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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 σ tend to creep, while high σ regions 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 t is 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/t acceleration 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. Plain Language Summary: 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. Key Points: 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/t acceleration prior to the mainshockThe precursory phase is characterized by migratory seismicity and creep over an extended region [ABSTRACT FROM AUTHOR]

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. [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]

In the aftermath of a devastating earthquake, the ensuing cascade of aftershocks can turn out to be even more destructive than the mainshock. Operational earthquake forecasting seeks to provide reliable real-time information about the time-dependence of earthquake hazard to the public. Two forecasting approaches tend to be used: statistical modelling, which employs empirical laws to predict the clustering characteristics (e.g. ETAS); and physics-based modelling, that combines the stress transfer hypothesis with rate-and-state friction laws (CRS models). Our work is partly motivated by recent retrospective experiments that reveal the comparable performance of physics-based and statistical forecasts.Here, we assess the influence of individual CRS model choices driven by real-time conditions, and clarify the effect of input data quality on the predictive skills of CRS forecasts during the 2016-17 Central Italy earthquake cascade. In the first year of the 2016-17 Central Italy sequence 1160 M ≥ 3.0 events were detected, spreading along a 60-km long normal fault system. Among these, seven M ≥ 5.4 earthquakes occurred from a few hours to five months after the first Mw 6.0 Amatrice event. We develop seven physics-based models with gradually increasing level of refinement as part of a pseudo-prospective experiment. The preliminary forecasts include data available just a few minutes after each M ≥ 5.4 event, featuring synthetic source models with empirically determined fault length and fault constitutive parameters from previous regional studies. Increasingly complex models incorporate: (1) optimized rate-state parameters, (2) spatially heterogeneous receiver fault planes, (3) best available slip models, and (4) secondary triggering effects from M ≥ 3.0 aftershocks. We evaluate and track models' performance in space and time using CSEP log-likelihood statistics for a 1-year time horizon after the Mw 6.0 Amatrice earthquake and compare against a benchmark ETAS model. When including refined input data and increasing model complexity, we observe a significant performance improvement of physics-based forecasts. The most enhanced CRS forecasts reach dramatic probability gains per earthquake of up to 1,000 over the preliminary physics-based realizations. The results confirm that CRS models are about as informative as ETAS only when secondary triggering effects are included together with realistic slip models, spatially heterogeneous receiver fault planes and optimized fault constitutive parameters. [ABSTRACT FROM AUTHOR]