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Abstract: We discuss general structural features of the Banning and Mission Creek strands (BF and MCF) of the southern San Andreas fault (SSAF) in the Coachella Valley, based on ambient noise and earthquake wavefields recorded by a seismic array with >300 nodes. Earthquake P arrivals show rapid changes in waveform characteristics over 20–40 m zones that coincide with the surface BF and MCF. These variations indicate that the BF and MCF are high-impedance contrast interfaces—an observation supported by the presence of seismic reflections. Another prominent but more diffuse change in SSAF structure is found ∼1 km northeast of the BF. This feature has average-to-low arrival times (P and S) and ambient noise levels (at 2 times median) are consistently observed southwest of the BF—a combined effect of Coachella Valley sediments and rock damage on that side. Immediately northeast of the MCF, large S minus P delays suggest a broad high VP/VS zone associated with asymmetric rock damage across the SSAF. This general overview shows the BF and MCF as mature but distinctly different fault zones. Future analyses will further clarify these and other SSAF features in greater detail.

Abstract: The Mw 6.4 and Mw 7.1 Ridgecrest, California, earthquakes of July 2019 occurred within 34 hr 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.

Numerical simulations of sequences of earthquakes and aseismic slip (SEAS) have made great progress over past decades to address important questions in earthquake physics. However, significant challenges in SEAS modeling remain in resolving multiscale interactions between earthquake nucleation, dynamic rupture, and aseismic slip, and understanding physical factors controlling observables such as seismicity and ground deformation. The increasing 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. Here, we report the findings from our first two benchmark problems (BP1 and BP2), designed to verify different computational methods in solving a mathematically well-defined, basic faulting problem. We consider a 2D antiplane problem, with a 1D planar vertical strike-slip fault obeying rate-and-state friction, embedded in a 2D homogeneous, linear elastic halfspace. Sequences of quasi-dynamic earthquakes with periodic occurrences (BP1) or bimodal sizes (BP2) and their interactions with aseismic slip are simulated. The comparison of results from 11 groups using different numerical methods show excellent agreements in long-term and coseismic fault behavior. In BP1, we found that truncated domain boundaries influence interseismic stressing, earthquake recurrence, and coseismic rupture, and that model agreement is only achieved with sufficiently large domain sizes. In BP2, we found that complexity of fault behavior depends on how well physical length scales related to spontaneous nucleation and rupture propagation are resolved. Poor numerical resolution can result in artificial complexity, impacting simulation results that are of potential interest for characterizing seismic hazard such as earthquake size distributions, moment release, and recurrence times. These results inform the development of more advanced SEAS models, contributing to our further understanding of earthquake system dynamics.

Brittle deformation in the upper crust is thought to occur primarily via faulting. The fault length‐frequency distribution determines how much deformation is accommodated by numerous small faults versus a few large ones. To evaluate the amount of deformation due to small faults, we analyze the fault length distribution using high‐quality fault maps spanning a wide range of spatial scales from a laboratory sample to an outcrop to a tectonic domain. We find that the cumulative fault length distribution is well approximated by a power law with a negative exponent close to 2. This is in agreement with the earthquake magnitude‐frequency distribution (the Gutenberg‐Richter law with b‐value of 1), at least for faults smaller than the thickness of the seismogenic zone. It follows that faulting is a self‐similar process, and a substantial fraction of tectonic strain can be accommodated by faults that don't cut through the entire seismogenic zone, consistent with inferences of "hidden strain" from natural and laboratory observations. A continued accumulation of tectonic strain may eventually result in a transition from distributed fault networks to localized mature faults. Plain Language Summary: The Earth's crust is pervasively damaged, and contains faults, fractures, and joints of various sizes and orientations. We use mapped fault traces from multiple data sets spanning a wide range of scales to investigate how much deformation is accommodated by small versus large faults. The fault length distribution is often assumed to be fractal, that is, following a power law. The power‐law exponent α $\alpha $ quantifies the relative contributions of many small faults relative to a few large ones. For α≤1 $\alpha \le 1$, the contribution of small faults is negligible, while for α≥2 $\alpha \ge 2$, strains accommodated by small faults become significant. We find that the cumulative fault length distribution approximately follows a power law with an exponent α≥2 $\alpha \ge 2$. This implies that small faults in developing shear zones can accommodate a substantial fraction of tectonic strain. Key Points: We analyze the fault length‐frequency distribution in developing (structurally immature) fault systemsThe cumulative frequency distribution follows a power law over a range of fault length spanning 8 orders of magnitude, with a negative exponent of ∼2, consistent with the Gutenberg‐Richter lawSmall faults within the brittle upper crust can accommodate a substantial (>30%) fraction of tectonic strain [ABSTRACT FROM AUTHOR]

