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We evaluate seismicity and tectonics along the San Andreas Fault (SAF) in southern California to elucidate ongoing near‐field crustal deformation processes. The principal slip surfaces (PSSs) or the fault core that accommodate major earthquakes, form the boundary between the tectonic plates. We analyze seismicity catalogs extending back to 1857, 1932, and 1981 with progressively improved magnitude of completeness and spatial resolution. The 1857 to present statewide catalog that is complete at M5.5+ documents minimal aftershock activity for the Mw7.9 1857 and 1906 Mw7.8 San Francisco earthquakes. The higher quality 1932 and 1981 catalogs show that the PSSs (the rupture zone) of the 1857 Mw7.9 Fort Tejon earthquake exhibits remarkable seismic quiescence both in the core and in the adjacent extended‐damage zone. Further south, the fault core is still aseismic but the shape of the SAF is more complex, and the rate of adjacent seismicity is much higher. This fault complexity and the seismicity rate are larger the more the strike of the SAF deviates from the Pacific plate velocity‐vector direction. The focal mechanisms of the SAF adjacent earthquakes are also heterogeneous and rarely have strikes and dips that are consistent with slip on the nearby PSSs. We infer that the southern SAF is locked, and a lack of seismicity at the core of the fault may be a standard feature of faults that almost exclusively accommodate high‐slip rates by producing major earthquakes. Correspondingly future aftershock sequences of major earthquakes on the southern SAF will likely have below average aftershock productivity. Plain Language Summary: The fast‐moving San Andreas Fault (SAF) that runs up the spine of California from the Salton Sea to Cape Mendocino, forms the plate boundary between the Pacific and North America plates. It has accommodated California's two largest earthquakes, the 1857 Mw7.9 Fort Tejon and the 1906 Mw7.8 San Francisco, which ruptured the fault for hundreds of kilometers. We focus on analyzing the seismicity of the southern SAF from 1857 to present. Almost no small earthquakes occur along the core of the southern SAF, from south of Parkfield to the Salton Sea. To the north, the 1857 Mw7.9 earthquake was followed by relatively few immediate aftershocks, and since 1932 this section of the fault has been very seismically quiescent. To the south of the 1857 rupture, the rate of extended‐damage zone seismicity inversely correlates with the SAF strike deviation from the Pacific plate vector relative to the SAF strike. In particular, the east‐west striking 28 km long Mill Creek segment may impede SAF slip, which results in abundant off‐fault seismicity and transfer of tectonic strain into the eastern California shear zone and the Peninsular Ranges. Key Points: The near‐field seismicity of the southern San Andreas Fault (SAF) is limited to the adjacent extended‐damage zone and mostly absent from the fault coreStep changes in the 95% depth and in the texture of seismicity depth distribution likely define the presence of the SAF fault coreSeismicity rate in the damage zone positively correlates with SAF deviation from Pacific plate vector except in the quiescent 1857 rupture [ABSTRACT FROM AUTHOR]

Scientists have observed the surface expression of creep events along the San Andreas Fault since the 1960s. However, the evolution of slip at depth has been examined relatively little. So here we probe that deep slip by analyzing strain observations just before and during hours‐ to day‐long creep events at the northern end of the creeping section of the San Andreas Fault. We identify 71 strain offsets that are likely produced by few‐hour bursts of slip at depth. Then, we grid search to determine the location, depth, and magnitude of these slip bursts. We find that the slip bursts occur at a range of along‐strike locations, from 0 to 7 km away from the surface slip observations. Slip occurs at depths from 0 to 10 km; 42%–55% of the bursts are likely below 4 km depth. The bursts typically have moments equivalent to Mw 3.2–4.1 earthquakes. These findings suggest that creep events are not just small shallow events; they are relatively large events that nucleate at significant depths and could play a prominent role in the slip dynamics of the creeping section. Plain Language Summary: Scientists have observed bursts of slip, or creep events, along the creeping section of the San Andreas Fault since the 1960s. Despite this, we still need to learn how they grow spatially or how much energy they release. We identify strain offsets associated with creep events and estimate where the fault has slipped to produce these strain offsets and the event's magnitude. We identify 71 strain offsets associated with creep events at the northern end of the creeping section of the San Andreas Fault. We then grid search for the slip locations that cause the observed strain offsets and estimate their magnitude. We find a range of along‐strike locations where slip occurs, most of which are below 4 km and have a magnitude of 3.2–4.1. These findings suggest that creep events are much larger and deeper than previous estimates and may play an important role in the slip dynamics of the creeping section of the San Andreas Fault. Key Points: We identify 71 strain offsets that result from slip bursts during creep eventsSlip bursts originate from various along‐strike locations, often below 4 km deepSlip bursts typically have a moment equivalent to M3.2–4.1 earthquakes [ABSTRACT FROM AUTHOR]

The Parkfield transitional segment of the San Andreas Fault (SAF) is characterized by the production of frequent quasi-periodical M6 events that break the very same asperity. The last Parkfield mainshock occurred on 28 September 2004, 38 years after the 1966 earthquake, and after the segment showed a ~22 years average recurrence time. The main reason for the much longer interevent period between the last two earthquakes is thought to be the reduction of the Coulomb stress from the M6.5 Coalinga earthquake of 2 May 1983, and the M6 Nunez events of June 11th and 22 July 1983. Plausibly, the transitional segment of the SAF at Parkfield is now in the late part of its seismic cycle and current observations may all be relative to a state of stress close to criticality. However, the behavior of the attenuation parameter in the last few years seems substantially different from the one that characterized the years prior to the 2004 mainshock. A few questions arise: (i) Does a detectable preparation phase for the Parkfield mainshocks exist, and is it the same for all events? (ii) How dynamically/kinematically similar are the quasi-periodic occurrences of the Parkfield mainshocks? (iii) Are some dynamic/kinematic characteristics of the next mainshock predictable from the analysis of current data? (e.g., do we expect the epicenter of the next failure to be co-located to that of 2004?) (iv) Should we expect the duration of the current interseismic period to be close to the 22-year "undisturbed" average value? We respond to the questions listed above by analyzing the non-geometric attenuation of direct S-waves along the transitional segment of the SAF at Parkfield, in the close vicinity of the fault plane, between January 2001 and November 2023. Of particular interest is the preparatory behavior of the attenuation parameter as the 2004 mainshock approached, on both sides of the SAF. We also show that the non-volcanic tremor activity modulates the seismic attenuation in the area, and possibly the seismicity along the Parkfield fault segment, including the occurrence of the mainshocks. [ABSTRACT FROM AUTHOR]

I investigate the long‐term, rigid motions of the 20 microplates identified by McCaffrey (2005, https://doi.org/10.1029/2004jb003307) within the Pacific‐North America plate boundary in southwestern United States. Those motions are described by the Euler vectors (Ω0i ${\boldsymbol{\Omega }}_{\mathbf{0}}^{\boldsymbol{i}}$ for the ith microplate) given by McCaffrey for each microplate. McCaffrey noticed that the Euler poles for those microplates were aligned along the great circle that connects the geometric center of the microplate distribution with the PACI pole, the pole of rotation of the Pacific Plate (PA) about the North American Plate (NA). To explain that alignment, Thatcher et al. (2016, https://doi.org/10.1002/2015jb0126678.0) proposed replacing each Ω0i ${\boldsymbol{\Omega }}_{\mathbf{0}}^{\boldsymbol{i}}$ by two, vertical‐axis rotations of the microplate, one ΩRi ${\boldsymbol{\Omega }}_{\boldsymbol{R}}^{\boldsymbol{i}}$ describing the trajectory (orbit) of its center of mass (CM) and the other ΩSi ${\boldsymbol{\Omega }}_{\boldsymbol{S}}^{\boldsymbol{i}}$ its rotation (spin) about that CM, where ΩRi+ΩSi=Ω0i ${\boldsymbol{\Omega }}_{\boldsymbol{R}}^{\boldsymbol{i}}+{\boldsymbol{\Omega }}_{\boldsymbol{S}}^{\boldsymbol{i}}={\boldsymbol{\Omega }}_{\mathbf{0}}^{\boldsymbol{i}}$. Moreover, they suggested that the orbital motion was being driven by drag from the rotating PA, which suggests that the ΩRi ${\boldsymbol{\Omega }}_{\boldsymbol{R}}^{\boldsymbol{i}}$ poles coincide with the PACI pole. Then rotation vectors ΩRiandΩSi ${\boldsymbol{\Omega }}_{\boldsymbol{R}}^{\boldsymbol{i}}\,\text{and}\,{\boldsymbol{\Omega }}_{\boldsymbol{S}}^{\boldsymbol{i}}$ consistent with the given Ω0i ${\boldsymbol{\Omega }}_{\mathbf{0}}^{\boldsymbol{i}}$ can be found for 17 of the microplates; the other 3 microplates are apparently affected by Basin‐and‐Range extension as well as PA relative motion. The long‐term motion of each of the 17 microplates then can be described as an orbital rotation about the PACI pole plus spin about the CM of the microplate. The closer the microplate CM is to the PA, the more nearly its orbital rotation rate approaches the rotation rate of the PA about the PACI pole. Plain Language Summary: I investigate the long‐term, rigid motions of the 20 microplates identified by McCaffrey (2005, https://doi.org/10.1029/2004jb003307) along the Pacific‐North America plate boundary in southwestern United States. The motion of each microplate is a rotation specified by its Euler vector (i.e., angular velocity and latitude and longitude of its pole of rotation). It is observed that the Euler poles of the various microplates are aligned along the great circle that connects the geometric center of microplate distribution with the PACI pole, the pole of rotation of the Pacific Plate (PA) about the North American Plate (NA). An alternative description of the microplate motion is to describe the motion of each microplate as the sum of two rotations, one of which describes the orbit of its center of mass (CM) and the other its spin about that CM. The vector sum of the orbital and spin rotations must equal the Euler rotation. The observed alignment of 17 of the Euler poles is explained by requiring that the poles of the orbital rotations of those microplates coincide with the PACI pole (i.e., those microplates are being partially dragged along with the motion of PA). The other three microplates involve Basin and Range extension. Key Points: The long‐term motions of the 20 microplates in the PA‐NA plate boundary in southwestern USA are described by their Euler vectorsEach Euler vector can be resolved into two rotations, one describing the orbital rotation of the microplate and the other its spinThe inferred orbital pole for each microplate coincides with the PA‐NA pole, implying that the PA plate partly drags each microplate along [ABSTRACT FROM AUTHOR]

