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Tectonic stress in the crust evolves during a seismic cycle, with slow stress accumulation over interseismic periods, episodic stress steps at the time of earthquakes, and transient stress readjustment during a postseismic period that may last months to years. Static stress transfer to surrounding faults has been well documented to alter regional seismicity rates over both short and long time scales. While static stress transfer is instantaneous and long lived, postseismic stress transfer driven by viscoelastic relaxation of the ductile lower crust and mantle leads to additional, slowly varying stress perturbations. Both processes may be tested by comparing a decade‐long record of regional seismicity to predicted time‐dependent seismicity rates based on a stress evolution model that includes viscoelastic stress transfer. Here we explore crustal stress evolution arising from the seismic cycle in Southern California from 1981 to 2014 using five M≥6.5 source quakes: the M7.3 1992 Landers, M6.5 1992 Big Bear, M6.7 1994 Big Bear, M7.1 1999 Hector Mine, and M7.2 2010 El Mayor‐Cucapah earthquakes. We relate the stress readjustment in the surrounding crust generated by each quake to regional seismicity using rate‐and‐state friction theory. Using a log likelihood approach, we quantify the potential to trigger seismicity of both static and viscoelastic stress transfer, finding that both processes have systematically shaped the spatial pattern of Southern California seismicity since 1992. A catalog spanning three decades of Southern California seismicity is consistent with the Coulomb rate‐state stressing modelSpatial patterns of seismicity evolved systematically following the 1992 Landers, California, earthquakeBoth static and viscoelastic stress transfer mechanisms are effective at triggering seismicity

Magmatic intrusions release extensional strain in the Earth's crust upon availability of magma. Intrusions are typically accompanied by earthquake swarms and by surface faulting that is often larger than what is expected from the magnitude of the induced earthquakes. The 2000 Miyakejima dike intrusion triggered the largest volcanic earthquake swarm monitored so far, with five Ml>6 earthquakes. We analyze the seismicity and deformation induced by the Miyakejima dike with the aim of constraining the timescale and mechanisms of slow strain release during the episode. In six earthquake bursts lasting few hours and migrating at ~1 km h-1we find candidates for slow earthquakes. Each burst nucleated at the tips of previous bursts, suggesting stress interaction. The variability of fault plane solutions indicates that the bursts occurred on a complex system of fractures, consistent with weakly consolidated surface layers strained by spatially inhomogneous stresses that change in time, such as those induced by a dike. Based on dislocation models, we find that deformation is best explained by aseismic slip (in addition to the seismic burst), with a moment 1.3 to 2.3 times larger than the earthquakes' seismic moment, and opening of 0.20 ± 0.07 m on the dike. The aseismic slip occurred over a few hours, with moment, duration, and migration velocity consistent with that of previously observed slow slip events. We argue that the seismic bursts are likely driven by slow slip, sharing most properties with tectonic slow slip events and swarms, but occurring on a set of nonaligned faults. During the 2000 Miyakejima dike intrusion we detect short bursts of seismicity lasting a few hours and propagating along the dikeThe surface deformation during the largest burst is best explained by aseismic slip taking place on the same faults as the earthquakesIntermittent slip episodes with a large aseismic component may accommodate a significant fraction of strain during diking

While dynamic earthquake triggering has been reported in several continental settings, offshore observations are rare. Oceanic transform faults share properties with continental geothermal areas known for dynamic triggering: high geothermal gradients, high seismicity rates, and frequent swarms. We study dynamic triggering along the East Pacific Rise by analyzing 1 year of seismicity recorded by Ocean Bottom Seismographs. By comparing the response to teleseismic waves from global earthquakes, we find triggering to be most sensitive to changes in normal stress and to preferentially occur above 0.25 kPa. The clearest example of triggering occurs on the Quebrada and Gofar faults after the Mw8.0 Wenchuan earthquake. On Gofar, triggered seismicity occurs between the rupture areas of large earthquakes, within a zone characterized by aseismic slip, abundant microseismicity, frequent swarms, and low Vp. We infer that lithological properties inhibiting rupture propagation, such as high porosity and fluid content, also favor dynamic triggering. We perform a systematic search for dynamic triggering on three transform faults located along the equatorial East Pacific RiseFault segments with low seismic coupling and high rates of microseismicity are more sensitive to dynamic triggeringSurface waves that produce the highest normal stress changes and the largest first strain invariant appear more effective at triggering

The spatiotemporal clustering of earthquakes is a feature of medium‐ and short‐term seismicity, indicating that earthquakes interact. However, controversy exists about the physical mechanism behind aftershock triggering: static stress transfer and reloading by postseismic processes have been proposed as explanations. In this work, we use a Coulomb rate‐and‐state model to study the role of coseismic and postseismic stress changes on aftershocks and focus on two processes: creep on the main shock fault plane (afterslip) and secondary aftershock triggering by previous aftershocks. We model the seismic response to Coulomb stress changes using the Dieterich constitutive law and focus on two events: the Parkfield, Mw= 6.0, and the Tohoku, Mw= 9.0, earthquakes. We find that modeling secondary triggering systematically improves the maximum log likelihood fit of the sequences. The effect of afterslip is more subtle and difficult to assess for near‐fault events, where model errors are largest. More robust conclusions can be drawn for off‐fault aftershocks: following the Tohoku earthquake, afterslip promotes shallow crustal seismicity in the Fukushima region. Simple geometrical considerations indicate that afterslip‐induced stress changes may have been significant on trench parallel crustal fault systems following several of the largest recorded subduction earthquakes. Moreover, the time dependence of afterslip strongly enhances its triggering potential: seismicity triggered by an instantaneous stress change decays more quickly than seismicity triggered by gradual loading, and as a result we find afterslip to be particularly important between few weeks and few months after the main shock. Inclusion of secondary triggering improves the performance of aftershock modelsEffect of afterslip on near‐fault aftershocks is hard to asses due to uncertaintiesAfter megathrust earthquakes, afterslip enhances seismicity on crustal faults