Fluid injection can induce seismicity by altering stresses on pre‐existing faults. Here, we investigate minimizing induced earthquake potential by optimizing injection operations in a physics‐based forecasting framework. We built a 3D finite element model of the poroelastic crust for the Raton Basin, Central US, and used it to estimate time dependent Coulomb stress changes due to ∼ ${\sim} $25 years of wastewater injection in the region. Our finite element model is complemented by a statistical analysis of the seismogenic index (SI), a proxy for critically stressed faults affected by variations in the pore pressure. Forecasts of seismicity rate from our hybrid physics‐based statistical model suggest that induced seismicity in the Raton Basin, from 2001 to 2022, is still driven by wastewater injection despite declining injection rates since 2011. Our model suggests that pore pressure diffusion is the dominant cause of Coulomb stress changes at seismogenic depth, with poroelastic stress changes contributing about 5% to the driving force. Linear programming optimization for the Raton Basin reveals that it is feasible to reduce earthquake potential for a given amount of injected fluid (safety objective) or maximize fluid injection for a prescribed earthquake potential (economic objective). The optimization tends to spread out high‐rate injectors and shift them to regions of lower SI. The framework has practical importance as a tool to manage injection rate per unit field area to reduce induced earthquake potential. Our optimization framework is both flexible and adaptable to mitigate induced earthquake potential in other regions and for other types of subsurface fluid injection. Plain Language Summary: The Raton Basin, in the central United States, has had a remarkable increase in seismicity coincident with large wastewater injection since 2001. This seismicity primarily occurs at depths greater than several kilometers where preexisting faults in the crystalline basement are reactivated by increasing pore pressure due to fluid diffusion. The spatial extent and rate of the induced earthquakes can inform earthquake probability maps which display the probability of an earthquake occurrence within a specific time period. We use the physics‐based and statistical models to develop an optimization framework that may help inform well operations. The proposed method allows for the maximization of injected fluid (the economic objective) and the reduction of earthquake potential (the safety objective). Key Points: Poroelastic and statistical model suggests induced seismicity in Raton Basin is still primarily driven by wastewater injectionOptimization reduces earthquake potential for a given amount of injected fluid or maximizes injection for a prescribed earthquake potentialOptimization tends to spread out higher rate injection wells and may be a useful tool to reduce basin‐scale induced earthquake potential [ABSTRACT FROM AUTHOR]

We apply the Matrix Profile algorithm to 100 days of continuous data starting 10 days before the 2019 M 6.4 and M 7.1 Ridgecrest earthquakes from borehole seismic station B921 near the Ridgecrest aftershock sequence. We identify many examples of reversely polarized waveforms, but focus on one particularly striking earthquake pair with strongly negatively correlated P and S waveforms at B921 and several other nearby stations. Waveform‐cross‐correlation‐based relocation of these events indicates they are at about 10 km depth and separated by only 115 m. Individual focal mechanisms are poorly resolved for these events because of the limited number of recording stations with unambiguous P polarities. However, relative P and S polarity and amplitude information can be used to constrain the likely difference in fault plane orientation between the two events to be 5–20°. We explore possible models to explain these observations, including low effective coefficients of fault friction and short‐wavelength stress heterogeneity caused by prior earthquakes. Although definitive conclusions are lacking, we favor local stress heterogeneity as being more consistent with other observations for the Ridgecrest region. Plain Language Summary: Earthquake focal mechanisms are estimated from seismic observations and provide valuable information on fault geometry and crustal stress orientation at depth. Most focal mechanisms are spatially correlated, that is, mechanisms tend to be similar to those of neighboring earthquakes. However, on rare occasions earthquake pairs are observed that appear nearly opposite in orientation, as evidenced by seismograms that are flipped in polarity. These extreme examples of focal mechanism diversity are valuable because they provide strong constraints on fault and stress properties at depth. Here we identify and study a particularly well‐recorded reverse‐polarity earthquake pair among aftershocks of the 2019 M6.4 and M7.1 earthquakes at Ridgecrest, California. Our analysis shows that they are at 10 km depth in the crust but only 115 m apart and that their fault planes differ in orientation by less than 20°. This implies either unusually low values of fault friction, which permit faults to slip even when they are far from their optimal faulting orientation, or strong changes in stress orientation at depth, perhaps caused by residual stresses from prior earthquakes. Key Points: We identify a pair of 2019 Ridgecrest earthquake aftershocks at 10 km depth with reverse polarity P and S wavetrains at several stationsThe events are about 115 m apart and have opposing focal mechanisms with fault planes 10–20° different in orientationThese results can be explained by either locally low effective fault friction or, more likely, strong short‐wavelength stress heterogeneity [ABSTRACT FROM AUTHOR]