Low-frequency earthquakes (LFEs) are small magnitude earthquakes with frequencies of 1–10 Hertz which often occur in overlapping sequence forming persistent seismic tremors. They provide insights into large earthquake processes along plate boundaries. LFEs occur stochastically in time, often forming temporally recurring clusters. The occurrence times are typically modeled using point processes and their intensity functions. We demonstrate how to use hidden Markov models coupled with visualization techniques to model inter-arrival times directly, classify LFE occurrence patterns along the San Andreas Fault, and perform model selection. We highlight two subsystems of LFE activity corresponding to periods of alternating episodic and quiescent behavior. [ABSTRACT FROM AUTHOR]

The Parkfield transitional segment of the San Andreas Fault (SAF) is characterized by the production of frequent quasi-periodical M6 events that break the very same asperity. The last Parkfield mainshock occurred on 28 September 2004, 38 years after the 1966 earthquake, and after the segment showed a ∼22 years average recurrence time. The main reason for the much longer interevent period between the last two earthquakes is thought to be the reduction of the Coulomb stress from the M6.5 Coalinga earthquake of 2 May 1983, and the M6 Nuñez events of June 11th and 22 July 1983. Plausibly, the transitional segment of the SAF at Parkfield is now in the late part of its seismic cycle and current observations may all be relative to a state of stress close to criticality. However, the behavior of the attenuation parameter in the last few years seems substantially different from the one that characterized the years prior to the 2004 mainshock. A few questions arise: (i) Does a detectable preparation phase for the Parkfield mainshocks exist, and is it the same for all events? (ii) How dynamically/kinematically similar are the quasi-periodic occurrences of the Parkfield mainshocks? (iii) Are some dynamic/kinematic characteristics of the next mainshock predictable from the analysis of current data? (e.g., do we expect the epicenter of the next failure to be co-located to that of 2004?) (iv) Should we expect the duration of the current interseismic period to be close to the 22-year “undisturbed” average value? We respond to the questions listed above by analyzing the non-geometric attenuation of direct S-waves along the transitional segment of the SAF at Parkfield, in the close vicinity of the fault plane, between January 2001 and November 2023. Of particular interest is the preparatory behavior of the attenuation parameter as the 2004 mainshock approached, on both sides of the SAF. We also show that the non-volcanic tremor activity modulates the seismic attenuation in the area, and possibly the seismicity along the Parkfield fault segment, including the occurrence of the mainshocks.

Natural hydrogen has generated great interest as a potential clean and renewable energy source. To understand the occurrence of natural hydrogen, 103 1-m deep soil gas samples were acquired near the San Andreas Fault at Jasper Ridge and Portola Valley, California, USA. The gas samples were analyzed for hydrogen, helium, carbon dioxide, light hydrocarbons, and fixed gas concentrations. Statistical data analysis was carried out to group samples, reveal their spatial distribution, and understand possible sources of the gases. High concentrations of hydrogen up to 20.3 ppmv and 17.3 ppmv occur in Jasper Ridge and Portola Valley, respectively, ∼30–35 times greater than the atmospheric concentration. Most samples with high hydrogen concentrations fall on or near faults, suggesting an origin by serpentinization or geomechanical activation of catalytic sites in minerals, although a deep-seated primordial origin cannot be excluded. Elevated concentrations of carbon dioxide resulted from aerobic microbial degradation of organic matter and elevated concentrations of light hydrocarbons likely resulted from thermal cracking of organic matter. • High concentrations of hydrogen discovered to occur in Jasper Ridge and Portola Valley near faults. • Hierarchical cluster and principal component analyses reveal spatial clustering of gas samples. • Alternating least squares regression reveals pure components and sources of gases. • Geochemical soil sampling and data analysis is a tool for natural H 2 exploration. [ABSTRACT FROM AUTHOR]

Inversions of interseismic geodetic surface velocities often cannot uniquely resolve the three‐dimensional slip‐rate distribution along closely spaced faults. Microseismic focal mechanisms reveal stress information at depth and may provide additional constraints for inversions that estimate slip rates. Here, we present a new inverse approach that utilizes both surface velocities and subsurface stressing‐rate tensors to constrain interseismic slip rates and activity of closely spaced faults. We assess the ability of the inverse approach to recover slip rate distributions from stressing‐rate tensors and surface velocities generated by two forward models: (a) a single strike‐slip fault model and (b) a complex southern San Andreas fault system (SAFS) model. The single fault model inversions reveal that a sparse array of regularly spaced stressing‐rate tensors can recover the forward model slip distribution better than surface velocity inversions alone. Because focal mechanism inversions currently provide normalized deviatoric stress tensors, we perform inversions for slip rate using full, deviatoric or normalized deviatoric forward‐model‐generated stressing‐rate tensors to assess the impact of removing stress magnitude from the constraining data. All the inversions, except for those that use normalized deviatoric stressing‐rate tensors, recover the forward model slip‐rate distribution well, even for the SAFS model. Jointly inverting stressing rate and velocity data best recovers the forward model slip‐rate distribution and may improve estimates of interseismic deep slip rates in regions of complex faulting, such as the southern SAFS; however, successful inversions of crustal data will require methods to estimate stressing‐rate magnitudes. Key Points: Joint inversions of velocity and stressing‐rate data can better estimate slip rates along complex faults than individual inversionsInverting data at multiple depths can better estimate fault locking depth than inverting data at a single depthApplication of the new method requires estimates of crustal deviatoric stressing‐rate tensors with magnitude [ABSTRACT FROM AUTHOR]

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]

We use UAVSAR interferograms to characterize fault slip, triggered by the Mw 7.2 El Mayor-Cucapah earthquake on the 1 San Andreas Fault in the Coachella Valley providing comprehensive maps of short-term geodetic surface deformation that complement in situ measurements. Creepmeters and geological mapping of fault offsets on Durmid Hill recorded 4 and 8 mm of average triggered slip respectively on the fault, in contrast to radar views that reveal significant off-fault dextral deformation averaging 20 mm. Unlike slip in previous triggered slip events on the southernmost San Andreas fault, dextral shear in 2010 is not confined to transpressional hills in the Coachella valley. Edge detection and gradient estimation applied to the 50-m-sampled interferogram data identify the location (to 20 m) and local strike (to

Abstract Inversions of interseismic geodetic surface velocities often cannot uniquely resolve the three‐dimensional slip‐rate distribution along closely spaced faults. Microseismic focal mechanisms reveal stress information at depth and may provide additional constraints for inversions that estimate slip rates. Here, we present a new inverse approach that utilizes both surface velocities and subsurface stressing‐rate tensors to constrain interseismic slip rates and activity of closely spaced faults. We assess the ability of the inverse approach to recover slip rate distributions from stressing‐rate tensors and surface velocities generated by two forward models: (a) a single strike‐slip fault model and (b) a complex southern San Andreas fault system (SAFS) model. The single fault model inversions reveal that a sparse array of regularly spaced stressing‐rate tensors can recover the forward model slip distribution better than surface velocity inversions alone. Because focal mechanism inversions currently provide normalized deviatoric stress tensors, we perform inversions for slip rate using full, deviatoric or normalized deviatoric forward‐model‐generated stressing‐rate tensors to assess the impact of removing stress magnitude from the constraining data. All the inversions, except for those that use normalized deviatoric stressing‐rate tensors, recover the forward model slip‐rate distribution well, even for the SAFS model. Jointly inverting stressing rate and velocity data best recovers the forward model slip‐rate distribution and may improve estimates of interseismic deep slip rates in regions of complex faulting, such as the southern SAFS; however, successful inversions of crustal data will require methods to estimate stressing‐rate magnitudes.