Stress transfer between earthquakes is recognized as a fundamental mechanism governing aftershock sequences. A common approach to relate stress changes to seismicity rate changes is the rate‐and‐state constitutive law developed by Dieterich: these elements are the foundation of Coulomb‐rate‐and‐state (CRS) models. Despite the successes of Coulomb hypothesis and of the rate‐and‐state formulation, such models perform worse than statistical models in an operational forecasting context: one reason is that Coulomb stress is subject to large uncertainties and intrinsic spatial heterogeneity. In this study, we characterize the uncertainties in Coulomb stress inherited from different physical quantities and assess their effect on CRS models. We use a Monte Carlo method and focus on the following aspects: the existence of multiple receiver faults; the stress heterogeneity within grid cells, due to their finite size; and errors inherited from the coseismic slip model. We study two well‐recorded sequences from different tectonic settings: the Mw= 6.0 Parkfield and the Mw= 9.0 Tohoku earthquakes. We find that the existence of multiple receiver faults is the most important source of intrinsic stress heterogeneity, and CRS models perform significantly better when this variability is taken into account. The choice of slip model also generates large uncertainties. We construct an ensemble model based on published slip models and find that it outperforms individual models. Our findings highlight the importance of identifying sources of errors and quantifying confidence boundaries in the forecasts; moreover, we demonstrate that consideration of stress heterogeneity and epistemic uncertainty has the potential to improve the performance of operational forecasting models. High impact of Coulomb stress uncertainties on aftershock modelingThe variable orientation of receiver faults generates large stress heterogeneityEnsemble models based on a set of alternative slip models perform best

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. 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. 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 We pursue community efforts to develop code verification benchmarks for three‐dimensional earthquake rupture and crustal faulting problems We assess the agreement and discrepancies of seismic and aseismic fault behavior among simulations based on different numerical methods Our comparisons lend confidence to numerical codes and reveal sensitivities of model observables to major computational and physical factors

Foreshocks are not uncommon prior to large earthquakes, but their physical mechanism remains controversial. Two interpretations have been advanced: (1) foreshocks are driven by aseismic nucleation and (2) foreshocks are cascades, with each event triggered by earlier ones. Here, we study seismic cycles on faults with fractal roughness at wavelengths exceeding the nucleation length. We perform 2‐D quasi‐dynamic, elastic simulations of frictionally uniform rate‐state faults. Roughness leads to a range of slip behavior between system‐size ruptures, including widespread creep, localized slow slip, and microseismicity. These processes are explained by spatial variations in normal stress (σ) caused by roughness: regions with low σtend to creep, while high σregions remain locked until they break seismically. Foreshocks and mainshocks both initiate from the rupture of locked asperities, but mainshocks preferentially start on stronger asperities. The preseismic phase is characterized by feedback between creep and foreshocks: episodic seismic bursts break groups of nearby asperities, causing creep to accelerate, which in turns loads other asperities leading to further foreshocks. A simple analytical treatment of this mutual stress transfer, confirmed by simulations, predicts slip velocities and seismicity rates increase as 1/t, where tis the time to the mainshock. The model reproduces the observed migration of foreshocks toward the mainshock hypocenter, foreshock locations consistent with static stress changes, and the 1/tacceleration in stacked catalogs. Instead of interpreting foreshocks as either driven by coseismic stress changes or by creep, we propose that earthquake nucleation on rough faults is driven by the feedback between the two. Understanding premonitory seismicity leading up to large earthquakes has been a central problem in seismology for several decades. In spite of constantly improving observational networks and data analysis tools, we are still grappling with the fundamental question: what causes foreshocks? Do they represent a chain of isolated events, or are they driven by slow slip over a large fault area, gradually accelerating before the mainshock? In this study, we tackle this question with numerical simulations of slip on a fault with a realistic (fractal) geometry. This geometrical complexity causes spatial variations in stress: compression or extension occur as irregularities on opposite sides of the fault are pressed closer together or pulled apart. This spatial heterogeneity modulates slip stability across the fault, causing simultaneous occurrence of slow slip and foreshocks. The two processes are linked by a positive feedback, since each increases stress at the location of the other; under certain conditions, this can culminate in a large earthquake. Our model reproduces a number of observed foreshock characteristics, and offers new insights on the physical mechanism driving them. Rough fault simulations exhibit simultaneous foreshocks and creep caused by heterogeneity in normal stress induced by roughnessStress transfer between foreshocks and creep produces a positive feedback and 1/tacceleration prior to the mainshockThe precursory phase is characterized by migratory seismicity and creep over an extended region Rough fault simulations exhibit simultaneous foreshocks and creep caused by heterogeneity in normal stress induced by roughness Stress transfer between foreshocks and creep produces a positive feedback and 1/tacceleration prior to the mainshock The precursory phase is characterized by migratory seismicity and creep over an extended region