Precursory Slow Slip and Foreshocks on Rough Faults

Abstract

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.

Possible Precursory Slow‐Slip to Two ML∼3 Mainevents of the Diemtigen Microearthquake Sequence, Switzerland

“…Our observations are reminiscent of those of acoustic emissions in laboratory experiments, where the underlying precursory aseismic fault slip was measured directly (e.g., Bolton et al, 2019;Goebel et al, 2013;Rouet-Leduc et al, 2017). Recent theoretical models (e.g., Cattania & Segall, 2021) further substantiate this interpretation.…”

Scale Dependence of Earthquake Rupture Prestress in Models With Enhanced Weakening: Implications for Event Statistics and Inferences of Fault Stress

“…Our modeling shows that increasingly efficient dynamic weakening leads to different earthquake statistics, with fewer small events and increasing number of large events. Another factor that can significantly affect the ability of earthquake ruptures to propagate is fault heterogeneity, including variations in the rate-andstate frictional properties, effective normal stress, and fault roughness (e.g., Ampuero et al, 2006;Cattania & Segall, 2021;Heimisson, 2020;Hillers et al, 2006Hillers et al, , 2007Schaal & Lapusta, 2019). Some dynamic heterogeneity in shear stress spontaneously develops in our simulations, leading to a broad distribution of event sizes for cases with mild to moderate enhanced dynamic weakening.…”

“…Another factor that can significantly affect the ability of earthquake ruptures to propagate is fault heterogeneity, including variations in the rate-and-state frictional properties, effective normal stress, and fault roughness (e.g. Hillers et al, 2006Hillers et al, , 2007Ampuero et al, 2006;Schaal & Lapusta, 2019;Heimisson, 2020;Cattania & Segall, 2021). Some dynamic heterogeneity in shear stress spontaneously develops in our simulations, leading to a broad distribution of event sizes for cases with mild to moderate enhanced dynamic weakening.…”

Fine Structure of Microseismic Glacial Stick‐Slip

“…In this view, our "main asperity" is not a purely rate-weakening frictional patch surrounded by rate-strengthening material (T. Chen & Lapusta, 2009;Johnson & Nadeau, 2002), but a superposition of nearly simultaneous ruptures at many smaller asperities with a spatially constant location of rupture initiation. Some hypotheses explaining such behavior may be, for example, locally higher normal stress (Cattania & Segall, 2021) or regions that exhibit lower levels of fracture surface energy (Ide & Aochi, 2005;Noda et al, 2013). This is supported by one event at the end of the main asperity's active period that shows a magnitude 20 times weaker than typical events, indicating that it may have only ruptured a subset of weak asperities.…”