Elastic strain energy and pore-fluid pressure control of aftershocks

Abstract

Aftershocks are well-known, however their triggering mechanisms remain unclear. The Coulomb failure stress change (ΔCFS) has been widely implemented to understand the spatial distribution of aftershocks. Here we propose a new metric for evaluating aftershock generation based on the energetics of shear faulting in a prestressed state. Unlike ΔCFS, the new metric depends not only on coseismic stress changes but also on background crustal stresses. The energetics-based formulation expands the ΔCFS defined on a specific plane into a generalized failure stress defined in a three-dimensional space. We examined the diagnostic ability of the new metric by applying receiver operating characteristic analysis to 12,175 aftershocks (M≥0) that followed the 1992 Landers earthquake. With a realistic background stress field inferred from thousands of earthquake focal mechanisms, the new metric was able to discriminate the triggering mechanisms of the aftershocks: two-thirds were direct responses to coseismic stress changes and one-fifth resulted from strength decreases owing to pore-fluid pressure increases.