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Abstract 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. 

Abstract Numerical simulations of Sequences of Earthquakes and Aseismic Slip (SEAS) have rapidly progressed to address fundamental problems in fault mechanics and provide self‐consistent, physics‐based frameworks to interpret and predict geophysical observations across spatial and temporal scales. To advance SEAS simulations with rigor and reproducibility, we pursue community efforts to verify numerical codes in an expanding suite of benchmarks. Here we present code comparison results from a new set of quasi‐dynamic benchmark problems BP6‐QD‐A/S/C that consider an aseismic slip transient induced by changes in pore fluid pressure consistent with fluid injection and diffusion in fault models with different treatments of fault friction. Ten modeling groups participated in problems BP6‐QD‐A and BP6‐QD‐S considering rate‐and‐state fault models using the aging (‐A) and slip (‐S) law formulations for frictional state evolution, respectively, allowing us to better understand how various computational factors across codes affect the simulated evolution of pore pressure and aseismic slip. Comparisons of problems using the aging versus slip law, and a constant friction coefficient (‐C), illustrate how aseismic slip models can differ in the timing and amount of slip achieved with different treatments of fault friction given the same perturbations in pore fluid pressure. We achieve excellent quantitative agreement across participating codes, with further agreement attained by ensuring sufficiently fine time‐stepping and consistent treatment of boundary conditions. Our benchmark efforts offer a community‐based example to reveal sensitivities of numerical modeling results, which is essential for advancing multi‐physics SEAS models to better understand and construct reliable predictive models of fault dynamics. 

Hydrologic loads can stimulate seismicity in the Earth’s crust1. However, evidence for the triggering of large earthquakes remains elusive. The southern San Andreas Fault (SSAF) in Southern California lies next to the Salton Sea2, a remnant of ancient Lake Cahuilla that periodically filled and desiccated over the past millennium3,4,5. Here we use new geologic and palaeoseismic data to demonstrate that the past six major earthquakes on the SSAF probably occurred during highstands of Lake Cahuilla5,6. To investigate possible causal relationships, we computed time-dependent Coulomb stress changes7,8 due to variations in the lake level. Using a fully coupled model of a poroelastic crust9,10,11 overlying a viscoelastic mantle12,13, we find that hydrologic loads increased Coulomb stress on the SSAF by several hundred kilopascals and fault-stressing rates by more than a factor of 2, which is probably sufficient for earthquake triggering7,8. The destabilizing effects of lake inundation are enhanced by a nonvertical fault dip14,15,16,17, the presence of a fault damage zone18,19 and lateral pore-pressure diffusion20,21. Our model may be applicable to other regions in which hydrologic loading, either natural8,22 or anthropogenic1,23, was associated with substantial seismicity. 

The destructive 2023 moment magnitude ( M w ) 7.8-7.7 earthquake doublet ruptured multiple segments of the East Anatolian Fault system in Turkey. We integrate multi-scale seismic and space-geodetic observations with multi-fault kinematic inversions and dynamic rupture modeling to unravel the events’ complex rupture history and stress-mediated fault interactions. Our analysis reveals three sub-shear slip episodes during the initial M w 7.8 earthquake with delayed rupture initiation to the southwest. The M w 7.7 event occurred 9 hours later with larger slip and supershear rupture on its western branch. Mechanically consistent dynamic models accounting for fault interactions can explain the unexpected rupture paths, and require a heterogeneous background stress. Our results highlight the importance of combining near- and far-field observations with data-driven and physics-based models for seismic hazard assessment. 

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 <30 Hz), and likely represents a relatively fast velocity block sandwiched between broader MCF and BF zones. The maximal arrival delays (P ∼0.1 s and S ∼0.25 s) and the highest ambient noise levels (>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 Landslides commonly occur in areas with steep topography and abundant precipitation and pose a significant hazard to local communities. Some of the largest known landslides occur in Alaska, including several that caused local tsunamis. Many landslides may have gone undetected in remote areas due to lack of observations. Here, we develop a semiautomated workflow using both seismic and geodetic observations to detect, locate, validate, and characterize landslides in Alaska. Seismic observations have shown promise in continuously monitoring landslide occurrence, while remote sensing techniques are well suited for verification and high‐resolution imaging of landslides. We validate our procedure using the 28 June 2016, Lamplugh Glacier landslide. We also present observations of a previously unknown landslide occurred on 22 September 2017 in the Wrangell Mountains region. The Wrangell Mountains landslide generated a coherent surface wavefield recorded across Alaska and the contiguous United States. We used Sentinel‐1 Synthetic Aperture Radar and Sentinel‐2 optical imagery to map the respective mass deposit. To investigate the landslide dynamics, we inverted regional seismic surface wave data for a centroid single force failure model. Our model suggests that the Wrangell Mountains landslide lasted for about 140 s and had two subevents involving at least five distinct stages. We estimate that the landslide had displaced 3.1–13.4 million tons of rocks over a distance of ∼2 km. Our results suggest that combining seismic and geodetic observations can vastly improve the detection and characterization of landslides in remote areas in Alaska and elsewhere, providing new insights into the landslide dynamics. 

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

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.