We report the results of a series of scaled laboratory experiments that investigate the development of fault systems in plate boundary transition zones, where deformation is distributed between a continental transform fault (e.g., Alpine Fault, New Zealand; North Anatolian Fault (NAF), Turkey; San Andreas Fault (SAF), USA) and its connecting plate boundary. In these transition zones, continental transform faults are observed to branch into multiple subsidiary faults. Here we show that large‐scale transition zone fault networks comprise crustal‐scale Riedel shears that develop sequentially outwards and away from the parent transform fault. Such fault networks form within brittle upper crust that overlies a ductile lower crust that deforms by large‐scale distributed simple shear. We argue that large‐scale distributed deformation of the lower crust occurs when continental transform faults are "misaligned" relative to their connecting plate boundaries. This misalignment may occur: (a) where a transform fault does not directly connect with a convergent or divergent plate boundary, as in the northern termination of the Alpine Fault and the western termination of the NAF; and (b) where a significant bend in the transform fault occurs in the plate boundary transition zone, as in the southern termination of the SAF. The development of such plate boundary misalignments appears to occur when a plate boundary transition zone develops as a continental transform fault propagates toward a different type of "connecting" plate boundary. Plain Language Summary: Continental transform faults develop where one tectonic plate slides past another, and their role is to link plate motion between other types of plate boundaries. In some instances, the transform fault does not link directly to its connecting plate boundary. Where this occurs, there is no focussed plate boundary to accommodate plate motion and a region of distributed deformation develops. Here we investigate how plate motion is transferred across such regions by recreating plate boundary conditions in scaled laboratory experiments. Our results show that a network of faults develop within these transitional regions in a consistent pattern to accommodate plate motion. The fault pattern develops due to deformation in the ductile lower crust becoming distributed between the two adjoining plate boundaries. Key Points: Analog experiments of strike‐slip shear investigate the development of fault networks associated with the termination of continental transform faultsLarge scale distributed simple shear occurs when the continental transform fault and its connecting plate boundary are "misaligned""Misaligned" plate boundaries occur when a transform fault propagates toward and eventually connects with a different plate boundary type [ABSTRACT FROM AUTHOR]

The Eastern Transverse Ranges (ETR) province of California contains a system of E‐W‐trending left‐lateral faults accommodating clockwise block rotation between the San Andreas Fault (SAF) system in the Coachella Valley and the Eastern California Shear Zone (ECSZ) in the Mojave Desert. Building upon established geometric relationships, we estimate that this rotation across the ETR may transfer right‐lateral strain from the SAF to the ECSZ at a time‐averaged rate of ∼4.3–7.7 mm/yr. Through geomorphic mapping and the analysis of 38 10Be surface exposure samples, we derive a long‐term slip rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, yet analysis of the displacement record indicates a rate decrease ∼71 kya. While a rate change on this fault could have implications for possible plate boundary reorganization in this area, we argue that this slip rate variability more likely reflects routine fluctuation about a steady lifetime slip rate. Plain Language Summary: Neighboring zones of faulting within a tectonic plate boundary can play different roles in the accommodation and transfer of strain over time. Here, we investigate a zone of faulting that, in the past, has experienced clockwise block rotation, which can serve as a mechanism to communicate strain from one zone to another. We use previously published geometric relationships to determine that the Eastern Transverse Ranges (ETR) province may be transferring 4.3–7.7 mm/yr of strain from the San Andreas Fault zone to the Eastern California Shear Zone. In addition, using mapping and surface exposure geochronology, we estimate the slip‐rate of a specific fault within the ETR to have a long‐term rate of 1.26 ± 0.50 (2σ) mm/yr over the late Pleistocene, with slight variation about that long‐term rate, indicating that seismic hazard may vary over time. Key Points: We estimate a time‐averaged rate of strain transfer through block rotation in the Eastern Transverse Ranges to be between ∼4.3 and 7.7 mm/yrNew geologic slip rate points to a best fit variable slip rate, with a long‐term average slip rate that agrees with regional kinematicsThe Blue Cut Fault presents another example of short‐term strain rate variability being preserved in the geologic and geomorphic record [ABSTRACT FROM AUTHOR]

Several large strike slip faults in central and northern California accommodate plate motions through aseismic creep. Although there is no consensus regarding the underlying cause of aseismic creep, aqueous fluids and mechanically weak, velocity‐strengthening minerals appear to play a central role. This study integrates field observations and thermodynamic modeling to examine possible relationships between the occurrence of serpentinite, silica‐carbonate rock, and CO2‐rich aqueous fluids in creeping faults of California. Our models predict that carbonation of serpentinite leads to the formation of talc and magnesite, followed by silica‐carbonate rock. While abundant exposures of silica‐carbonate rock indicate complete carbonation, serpentinite‐hosted CO2‐rich spring fluids are strongly supersaturated with talc at elevated temperatures. Hence, carbonation of serpentinite is likely ongoing in parts of the San Andres Fault system and operates in conjunction with other modes of talc formation that may further enhance the potential for aseismic creep, thereby limiting the potential for large earthquakes. Plain Language Summary: Constraining the relationships between the mechanical properties of rocks and lubricating fluids in tectonic fault zones is critical to understanding their seismogenic potential. We examined the distribution of exhumed mantle rocks and CO2‐rich springs in the San Andreas Fault area and find ubiquitous evidence for carbonate alteration in the creeping sections. Although carbonate alteration has gone to completion in exposed silica‐carbonate rocks, thermodynamic calculations suggest that CO2‐rich spring fluids are supersaturated with talc and magnesite at greater depth where temperatures are high. Because wet talc is a mechanically weak mineral, its formation through carbonation promotes tectonic movements without large earthquakes. Key Points: Carbonate‐altered mantle rocks and CO2‐rich springs are ubiquitous in the creeping section of the San Andreas Fault systemCO2‐rich spring fluids are saturated with talc and magnesite at seismogenic depths where mineral carbonation is likely ongoing todayThe formation of talc facilitates aseismic creep in wet faults and promotes tectonic movements without large earthquakes [ABSTRACT FROM AUTHOR]

Deformation in transform systems is accommodated by discrete fault slip and distributed off‐fault deformation. Here, we consider how a change in slip behavior along a fault can influence the distribution between on‐ and off‐fault deformation. We use a physical experiment to simplify the geometry, material properties, boundary conditions, and slip history along a strike‐slip fault to directly observe patterns of off‐fault deformation. We document deformation of a silicone slab on a simple shear apparatus using particle image velocimetry (2D) and photogrammetry (3D). The experimental results show regions of topographic highs and lows on either side of the slip transition that grow, evolve, and are displaced with progressive strain. The experimental dilatation field shares similarities with strain fields in central California along the San Andreas fault, which suggests that a change in slip behavior may explain some of the real‐world patterns in short‐ and long‐term deformation. Plain Language Summary: Understanding where and why deformation occurs along a fault is important for quantifying future earthquake hazards and interpreting existing geological structures. In nature, creeping sections of strike‐slip faults move slowly and continually, while locked sections are stationary for long periods and then release large amounts of energy rapidly in earthquake events. A portion of a strike‐slip fault's total motion can be accommodated away from the main trace of the fault leading to permanent deformation. Here, we investigate how a change from locked slip to creeping slip affects deformation away from the fault trace by deforming silicone, an analog for the Earth's crust. Physical models are useful because they allow us to simplify the fault system and isolate a single variable affecting deformation. We track deformation in the experiments in two and three‐dimensions. Our results show a pattern of extension and contraction which can be compared to a real‐world change in slip behavior on the San Andreas fault. Key Points: Physical models are useful for understanding where and why off‐fault deformation may occur on strike‐slip faultsA change in slip behavior causes off‐fault deformation patterns to developExperimental dilatation fields share similarities with strain fields in central California along the San Andreas fault [ABSTRACT FROM AUTHOR]

