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Seismologic studies have reported seismic velocities reduction and Vp/Vs ratio changes over damage zones associated with seismogenic faults. The structure and elastic properties of these damage zones indicate the maturity of faults and affect the rupture dynamics of future seismic events. Therefore, they contain critical information about fault properties that could inform seismic hazards. Here we present a geodetic modeling approach to constrain velocity changes and elastic properties of fault damage zones under stress perturbation from nearby earthquakes. Compared to seismic tomographic analysis that is usually limited by resolution, this geodetic approach provides tighter constraints on the elastic properties and geometry of the damage zone at shallow depths. Our results imply that a major component of the shallow strain release is distributed and inelastic. The existence of numerous shallow faults either may indicate a locally detached shallow layer or they are remnants from earlier fault evolution. Plain Language Summary: Ruptures of large earthquakes tend to fracture the materials nearby, thus creating a zone with lower seismic velocity, which is known as fault damage zone. Understanding the structure and mechanics of this zone will help reveal the status of fault evolution and seismogenic depth for future ruptures. To achieve that, we developed a geodetic modeling approach to determine the geometric and physical parameters of fault damage zones, utilizing observations from high‐resolution Interferometric Synthetic Aperture Radar and strain release estimates from kinematic fault slip models. By searching the parameters in a specific order, each unknown could be uniquely constrained from the observed deformation profiles. The approach works well with fault damage zones under compliant motion induced by the stress from nearby earthquakes. The method is straightforward, computationally efficient and is applicable even to a fault damage zone with 250 m width near the 2019 Ridgecrest earthquake sequence. Our study suggests that these structures are quite shallow and accommodate a major component of shallow strain release. Key Points: We developed a geodetic modeling approach to constrain structural and mechanical properties of fault damage zonesThe approach is straightforward, computationally efficient, and works well even over a small fault damage zone with 250 m widthThe existence of many shallow fractures indicates distributed shallow strain release over transtensional shear zones [ABSTRACT FROM AUTHOR]

Dynamic modeling of sequences of earthquakes and aseismic slip (SEAS) provides a self‐consistent, physics‐based framework to connect, interpret, and predict diverse geophysical observations across spatial and temporal scales. Amid growing applications of SEAS models, numerical code verification is essential to ensure reliable simulation results but is often infeasible due to the lack of analytical solutions. Here, we develop two benchmarks for three‐dimensional (3D) SEAS problems to compare and verify numerical codes based on boundary‐element, finite‐element, and finite‐difference methods, in a community initiative. Our benchmarks consider a planar vertical strike‐slip fault obeying a rate‐ and state‐dependent friction law, in a 3D homogeneous, linear elastic whole‐space or half‐space, where spontaneous earthquakes and slow slip arise due to tectonic‐like loading. We use a suite of quasi‐dynamic simulations from 10 modeling groups to assess the agreement during all phases of multiple seismic cycles. We find excellent quantitative agreement among simulated outputs for sufficiently large model domains and small grid spacings. However, discrepancies in rupture fronts of the initial event are influenced by the free surface and various computational factors. The recurrence intervals and nucleation phase of later earthquakes are particularly sensitive to numerical resolution and domain‐size‐dependent loading. Despite such variability, key properties of individual earthquakes, including rupture style, duration, total slip, peak slip rate, and stress drop, are comparable among even marginally resolved simulations. Our benchmark efforts offer a community‐based example to improve numerical simulations and reveal sensitivities of model observables, which are important for advancing SEAS models to better understand earthquake system dynamics. Plain Language Summary: Earthquakes and fault zone processes occur over time scales ranging from milliseconds to millennia and longer. Computational models are increasingly used to simulate sequences of earthquakes and aseismic slip (SEAS). These simulations can be connected to diverse geophysical observations, offering insights into earthquake system dynamics. To improve these simulations, we pursue community efforts to design benchmarks for 3D SEAS problems. We involve earthquake researchers around the globe to compare simulation results using different numerical codes. We identify major factors that contribute to the discrepancies among simulations. For example, the spatial dimension and resolution of the computational model can affect how earthquakes start and grow, as well as how frequently they recur. Code comparisons are more challenging when we consider the Earth's surface in the simulations. Fortunately, we find that several key characteristics of earthquakes are accurately reproduced in simulations, such as the duration, total movement, maximum speed, and stress change on the fault, even when model resolutions are not ideal. These exercises are important for promoting a new generation of advanced models for earthquakes. Understanding the sensitivity of simulation outputs will help test models against real‐world observations. Our community efforts can serve as a useful example to other geoscience communities. Key Points: We pursue community efforts to develop code verification benchmarks for three‐dimensional earthquake rupture and crustal faulting problemsWe assess the agreement and discrepancies of seismic and aseismic fault behavior among simulations based on different numerical methodsOur comparisons lend confidence to numerical codes and reveal sensitivities of model observables to major computational and physical factors [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]