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.

The Superstition Hills Fault (SHF) exhibits a rich spectrum of slip modes, including M 6+ earthquakes, afterslip, quasi‐steady creep, and both triggered and spontaneous slow slip events (SSEs). Following 13 years of quiescence, creepmeters recorded 25 mm of slip during 16–19 May 2023. Additional sub‐events brought the total slip to 41 mm. The event nucleated on the northern SHF in early‐May and propagated bi‐laterally at rates on the order of kilometers per day. Surface offsets reveal a bi‐modal slip distribution, with slip on the northern section of the fault being less localized and lower amplitude compared to the southern section. Kinematic slip models confirm systematic variations in the slip distribution along‐strike and with depth and suggest that slip is largely confined to the shallow sedimentary layer. Observations and models of the 2023 SSE bear a strong similarity to previous slip episodes in 1999, 2006, and 2010, suggesting a characteristic behavior. Plain Language Summary: Studying the mechanical properties and behavior of faults is essential for understanding earthquake ruptures. In this study, we investigate a recent slip event on the Superstition Hills Fault (SHF), which has a well‐documented record of slip. A notable aspect of the SHF is that it periodically undergoes "slow slip events" (SSEs), where the fault slips and releases energy without any accompanied ground shaking. During May‐July 2023, the SHF experienced a major SSE for the first time in 13 years. Our analysis shows that it was the largest documented SSE on the SHF and released equivalent energy to a magnitude 4.5 earthquake. We also find that the spatial pattern of fault slip is very similar to several previous slip events in 1999, 2006, and 2010, suggesting that the SHF has a tendency to slip in a characteristic manner. Key Points: We document a recent spontaneous slow slip event (SSE) on the Superstition Hills Fault using creepmeter, Interferometric Synthetic Aperture Radar, Global Navigation Satellite System, and field measurementsOver 41 mm of slip occurred from mid‐May to mid‐July 2023, with moment release corresponding to a Mw 4.5 earthquakeThe kinematics of the 2023 event are remarkably similar to several previous SSEs, suggesting a characteristic rupture process [ABSTRACT FROM AUTHOR]

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.

Our understanding of the earthquake process requires detailed insights into how the tectonic stresses are accumulated and released on seismogenic faults. We derive the full vector displacement field due to the Bam, Iran, earthquake of moment magnitude 6.5 using radar data from the Envisat satellite of the European Space Agency. Analysis of surface deformation indicates that most of the seismic moment release along the 20-km-long strike-slip rupture occurred at a shallow depth of 4 - 5 km, yet the rupture did not break the surface. The Bam event may therefore represent an end-member case of the 'shallow slip deficit' model, which postulates that coseismic slip in the uppermost crust is systematically less than that at seismogenic depths ( 4 - 10 km). The InSAR-derived surface displacement data from the Bam and other large shallow earthquakes suggest that the uppermost section of the seismogenic crust around young and developing faults may undergo a distributed failure in the interseismic period, thereby accumulating little elastic strain.