Tectonic faults show rheological heterogeneity in interfaces, and the spectrum of their sliding regimes span a continuum from the slow-slip events to dynamic ruptures. The heterogeneity of the fault interface is crucial for the mechanics of faulting. By using the earthquake source locations, the complex structure of a fault interface can be reproduced at a resolution down to 50–100 m. Here, we use a declustered seismic catalog of Northern California to investigate structures of 11 segments of San Andreas, Calaveras, and Hayward faults. The cumulative length of all the segments is about 500 km. All the selected segments belong to subvertical strike–slip faults. A noticeable localization of sources near the fault cores is observed for all segments. The projection of earthquake sources to the fault plane shows severe inhomogeneity. Topologically dense clusters (seismogenic patches (SPs)) can be detected in fault planes. The longer the observation are, the more distinct are the clusters. The SPs usually cover about 10%–20% of the fault interface area. It is in the vicinity of SPs that earthquakes of magnitudes above 5 are usually initiated. The Voronoi tessellation is used to determine the orderliness of SPs. Distributions of areas of Voronoi cells of all the SPs obey the lognormal law, and the value of Voronoi entropy of 1.6–1.9 prevails. The findings show the informativeness of the background seismicity in revealing the heterogenous structure of a tectonic fault interface.

In crustal fault zones, regions of damaged rock characterized by reduced elastic shear modulus can influence patterns of near-field interseismic deformation. In order to study these compliant fault zones (CFZs) and how they might develop over the lifetimes of faults, we compare two fault segments with contrasting fault age and lithology along the San Andreas Fault in the San Francisco Bay Area. New geodetic measurements of the interseismic velocity fields at each location are used to constrain fault zone parameters through a Markov chain Monte Carlo method. At Black Mountain, in the Santa Cruz Mountains of the San Francisco Peninsula, we do not find evidence for a compliant fault zone; instead, we find that the geodetic data are more consistent with a model of a single fault in a homogeneous elastic half-space. At Lake San Andreas, a younger fault segment 35 km farther north, we find evidence for a compliant fault zone about 3.4 +1.1/−1.4 km wide, containing a shear modulus of about 40% of the shear modulus of the surrounding rock. We also find that the best fitting CFZ model at this location, unlike the best fitting homogeneous half-space model, has a locking depth that agrees well with the observed depth of microseismicity. Based on differences in fault age, cumulative displacement, and lithology between Black Mountain and Lake San Andreas, we infer that lithology plays an important and, in this case, perhaps a dominant role in the accumulation of fault zone damage structures and the development of CFZs over the lifetime of a fault.

Changes in groundwater storage in California's Central Valley (CV) are considered partly responsible for vertical uplift surrounding the southern CV and for stress changes on nearby faults. Questions remain regarding the distribution of stress on the central San Andreas fault (CSAF) from recent and historical drawdown of the CV aquifer over the past 150 years (1860–2010). We combine groundwater storage change estimates for the 2006–2010 drought with a three‐dimensional finite element model to estimate Coulomb failure stress change (ΔCFS) on the CSAF. We combine a simple parameterization of historical hydraulic head change for 1860–1960 with a CV hydrological model (1961–2003) to estimate visco‐elastic effects on ΔCFS total and 2010 rate. We find that ΔCFS on the CSAF correlates positively with shallow seismicity and low frequency earthquakes for both short‐term and long term stressing. Unloading uplift rates surrounding the southern CV only partially account for the observed vertical GPS rates. Plain Language Summary: We investigate the spatial and temporal effects of groundwater removal in California's Central Valley on ground surface deformation and on stress changes on the nearby central San Andreas fault (CSAF). In particular, we focus on two outstanding questions: (a) What are the spatial distributions of Coulomb failure stress (ΔCFS) on the CSAF? (b) What are the effects of viscoelasticity on uplift and ΔCSF over time? We focus on the well studied 2006–2010 drought, estimating the change in groundwater storage and the resulting uplift and stress change through a numerical model for the Earth. Viscoelastic effects are examined through an estimate of historical groundwater change from 1860‐1961 and a hydrological model from 1961‐2003 added to the 2006–2010 drought estimates. Here we find that lower crust or upper mantle viscosity variations enhance uplift and stress changes and that CSAF stress changes may accumulate with time, especially with droughts and groundwater consumption. Key Points: We estimate Central Valley groundwater unloading viscoelastic effects on present‐day central San Andreas (CSAF) fault stressingCoulomb stress change for the CSAF during the 2006–2010 drought corresponds positively with seismicity and low frequency earthquakesCumulative Coulomb stress change on the CSAF for 1860–2010 is on the order of ∼0.01–0.02 MPa depending on the lower crustal viscosity [ABSTRACT FROM AUTHOR]

Understanding mechanical conditions that lead to complexity in earthquakes is important to seismic hazard analysis. In this study, we simulate physics‐based multicycle dynamic models of the San Andreas fault (Carrizo through San Bernardino sections) and the San Jacinto fault (Claremont and Clark strands). We focus on a complex fault geometry based on the Southern California Earthquake Center Community Fault Model and its effect over multiple earthquake cycles. Using geodetically derived strain rates, we validate the models against geologic slip rates and recurrence intervals at various paleoseismic sites. We find that the interactions among fault geometry, dynamic rupture and interseismic stress accumulation produce stress heterogeneities, leading to rupture segmentation and variability in earthquake recurrence. Our models produce earthquakes with rupture extents similar to a recent comprehensive paleoseismic catalog. The "earthquake gates" of the Big Bend and the Cajon Pass occasionally impede dynamic ruptures. The angle of compression, which is the subtraction of the maximum shear strain rate direction from the local fault strike, can better determine the likelihood of the impedance of restraining bends to dynamic ruptures. Because the Big Bend has an angle of compression of ∼20°, ruptures that traverse the Big Bend, like the 1857 Fort Tejon earthquake, are more frequent than expected based on empirical relations which predict the ∼40° restraining bend to terminate most ruptures. Our models indicate that large ruptures tend to initiate north of the Big Bend and propagate southwards, similar to the 1857 earthquake, providing critical information for ground shaking assessment in the region. Plain Language Summary: The historical and prehistoric earthquake record from the southern San Andreas and San Jacinto faults contains ruptures that have both variable lengths and recurrence times. Understanding the causes behind the complexity would advance our knowledge of earthquake dynamics and could improve earthquake forecasts. To achieve this, we need numerical models to integrate various datasets from geodesy, structural geology, seismology, and paleoseismology. In this study, we simulate earthquake cycles using a complex 2D fault geometry. We find that fault geometry interacts with other mechanical factors to produce heterogeneous stresses, leading to simulated earthquakes with various rupture extents and irregular return times that mimic quakes known from the geologic record. In our model we find that the Big Bend on the San Andreas fault controls rupture extent and large ruptures tend to initiate north of the Big Bend and propagate southwards, similar to the 1857 Fort Tejon earthquake. Continued research is needed to understand rupture extent variability near Cajon Pass and we identify promising research possibilities. Key Points: Multicycle dynamic models with realistic 2D fault geometry of the southern San Andreas and the northern San Jacinto faults are simulatedStress heterogeneity from interactions between fault geometry and dynamic ruptures produces diverse rupture extent and recurrenceLarge ruptures tend to initiate north of the Big Bend and propagate southwards, similar to the 1857 Fort Tejon earthquake [ABSTRACT FROM AUTHOR]

©2015. American Geophysical Union. All Rights Reserved. Campaign GPS data collected from 2002 to 2014 result in 41 new site velocities from the San Bernardino Mountains and vicinity. We combined these velocities with 93 continuous GPS velocities and 216 published velocities to obtain a velocity profile across the Pacific-North America plate boundary through the San Bernardino Mountains. We modeled the plate boundary-parallel, horizontal deformation with 5-14 parallel and one obliquely oriented screw dislocations within an elastic half-space. Our rate for the San Bernardino strand of the San Andreas Fault (6.5±3.6mm/yr) is consistent with recently published latest Quaternary rates at the 95% confidence level and is slower than our rate for the San Jacinto Fault (14.1±2.9mm/yr). Our modeled rate for all faults of the Eastern California Shear Zone (ECSZ) combined (15.7±2.9mm/yr) is faster than the summed latest Quaternary rates for these faults, even when an estimate of permanent, off-fault deformation is included. The rate discrepancy is concentrated on faults near the 1992 Landers and 1999 Hector Mine earthquakes; the geodetic and geologic rates agree within uncertainties for other faults within the ECSZ. Coupled with the observation that postearthquake deformation is faster than the pre-1992 deformation, this suggests that the ECSZ geodetic-geologic rate discrepancy is directly related to the timing and location of these earthquakes and is likely the result of viscoelastic deformation in the mantle that varies over the timescale of an earthquake cycle, rather than a redistribution of plate boundary slip at a timescale of multiple earthquake cycles or longer.

