pinto Mountain faults in the Eastern California Shear Zone (ECSZ) can explain the coseismic InSAR data from both the Landers and the Hector Mine earthquakes. For the Pinto Mountain fault zone, InSAR data suggest a 50 per cent reduction in effective shear modulus and no significant change in Poisson's ratio compared to the ambient crust. The large wavelength of coseismic line-of-sight displacements around the Pinto Mountain fault requires a fairly wide (∼1.9 km) CZ extending to a depth of at least 9 km. Best fit for the Calico CZ, north of Galway Dry Lake, is obtained for a 4 km deep structure, with a 60 per cent reduction in shear modulus, with no change in Poisson's ratio. We find that the required effective rigidity of the Calico fault zone south of Galway Dry Lake is not as low as that of the northern segment, suggesting along-strike variations of effective elastic moduli within the same fault zone. The ECSZ InSAR data is best explained by CZ models with reduction in both shear and bulk moduli. These observations suggest pervasive and widespread damage around active crustal faults.

During earthquakes, slip is often localized on preexisting faults, but it is not well understood how the structure of crustal faults may contribute to slip localization and energetics. Growing evidence suggests that the crust along active faults undergoes anomalous strain and damage during large earthquakes. Seismic and geodetic data from the Calico fault in the eastern California shear zone reveal a wide zone of reduced seismic velocities and effective elastic moduli. Using seismic traveltimes, trapped waves, and interferometric synthetic aperture radar observations, we document seismic velocities reduced by 40%‐ 50% and shear moduli reduced by 65% compared to wall rock in a 1.5-km-wide zone along the Calico fault. Observed velocity reductions likely represent the cumulative mechanical damage from past earthquake ruptures. No large earthquake has broken the Calico fault in historic time, implying that fault damage persists for hundreds or perhaps thousands of years. These fi ndings indicate that faults can affect rock properties at substantial distances from primary fault slip surfaces, and throughout much of the seismogenic zone, a result with implications for the amount of energy expended during rupture to drive cracking and yielding of rock and development of fault systems.