The major bends of the San Andreas fault in California are associated with significant variations in the along-fault topography. The topography-induced perturbations in the intermediate principal stress may result in the rotation of the fault with respect to the maximum compression axis provided that the fault is non-vertical, and the slip is horizontal. The progressive fault rotation may produce additional topography via thrust faulting in the adjacent crust, resulting in a positive feedback. The observed rotation of the fault plane due to the along-fault variations in topography is used to infer the magnitude of the in situ differential stress. Our results suggest that the average differential stress in the upper crust around the San Andreas fault is of the order of 50 MPa, implying that the effective fault strength is about a factor of two lower than predictions based on Byerlee's law and the assumption of hydrostatic pore pressure.

Postseismic deformation due to the 1992 M(w)7.3 Landers, southern California, earthquake is investigated using the entire catalog of the ERS synthetic aperture radar (SAR) data, and GPS measurements made between 1992 and 1999. The stacked interferometric SAR (InSAR) data spanning the time period of 7 years between the Landers and the Hector Mine earthquakes reveal a transient postseismic deformation with a characteristic decay time of several years. The horizontal displacements measured with GPS exhibit somewhat smaller decay times of 1-2 years. I use a slip model of the Landers earthquake that fits all available geodetic data [Fialko, 2004] to calculate and compare permanent postseismic displacements due to viscoelastic and poroelastic relaxation. Viscoelastic models assuming weak mantle or lower crust do not agree with the InSAR data in the limit of complete relaxation, implying large (>10 years) relaxation times, essentially nonlinear rheology, or an appreciable yield strength of the lower lithosphere. A combination of poroelastic relaxation above the brittle-ductile transition and localized shear deformation on and below the Landers rupture is able to explain most of the available geodetic data. The InSAR data suggest that pore fluids and interconnected pore space are ubiquitously present throughout the seismogenic layer up to depth of 15 km or greater. The effective hydraulic diffusivity of the upper crust inferred from the kinetics of surface deformation is of the order of 0.1-1 m(2)/s, consistent with the laboratory, field, and deep borehole measurements. The post-Landers geodetic data suggest that discrete narrow fault zones extend into the lower crust and perhaps the uppermost mantle, thus lending support to a "block tectonics'' model of the Eastern California Shear Zone.

[1] The coseismic deformation due to the 1992 M(w)7.3 Landers earthquake, southern California, is investigated using synthetic aperture radar (SAR) and Global Positioning System (GPS) measurements. The ERS-1 satellite data from the ascending and descending orbits are used to generate contiguous maps of three orthogonal components ( east, north, up) of the coseismic surface displacement field. The coseismic displacement field exhibits symmetries with respect to the rupture plane that are suggestive of a linear relationship between stress and strain in the crust. Interferometric synthetic aperture radar (InSAR) data show small-scale deformation on nearby faults of the Eastern California Shear Zone. Some of these faults ( in particular, the Calico, Rodman, and Pinto Mountain faults) were also subsequently strained by the 1999 M(w)7.1 Hector Mine earthquake. I test the hypothesis that the anomalous fault strain represents essentially an elastic response of kilometer-scale compliant fault zones to stressing by nearby earthquakes [Fialko et al., 2002]. The coseismic stress perturbations due to the Landers earthquake are computed using a slip model derived from inversions of the InSAR and GPS data. Calculations are performed for both homogeneous and transversely isotropic half-space models. The compliant zone model that best explains the deformation on the Calico and Pinto Mountain faults due to the Hector Mine earthquake successfully predicts the coseismic displacements on these faults induced by the Landers earthquake. Deformation on the Calico and Pinto Mountain faults implies about a factor of 2 reduction in the effective shear modulus within the similar to 2 km wide fault zones. The depth extent of the low-rigidity zones is poorly constrained but is likely in excess of a few kilometers. The same type of structure is able to explain high gradients in the radar line of sight displacements observed on other faults adjacent to the Landers rupture. In particular, the Lenwood fault north of the Soggy Lake has likely experienced a few centimeters of left-lateral motion across < 1-km-wide compliant fault zone having the rigidity reduction of more than a factor of 2. The inferred compliant fault zones are interpreted to be a result of extensive damage due to past earthquakes.