©2015. American Geophysical Union. All Rights Reserved. Campaign GPS data collected from 2002 to 2014 result in 41 new site velocities from the San Bernardino Mountains and vicinity. We combined these velocities with 93 continuous GPS velocities and 216 published velocities to obtain a velocity profile across the Pacific-North America plate boundary through the San Bernardino Mountains. We modeled the plate boundary-parallel, horizontal deformation with 5-14 parallel and one obliquely oriented screw dislocations within an elastic half-space. Our rate for the San Bernardino strand of the San Andreas Fault (6.5±3.6mm/yr) is consistent with recently published latest Quaternary rates at the 95% confidence level and is slower than our rate for the San Jacinto Fault (14.1±2.9mm/yr). Our modeled rate for all faults of the Eastern California Shear Zone (ECSZ) combined (15.7±2.9mm/yr) is faster than the summed latest Quaternary rates for these faults, even when an estimate of permanent, off-fault deformation is included. The rate discrepancy is concentrated on faults near the 1992 Landers and 1999 Hector Mine earthquakes; the geodetic and geologic rates agree within uncertainties for other faults within the ECSZ. Coupled with the observation that postearthquake deformation is faster than the pre-1992 deformation, this suggests that the ECSZ geodetic-geologic rate discrepancy is directly related to the timing and location of these earthquakes and is likely the result of viscoelastic deformation in the mantle that varies over the timescale of an earthquake cycle, rather than a redistribution of plate boundary slip at a timescale of multiple earthquake cycles or longer.

Aim: Phylogeographical structure in the black salamander (Aneides flavipunctatus) was inferred using two independent genetic datasets. Concordance between the datasets was sought in order to evaluate earlier suggestions of species-level breaks and evidence of vicariance and long-term isolation within the complex. We hypothesized that major phylogeographical breaks would either correspond to current tectonic plate boundaries or to historical geological processes. Location: North-western California and southern Oregon (USA). Methods: Three mitochondrial DNA (mtDNA) genes were sequenced for 240 black salamanders from 136 localities, and up to 13 nuclear DNA loci were sequenced for 145 black salamanders representing 93 localities. Phylogenetic analysis of our mitochondrial dataset was performed to recover major lineages, while spatial clustering analysis of our nuclear dataset was utilized to identify points of concordance with our mtDNA phylogeny. Levels of gene flow were estimated for all contact zones. Results: Our mitochondrial phylogeny recovered four major lineages. Cluster analysis of our nuclear dataset is consistent with a four-population scheme, with the boundaries matching those of the mtDNA lineages. Gene flow across a contact zone in southern Humboldt between two of the populational units is extremely limited (2Nm < 1). In what is identified as the Central Core population, two distinctive subpopulations were delineated based on nuclear data, but mitochondrial data are discordant. Main conclusions: The Aneides flavipunctatus complex comprises at least four species-level units. Two of the boundaries between these units are associated with current tectonic plate boundaries. The contact zone between our Northwest and Central Core populations lies adjacent to the Mendocino Triple Junction (MTJ), where the Humboldt and North American plates meet, while the area separating the Santa Cruz and Central Core populations corresponds to the boundary between the Pacific and Humboldt tectonic plates. The phylogeographical break within the Central Core population lies in a region in which uplift occurred that is associated with the historical position of the migrating MTJ.

Abstract Geologic slip rates are a time‐averaged measurement of fault displacement calculated over hundreds to million‐year time scales and are a primary input for probabilistic seismic hazard analyses, which forecast expected ground shaking in future earthquakes. Despite their utility for seismic hazard calculations, longer‐term geologic slip rates represent a time‐averaged measure of the tempo of strain release and do not measure variability across earthquake cycles. We have developed a numerical approach called STEPS (Slip Time Earthquake Path Simulations), which is built upon field‐based observations and explicitly incorporates realistic variations in displacement per event and variability in the recurrence interval between earthquakes. The STEPS approach, which simulates strain release through time, relies on representing earthquake cycles as stairsteps, rather than straight‐line paths, connecting per earthquake time‐displacement coordinates. We simulate earthquake histories based on these input constraints using two examples: the Carrizo section of the San Andreas fault and the Toe Jam Hill fault of the Seattle fault zone. We find that modeled slip rate distributions agree with slip rates reported for the sites of interest by the original investigators, while providing a slip rate distribution that reflects the variability of earthquake frequency and displacement. The STEPS approach provides an estimate of fault slip rate uncertainty based on a synthetic suite of plausible time‐displacement paths resulting from individual earthquakes, rather than measurement uncertainties associated with offset features. When considering this simulated earthquake behavior between measurements, the uncertainty associated with earthquake paths is greater than that calculated by the long‐term rate.

Abstract We use UAVSAR interferograms to characterize fault slip, triggered by the Mw 7.2 El Mayor‐Cucapah earthquake on the 1 San Andreas Fault in the Coachella Valley providing comprehensive maps of short‐term geodetic surface deformation that complement in situ measurements. Creepmeters and geological mapping of fault offsets on Durmid Hill recorded 4 and 8 mm of average triggered slip respectively on the fault, in contrast to radar views that reveal significant off‐fault dextral deformation averaging 20 mm. Unlike slip in previous triggered slip events on the southernmost San Andreas fault, dextral shear in 2010 is not confined to transpressional hills in the Coachella valley. Edge detection and gradient estimation applied to the 50‐m‐sampled interferogram data identify the location (to 20 m) and local strike (to

Geologic slip rates are a time‐averaged measurement of fault displacement calculated over hundreds to million‐year time scales and are a primary input for probabilistic seismic hazard analyses, which forecast expected ground shaking in future earthquakes. Despite their utility for seismic hazard calculations, longer‐term geologic slip rates represent a time‐averaged measure of the tempo of strain release and do not measure variability across earthquake cycles. We have developed a numerical approach called STEPS (Slip Time Earthquake Path Simulations), which is built upon field‐based observations and explicitly incorporates realistic variations in displacement per event and variability in the recurrence interval between earthquakes. The STEPS approach, which simulates strain release through time, relies on representing earthquake cycles as stairsteps, rather than straight‐line paths, connecting per earthquake time‐displacement coordinates. We simulate earthquake histories based on these input constraints using two examples: the Carrizo section of the San Andreas fault and the Toe Jam Hill fault of the Seattle fault zone. We find that modeled slip rate distributions agree with slip rates reported for the sites of interest by the original investigators, while providing a slip rate distribution that reflects the variability of earthquake frequency and displacement. The STEPS approach provides an estimate of fault slip rate uncertainty based on a synthetic suite of plausible time‐displacement paths resulting from individual earthquakes, rather than measurement uncertainties associated with offset features. When considering this simulated earthquake behavior between measurements, the uncertainty associated with earthquake paths is greater than that calculated by the long‐term rate. Plain Language Summary: The rate at which faults accommodate displacement at the Earth's surface during earthquakes may be variable through time. Such variability may result from differences between successive earthquakes, both in how much displacement occurs in each earthquake and the elapsed time between each earthquake. While this variability is challenging to measure, it may have a large impact for determining the activity of a fault and subsequently understanding seismic hazard posed by a fault. We have developed a set of calculations that provide an estimate of this earthquake cycle variability, which explore nuances of earthquake behavior and can combine what we know about past earthquakes into an understanding of when the next earthquake could occur. Key Points: Numerical models interpolate ground‐rupturing earthquakes between dated offset features observed in the fieldShort‐term slip rate uncertainty is estimated from variability in earthquake behavior, rather than only from measurement uncertaintyTest case model results from San Andreas and Toe Jam Hill faults confirm and augment preexisting calculated rates [ABSTRACT FROM AUTHOR]

Seismic interferometry is widely applied to retrieve wavefields propagating between receivers. Another version of seismic interferometry, called inter-source interferometry, uses the principles of seismic reciprocity and expands interferometric applications to retrieve waves that propagate between two seismic sources. Previous studies of inter-source interferometry usually involve surface-wave and coda-wave estimations. We use inter-source interferometry to estimate the P-waves propagating between two sources rather than the estimation of surface waves and coda waves. We show that the recovered arrival times are dependent on the accuracy of the earthquake catalog of the two sources. Using inter-source interferometry, one can recover the waveform of the direct body waves and potentially reconstruct the waveform of coda waves, depending on the source-receiver geometry. The retrieval of these waveforms is accurate only when the wavefield is sampled with approximately 4 receivers per wavelength in the stationary phase zone. We show that using only receivers inside the stationary phase region for inter-source interferometry introduces the phase error of approximately 0.3 radians. In our study, we show an example of the P-wavefield reconstruction between two earthquakes using the seismic records from an array along San Andreas Fault. The retrieved P waves give a qualitative estimation of the thickness of the low-velocity zone of San Andreas Fault of approximately 4 km. [ABSTRACT FROM AUTHOR]

