Foreshocks on Rough Rate-State Faults

Camilla Cattania

Stanford University

Date & Time
Yosemite Conference Room, Building 19, Moffett Field
Annemarie Baltay

Foreshocks are not uncommon prior to large earthquakes, but the physical mechanism driving them remains controversial. Two contrasting interpretations have been put forward: 1. foreshocks are driven by the aseismic nucleation process; 2. foreshocks occur as a cascade, with each event triggered by the previous ones and eventually triggering the mainshock.

Here we consider seismic cycles on a rate-state fault with fractal roughness at wavelengths exceeding the nucleation length and homogeneous velocity-weakening friction. We perform 2-D pseudo-dynamic simulations on a rough surface loaded by a uniform far-field stressing rate (with small perturbations due to local fault orientation).
Roughness leads to a rich slip behavior between large (system-size) ruptures, including widespread creep, localized slow slip, and microseismicity intensifying prior to the mainshock. These processes are well explained by the spatial variations in normal stress caused by roughness: regions with lower-than-average normal stress (such as releasing bends) experience interseismic creep, which in turn triggers seismicity on regions with higher-than-average normal stress (such as restraining bends). Most microseismicity occurs during, and is driven by, the nucleation phase. Creep velocities derived from a spring slider analysis predict seismicity rate to increase with time as 1/t, where t is the time to the mainshock, in agreement with the simulated stacked catalog. The relative location of the foreshocks is consistent with static stress triggering, and so is the presence of clusters deviating from the 1/t prediction. After each foreshock, asperities do not fully relock but instead they creep; this lowers the effective stiffness of the nucleation region, contributing to the acceleration. Even though nucleation is primarily aseismic in terms of moment release, foreshocks play an important role through feedbacks between asperity failure and the surrounding creep.

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