Thermal perturbations associated with seismic slip on faults may significantly affect the dynamic friction and the mechanical energy release during earthquakes. This paper investigates details of the coseismic temperature increases associated with the elastodynamic propagation of shear cracks and effects of fault heating on the dynamic fault strength. Self-similar solutions are presented for the temperature evolution on a surface of a mode II shear crack and a self-healing pulse rupturing at a constant velocity. The along-crack temperature distribution is controlled by a single parameter, the ratio of the crack thickness to the width of the conductive thermal boundary layer, (w) over bar. For "thick'' cracks, or at early stages of rupture ((w) over bar > 1), the local temperature on the crack surface is directly proportional to the amount of slip. For "thin'' cracks, or at later times ((w) over bar < 1), the temperature maximum shifts toward the crack tip. For faults having slip zone thickness of the order of centimeters or less, the onset of thermally induced phenomena (e.g., frictional melting, thermal pressurization, etc.) may occur at any point along the rupture, depending on the degree of slip localization and rupture duration. In the absence of significant increases in the pore fluid pressure, localized fault slip may raise temperature by several hundred degrees, sufficient to cause melting. The onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area, and high viscosity of silicate melts near solidus. The inferred transient increases in the dynamic friction ("viscous braking'') are consistent with results of high-speed rock sliding experiments and might explain field observations of the fault wall rip-out structures associated with pseudotachylites. Possible effects of viscous braking on the earthquake rupture dynamics include (1) delocalization of slip and increases in the effective fracture energy, (2) transition from a crack-like to a pulse-like rupture propagation, or (3) ultimate rupture arrest. Assuming that the pulse-like ruptures heal by incipient fusion, the seismologic observations can be used to place a lower bound on the dynamic fault friction. This bound is found to be of the order of several megapascals, essentially independent of the earthquake size. Further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength.

The Southern San Andreas Fault (SSAF) in California is one of the most thoroughly studied faults in the world, but its configuration at seismogenic depths remains enigmatic in the Coachella Valley. We use a combination of space geodetic and seismic observations to demonstrate that the relatively straight southernmost section of the SSAF, between Thousand Palms and Bombay Beach, is dipping to the northeast at 60–80° throughout the upper crust (<10 km), including the shallow aseismic layer. We constrain the fault attitude in the top 2–3 km using inversions of surface displacements associated with shallow creep, and seismic data from a dense nodal array crossing the fault trace near Thousand Palms. The data inversions show that the shallow dipping structure connects with clusters of seismicity at depth, indicating a continuous throughgoing fault surface. The dipping fault geometry has important implications for the long‐term fault slip rate, the intensity of ground shaking during future large earthquakes, and the effective strength of the southern SAF. Plain Language Summary: The San Andreas Fault (SAF) is capable of large, destructive earthquakes and poses significant seismic hazard in California. In the Coachella Valley, the geometry of the SAF beneath the Earth's surface has been the subject of much debate. Here, we present new constraints on the fault's structure in the uppermost 2–3 km of the crust from several new sets of observations. We measure slow movement of the fault at the surface using satellite‐based radar and perform simulations of what fault geometry is most likely to produce the observed motion. We also installed an array of sensors across the SAF to measure seismic waves that travel through or reflect off the fault. Analysis of both data sets indicates that the shallow portion of the SAF dips to the northeast in the Coachella Valley. Precise locations of small earthquakes at greater depths exhibit a similar trend—we thus suggest the fault dips throughout the Earth's crust. Better understanding the geometry of this section of the SAF has implications for earthquake hazards in Southern California, as well as the fault's geologic history. Key Points: InSAR measurements and models of shallow creep on the southernmost San Andreas Fault indicate a moderate‐to‐steep northeast fault dipSeismic imaging using data from a dense array deployed near Thousand Palms reveal northeast dipping fault damage zonesA joint interpretation of available data suggests that the northeast dip persists throughout the upper 10 km of the crust [ABSTRACT FROM AUTHOR]

Author(s): Dan Bassett (corresponding author) [1]; David T. Sandwell [1]; Yuri Fialko [1]; Anthony B. Watts [2] The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude M [...]

Several mechanisms have been proposed to explain the pre-melting weakening, including flash melting (7,8), lubrication by hydrous films coating the gouge particles (9,10), silica gel formation (2), nano-powder lubrication, thermal [...], Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (~0.1-1 m [s.sup.-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 (1-4). The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood (5,6). 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.sup.-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.

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 findings 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.

Author(s): Yuri Fialko (corresponding author) [1] The San Andreas fault in California is a mature continental transform fault that accommodates a significant fraction of motion between the North American and [...]