Rocks are heterogeneous materials that exhibit nonlinear elastic (anelastic) behavior at scales ranging from the laboratory to Earth. In the laboratory, typical, complex relationships exist between stress and strain that include hysteresis, finite relaxation times, strain rate, and history dependence. These behaviors are linked to important characteristics such as stress, porosity, permeability, material integrity, and material failure. We adopted a "pump‐probe" type experiment common in laboratory studies, using solid earth tides as the low‐frequency pump and empirical Green's function as the high‐frequency probe. By probing the velocity at different points in the pump cycle, we constrained important information about the strain‐modulus relationship. Near the San Andreas Fault, we observed strongly nonlinear elastic behavior that characterizes the damage zone. We also constrained important aspects of hysteretic behavior that are related to damage properties and possibly pore pressure. Away from the fault, the nonlinear behavior is diminished. Plain Language Summary: When intact materials are slightly squeezed or stretched, the amount of the material compresses or extends is typically some multiple of the applied force. Damaged materials have internal fractures, which makes this relationship more complicated. This is because fractures can be open or closed, each contributing differently to the overall material behavior. The more internal fractures exist, the more complicated the material behavior will be. By measuring complexity in how a material deforms when a force is applied, we can learn something about how damaged it is. We apply this analysis to rocks in the subsurface using seismic waves and solid earth tides. Solid earth tides are the stretching and compressing of the Earth due to the gravitational pull of the sun and moon. As the Earth is stretched and compressed, fractures in the Earth can open and close. We use seismic waves to measure how the Earth becomes softer or stiffer as these fractures open and close. The amount of this softening or stiffening tells us something about the material damage along the San Andreas Fault, and may also tell us something about the earthquake cycle. Key Points: The subsurface along the San Andreas Fault near Parkfield, California exhibits inhomogeneous nonlinear elastic behaviorThe magnitude of nonlinear response increases with decreasing distance to the San Andreas FaultWe constrain key properties of modulus‐strain hysteresis in the damage zone of the San Andreas Fault [ABSTRACT FROM AUTHOR]

The interaction between creep events and earthquakes at the regional scale has implications for earthquake cycles, yet few examples of such interaction have been found on continental faults. We report shallow triggered creep along the northern edge of the central creeping section of the San Andreas Fault following the 2003 San Simeon Mw 6.5 and 2004 Parkfield Mw 6.0 earthquakes. Following both earthquakes, we observe a delayed creep event beginning at depth on a strainmeter and continuing on a nearby creepmeter. In addition, between the regional earthquakes, the average slip rate at one creepmeter increased from 10 mm/yr to 30 mm/yr, and returned to 10 mm/yr after the Parkfield earthquake. Creep events and creep rate changes indicate that triggering connects behavior in the northern San Andreas Fault to the southern section and other regional faults. Plain Language Summary: Faults can move slowly or fail quickly in earthquakes. Episodic slip that occurs more slowly than ordinary earthquakes are known as creep events and are largely invisible without dedicated instrumentation. Here we use one of the longest available creep records to demonstrate that creep events can be affected by the seismic waves of regional earthquakes at distances >100 km. Importantly, one of these events was on the Southern San Andreas Fault. Furthermore, between the triggered creep events, the Northern San Andreas overall creep rate changed. The observations of dynamic triggering between the two sections suggests a linked San Andreas system. Key Points: Earthquakes on and near the Southern San Andreas Fault triggered creep events near San Juan Bautista on the Northern San Andreas FaultAverage creep rate also changes at the time of the regional earthquakesThe northern and southern ends of the creeping section communicate and can trigger each other via seismic waves [ABSTRACT FROM AUTHOR]

The PLUM (Propagation of Local Undamped Motion) earthquake early warning (EEW) algorithm differs from typical source‐based EEW algorithms as it predicts shaking directly from observed shaking without first deriving earthquake source information (e.g., magnitude and epicenter). Here, we determine optimal PLUM event detection thresholds for U.S. West Coast earthquakes using two data sets: 558 M3.5+ earthquakes (California, Oregon, Washington; 2012–2017) and the ShakeAlert test suite of historic and problematic signals (1999–2015). PLUM computes Modified Mercalli Intensity (IMMI) using velocity and acceleration data, leveraging co‐located sensors to avoid problematic signals. An event detection is issued when the observed IMMI exceeds a given threshold(s). We find a two‐station detection method using IMMI trigger thresholds of 4.0 and 3.0 for the first and second stations, respectively, is optimal for detecting M4.5+ earthquakes. PLUM detected 79 events in the 2012–2017 data set, reporting (not including telemetry or alert dissemination) detection times on par, and sometimes faster than current EEW methods (mean 8 s; median 6 s). As expected, detection times were slower for the older 1999–2015 earthquakes (N = 21; mean 11 s; median 6 s) when station coverage was sparser. Of the 31 PLUM detected M5+ events (10 2012–2017; 21 1999–2015), theoretically 20 (∼65%) could provide timely warnings. PLUM issued no false detections and avoided issuing detections for all calibration/anomalous signals, regional and teleseismic events. We conclude PLUM can successfully identify IMMI 4+ shaking from local earthquakes and could complement and enhance EEW in the U.S. Plain Language Summary: Earthquake early warning (EEW) detection schemes require (1) ample seismic information to identify where large ground motions are underway; (2) determining if these ground motions are significant enough to issue a detection; and (3) detecting large ground motions in a timely fashion. Some EEW methods estimate earthquake source parameters like magnitude and location and then input those parameters into a ground‐motion prediction equation, while other methods use observations of the ground motions to directly forecast shaking. We explore the latter approach using the PLUM (Propagation of Local Undamped Motion) method to detect earthquakes that produce shaking above a target value. In this work, we test PLUM's ability to detect earthquakes using two data sets: 558 earthquakes magnitude 3.5 and above from California, Oregon, and Washington (2012–2017) and a test suite of historic and problematic signals (1999–2015) curated by ShakeAlert. We find a two‐station detection method is preferred over a one‐station method as two‐stations can greatly minimize false detections. The PLUM method is also 100% successful at avoiding non‐earthquake anomalous signals and can successfully differentiate ground shaking from local and distant earthquakes. We conclude that PLUM may be a promising candidate for integration into the U.S. EEW system. Key Points: Propagation of Local Undamped Motion (PLUM) detects offshore events that produce at least moderate onshore ground motions, including an event problematic for other algorithmsPLUM's detection times are as timely, and sometimes faster, than other earthquake early warning detection timesPLUM correctly avoids erroneous detections for all teleseismic, calibration pulses, and anomalous signals in the test suite [ABSTRACT FROM AUTHOR]

Fault creep reduces seismic hazard and serves as a window into plate boundary processes; however, creep rates are typically constrained with sparse measurements. We use differential lidar topography (11–13 year time span) to measure a spatially dense surface deformation field along a 150 km section of the Central San Andreas and Calaveras faults. We use an optimized windowed‐iterative‐closest‐point approach to resolve independent creep rates every 400 m at 1–2 km apertures. Rates vary from <10 mm/year along the creeping fault ends to over 30 mm/year along much of the central 100 km of the fault. Creep rates are 3–8 mm/year higher than most rates from alignment arrays and creepmeters, likely due to the larger aperture of the topographic differencing. Creep is often focused along discrete fault traces, but strain is sometimes distributed in areas of complex fault geometry, such as Mustang Ridge. Our observations constrain shallow seismic moment accumulation and the location of the creeping fault trace. Key Points: Aseismic fault creep rates are measured over 11–13 years in central California by differencing repeat airborne lidar topographyThe Central San Andreas fault creeps at rates over 30 mm/year, indicating limited strain accumulation for release in earthquakesWe describe challenges and new insights regarding applying topographic differencing methods along creeping faults Plain Language Summary: Repeated dense measurements of topography reveal how the Earth's surface shifts and deforms at tectonic plate boundaries. Along some active faults, deformation and offset occur continuously even in the absence of large earthquakes; this is referred to as fault creep which generally proceeds at rates of millimeters to tens of millimeters per year. Along sections of the Central San Andreas and southern Calaveras faults in California, we map rates and patterns of fault creep over 150 km by comparing two older airborne lidar elevation data sets from 2005 and 2007, with newer data collected in 2018. This analysis shows that the San Andreas fault creeps at a mean rate of 22 mm/year between Parkfield, California, and San Juan Bautista, California. We use the mean rate and the variation along the length of the fault to better understand the seismic hazard along the fault, depth variations of fault creep, and how the geology surrounding the fault impacts the distribution of creep. [ABSTRACT FROM AUTHOR]

To better illuminate structural discontinuities around the San Andreas Fault Observatory at Depth (SAFOD) site, following the previous study of Zhang et al. (2009), we have extended the Generalized Radon Transform (GRT) method to jointly use scattered P and SH waves from abundant local earthquakes recorded by a local dense seismic network. A hybrid imaging condition is applied to extract consistent structures from scattered P and SH images. In this way, more robust structure information can be determined from separate images with potential artifacts. For the transverse component waveforms, coherent S‐S scattered waves after the direct SH waves can be clearly identified, and they are less interfered with by other scattered and converted waves compared to scattered P‐P waves on the vertical component. Similar to Zhang, Wang, et al. (2009), near‐vertical reflectors are imaged on both sides of the San Andreas Fault (SAF) with scattered SH waves, which is generally consistent with the imaging results using scattered P‐P waves from local earthquakes and a wide‐angle active seismic reflection profile. Compared to the P‐P scattering imaging result, the imaging result using scattered SH waves has higher resolution due to shorter S wavelength and cleaner S‐S scattered waveforms, and the SAF, as well as other reflectors around it, is better resolved. Although the resolution of the joint image obtained by combining separate imaging results from different scattered waves may be degraded, it is able to more robustly characterize strong structure discontinuities using passive seismic sources. Key Points: Separate P and SH inverse scattering images are obtained using local earthquake waveforms recorded by a local array around the SAFOD siteA joint P and SH scattering image is determined by a hybrid imaging condition that can suppress potential artifacts in separate imagesStrong vertical reflectors parallel to the SAF are imaged on both sides of the SAFOD in the direction normal to the fault [ABSTRACT FROM AUTHOR]

Tremor and low‐frequency earthquakes are presumed to be indicative of surrounding slow, aseismic slip that is often below geodetic detection thresholds. This study uses data from borehole seismometers and long‐baseline laser strainmeters to observe both the seismic and geodetic signatures of episodic tremor and slip on the Parkfield region of the San Andreas Fault near Cholame, CA. The observed occurrence rates of both the tremors and co‐located families of low‐frequency earthquakes are not steady but instead exhibit quasiperiodic bursts of increased activity. We show that these periods of elevated seismic activity correlate with statistically significant stacked strain signals consisting of 44 slow‐slip events. Modeled individual slow‐slip events and their total summed moment, which are constrained by seismic signals and stacked strain, respectively, indicate that the individual moment magnitudes of these events range from Mw 4.6–5.2. We find that the measured geodetic signal likely precedes the seismic signal by several hours, consistent with the aseismic slip preceding and driving the observed seismic tremor activity. We confirm that strike‐slip faults, in addition to subduction zones, are capable of producing episodic tremor and slip. Key Points: Stacked strain data reveal the average surface geodetic deformation associated with large bursts of tremor on the SAF near Cholame, CAThese geodetic measurements confirm that episodic LFE families provide reliable estimates of deep fault slipThe geodetic observations imply that the aseismic slip is preceding and driving the observed seismic activity [ABSTRACT FROM AUTHOR]

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.

We investigate the fine-scale geometry and structure of the San Andreas Fault near Parkfield, CA, and their role in the development of the 1966 and 2004 ∼M6 earthquakes. Long-term surface fault traces indicate that structural heterogeneities associated with secondary reverse and normal fault structures are present at both rupture tips, near Middle Mountain and Gold Hill. Detailed analysis of almost 50 years of high-resolution seismicity reveals a fault plane that has been twisted into a helicoid between Middle Mountain and Gold Hill. Numerical models support our conclusion that this shape is the result of long-term torqueing of a strong stuck patch surrounded by a weak creeping region. The changes in fault friction behavior and related geometric discontinuities act as barriers to rupture propagation of moderate size earthquakes at Parkfield, and as areas of concentrations where rupture initiates. Our study demonstrates also that smooth strike-slip faults with large cumulative offset can form new fault segments at a late stage in their evolution. • The San Andreas fault plane near Parkfield has been deformed into a helicoid over the long-term. • The torque results from the strength contrast between locked and creeping zones. • Complex structures at surface correspond with the tips of the stuck patch at depth. • Stress concentrations at the tips of the stuck patch control rupture development of M6 events. [ABSTRACT FROM AUTHOR]

The active plate boundary of the California margin includes the active San Andreas Fault System and the active mountain building of the California Coast Ranges where ultramafic rocks are also commonly found. There are numerous hypotheses for the origin and the timing and emplacement mechanisms of such rocks that ultimately came from Earth's mantle, but typically the mineralogical, structural, and geochemical features of fresh serpentinite, and their reaction and deformation histories were unknown due to heavy weathering of most of the exposures. We investigated a block-and-matrix serpentinite mélange in Redwood City, California on the San Francisco Peninsula that is proximal to an active strand of the San Andreas Fault. The mélange is composed of 6 m- to cm-sized serpentinite blocks surrounded by fine-grained serpentine matrix. The blocks are polygonal in shape and show a remarkable internal mineralogical structure that has recorded at least three stages of serpentinization of a clinopyroxene-bearing harzburgite protolith. The largest, freshest polygonal serpentinite blocks contain a core comprised of surviving peridotite minerals plus lizardite (lz) ± antigorite (atg) + brucite (brc) + magnetite (mag) domains. The cores are surrounded by a ~10 cm thick peripheral rim of lizardite + magnetite. The reaction boundaries between rims and cores are parallel to the external polygonal surfaces of the blocks. These blocks are also sheathed by thin skins of sheared lizardite + chrysotile (ctl). The higher-temperature lz/atg + brc + mag serpentinization represented by the core mineralogy (Stage A) probably occurred on a large scale in the mantle and the hydration metamorphism producing the lz + mag mineralogy in the rims (Stage B) was likely triggered by the growth of a system of approximately planar fractures under crustal conditions that was enabled by the ingress of water at high pressure and that defined the polygonal block shapes as well as the Stage-A to Stage-B reaction boundaries. Stage-A serpentinization reactions are internally balanced except for addition of H 2 O, Cs and Rb as indicated by core mineral composition. The Stage-B serpentinization was open system with CaO, Cs and Rb migrating outside the blocks with addition of H 2 O, U, Pb ± LREE. The sheared lizardite skin probably formed during the ascent of the RCS through the crust. Several classes of mineral-filled veins were found in both cores and rims and indicate fluid pressures approaching lithostatic pressures that enabled these opening- mode fractures to occur. We posit that such high-pressure hydrothermal conditions were important in promoting the serpentinization reactions as well as enabling a weak rheology consistent with a cold-intrusion/diapiric emplacement mechanism. These factors give insights into where these mantle rocks came from, the sources of the water causing serpentinization, and the deformation mechanisms by which they come to Earth's surface. Similarities of the structures of serpentinite blocks reported in this paper with those in blocks from other serpentinite bodies in the San Francisco Bay Area (Lewis and Kirby, 2015 AGU Abstract T13H-05) suggests that the processes inferred from our investigation are regional in spatial extent. • A first detailed description of a serpentinite body along the San Andreas Fault (SAF). • Sequential mantle and crustal serpentinization reactions from blueschist depths. • Fracture systems infiltrated water and controlled block shape and reaction fronts. • Network systems of fractures with opening mode veins suggest high-fluid pressures. • High-pressure hydrothermal conditions brought the serpentinite up along SAF. [ABSTRACT FROM AUTHOR]

The Coachella Valley in the northern Salton Trough is known to produce destructive earthquakes, making it a high seismic hazard area. Knowledge of the seismic velocity structure and geometry of the sedimentary basins and fault zones is required to improve earthquake hazard estimates in this region. We simultaneously inverted first P wave travel times from the Southern California Seismic Network (39,998 local earthquakes) and explosions (251 land/sea shots) from the 2011 Salton Seismic Imaging Project to obtain a 3‐D seismic velocity model. Earthquakes with focal depths ≤10 km were selected to focus on the upper crustal structure. Strong lateral velocity contrasts in the top ~3 km correlate well with the surface geology, including the low‐velocity (<5 km/s) sedimentary basin and the high‐velocity crystalline basement rocks outside the valley. Sediment thickness is ~4 km in the southeastern valley near the Salton Sea and decreases to <2 km at the northwestern end of the valley. Eastward thickening of sediments toward the San Andreas fault within the valley defines Coachella Valley basin asymmetry. In the Peninsular Ranges, zones of relatively high seismic velocities (~6.4 km/s) between 2‐ and 4‐km depth may be related to Late Cretaceous mylonite rocks or older inherited basement structures. Other high‐velocity domains exist in the model down to 9‐km depth and help define crustal heterogeneity. We identify a potential fault zone in Lost Horse Valley unassociated with mapped faults in Southern California from the combined interpretation of surface geology, seismicity, and lateral velocity changes in the model. Key Points: Low P wave velocities in our model characterize a detailed, 3‐D asymmetric Coachella Valley basinThe 3‐D basin shape and Z2.5 surface produced in this study are important for improving seismic hazard estimates in Southern CaliforniaWe identify a potential fault zone in Lost Horse Valley currently unassociated with mapped faults in Southern California [ABSTRACT FROM AUTHOR]

In this paper we investigate the dynamic behavior of a system of interconnected faults in the Brawley Seismic Zone (BSZ) in southern California. The system of faults includes the southern San Andreas Fault (SSAF), the Imperial Fault (IF), and a set of cross faults in the BSZ that may serve as connecting structures between the two larger faults. Geological and seismic evidence imply that the SSAF and IF may have buried extensions that link them together in a large‐scale step over, with the cross faults in the BSZ cutting between them. Such a configuration poses the question of whether through‐going rupture across the step over is possible in this region, leading to large, plate‐boundary scale earthquakes. We investigate potential earthquakes in this region through 3‐D dynamic finite element spontaneous rupture modeling. We find that under multiple assumptions about fault stress and fault geometry, through‐going rupture is possible, both from north to south and south to north. Participation of the cross faults is facilitated by two factors: absence of rupture on one of the main two faults and a contrast in prestress between the main faults and the cross faults, leading to slow propagation speed on the main faults while maintaining ease of failure on the cross faults. The pattern of rupture propagation and slip is strongly affected by fault‐to‐fault dynamic stress interactions during the rupture process. The results may have implications for both potential earthquakes in this region, as well as for understanding the dynamics of geometrically complex/branched faults in general. Key Points: Based on our current assumptions about fault geometry and prestress, through‐going rupture might be possible in the Brawley Seismic ZoneSlip on the cross faults can hinder rupture propagation on the main faultsCross‐fault rupture appears to be facilitated by a contrast in stress initial conditions with the main fault segments [ABSTRACT FROM AUTHOR]

Reproducibility of results from scientific studies is rarely demonstrated outside the laboratory. Measurements of fault slip rate underpin scientific models of active faulting and seismic hazard assessments widely used for policymaking and risk mitigation. We replicated a highly referenced study that measured the slip rate as 33.9 ± 2.9 mm/year along the San Andreas fault over the past ~3,700 years at Wallace Creek in the Carrizo Plain National Monument, USA. Our results provide a slip rate of slip 36 ± 1 mm/year since ~3,500 CalBP. We find that the late Holocene slip rate of the San Andreas fault, a key indicator of seismic hazard, is reproducible within measurement uncertainty. Geologic slip rate determinations are relatively insensitive to short‐term fluctuations and thus should be reproducible if the time interval of measurement is much greater than the average rupture interval. Plain Language Summary: We have replicated a keystone San Andreas fault slip rate measurement from the 1980s and showed that fault slip rate is reproducible if measured over the same geologic time interval and if the time interval is much greater than the average time interval between surface rupturing earthquakes. Key Points: Slip rate of the San Andreas fault was measured at a keystone site in the 1980s and remeasured in 2011‐2017 with newer methodsWithin measurement uncertainty, the 3,700‐year slip rate published in 1984 agrees with this 2019 slip rate measurementGenerally, fault slip rate is reproducible within measurement uncertainty if the time interval of measurement is much greater than the average rupture interval [ABSTRACT FROM AUTHOR]

Highlights • The nature of seismogenesis in South California, USA is examined. • Bivariate frequency distributions of magnitudes and interevent times are studied. • Hypotheses of Extensive vs. Non-Extensive Statistical Physics behaviour are tested. • Persistent sub-Extensive (complex) seismogenetic systems are identified. • Strong evidence of complexity and SOC dynamics in background seismicity is found. Abstract We examine the nature of the seismogenetic system in South California, USA, by searching for evidence of complexity and non-extensivity in the earthquake record. We attempt to determine whether earthquakes are generated by a self-excited Poisson process, in which case they are independent, or by a Critical process, in which long-range interactions in non-equilibrium states are expected (correlation). Emphasis is given to background seismicity, i.e. to the rudimentary expression of the seismogenetic system. We use the complete and homogeneous earthquake catalogue published by the South California Earthquake Data Centre, in which aftershocks are either included, or have been removed by a stochastic declustering procedure. We examine multivariate cumulative frequency distributions of earthquake magnitudes, interevent time and interevent distance in the context of Non-Extensive Statistical Physics, which generalizes the additive Boltzmann-Gibbs thermodynamics to non-additive (non-extensive) dynamic systems. Our results indicate that the seismogenetic systems of South California are generally sub-extensive complex and certainly non-Poissonian. Background seismicity exhibits long-range interaction as evidenced by the overall increase of correlation observed by declustering the earthquake catalogues, as well as by the high correlation observed for earthquakes separated by long interevent distances. The results compare very well with those obtained in previous work for the seismogenetic systems of North California. Specifically, the Walker Lane – Eastern California Shear Zone on one hand, and the San Andreas Fault system – south California Continental Borderland region on the other, appear to comprise fault networks with different dynamics and expression. The former exhibits persistent, very high correlation and has attributes of (stationary) Self-Organized Criticality (SOC). The latter exhibits high correlation mainly at long ranges and has attributes of evolutionary of (Self-Organized) Criticality. All in all, our results are compatible with simulations of small-world fault networks in which free boundary conditions at the surface allow for self-organization and criticality to develop. [ABSTRACT FROM AUTHOR]

Abstract: Within the San Bernardino basin, some focal mechanisms show normal slip that is inconsistent with the expected interseismic strike‐slip loading of the region. The discrepancy may owe to deep (>10‐km depth), creep along the nearby northern San Jacinto fault. The enigmatic normal slip microseismicity occurs to the northeast of the fault and primarily below 10‐km depth, consistent with off‐fault deformation due to spatially nonuniform ongoing slip. Consequently, if these normal focal mechanisms are included in stress inversions from the seismic catalog, the results may provide inaccurate information about fault loading. Here we show that off‐fault loading from models with deep interseismic creep on the northern San Jacinto fault match the first‐order pattern of observed normal slip focal mechanisms in the basin and that this deep creep cannot be detected with GPS data due to the proximity of the San Andreas fault. [ABSTRACT FROM AUTHOR]

The crust and upper mantle structure of central California have been modified by subduction termination, growth of the San Andreas plate boundary fault system, and small-scale upper mantle convection since the early Miocene. Here we investigate the contributions of these processes to the creation of the Isabella Anomaly, which is a high seismic velocity volume in the upper mantle. There are two types of hypotheses for its origin. One is that it is the foundered mafic lower crust and mantle lithosphere of the southern Sierra Nevada batholith. The alternative suggests that it is a fossil slab connected to the Monterey microplate. A dense broadband seismic transect was deployed from the coast to the western Sierra Nevada to fill in the least sampled areas above the Isabella Anomaly, and regional-scale Rayleigh and S wave tomography are used to evaluate the two hypotheses. New shear velocity (Vs) tomography images a high-velocity anomaly beneath coastal California that is sub-horizontal at depths of ∼40–80 km. East of the San Andreas Fault a continuous extension of the high-velocity anomaly dips east and is located beneath the Sierra Nevada at ∼150–200 km depth. The western position of the Isabella Anomaly in the uppermost mantle is inconsistent with earlier interpretations that the Isabella Anomaly is connected to actively foundering foothills lower crust. Based on the new Vs images, we interpret that the Isabella Anomaly is not the dense destabilized root of the Sierra Nevada, but rather a remnant of Miocene subduction termination that is translating north beneath the central San Andreas Fault. Our results support the occurrence of localized lithospheric foundering beneath the high elevation eastern Sierra Nevada, where we find a lower crustal low Vs layer consistent with a small amount of partial melt. The high elevations relative to crust thickness and lower crustal low Vs zone are consistent with geological inferences that lithospheric foundering drove uplift and a ∼3–4 Ma pulse of basaltic magmatism. [ABSTRACT FROM AUTHOR]