Cascading Foreshocks and Aftershocks in a Discrete Fault Network

Kyungjae Im, & Jean-Philippe Avouac

Published September 11, 2022, SCEC Contribution #12030, 2022 SCEC Annual Meeting Poster #134

Earthquakes come in clusters formed mostly of mainshock-aftershock sequences, including occasional foreshocks. This clustering is thought to result primarily from stress transfer among faults. Here, we analyze how the characteristics of foreshock and aftershock cascades can be explained by fault interactions, taking into account the spatial distribution of faults and the fact that earthquake nucleation is not instantaneous. We resort to analytical approximations and numerical simulations with a discrete network of faults governed by rate and state friction. A fault network including a high-density fault zone favorable to stress transfer produces realistic foreshocks and aftershocks. The Omori law, characterizing the temporal decay of aftershocks, emerges in all simulations independently of the assumed distribution of the initial state of faults. By contrast, the inverse Omori law, which characterizes the temporal evolution of foreshocks, emerges in some simulations. The heterogeneous stress change imparted by the mainshock and subsequent aftershocks overprints the influence of the initial condition and leads to the Omori law independently of the initial velocity condition on the faults. A high-density fault zone favors fault interactions and the emergence of an accelerating sequence of foreshocks. Fault interactions are reduced during aftershock sequences due to the depletion of the number of critically stressed faults. The productivity of the cascading process is, therefore, significantly higher in foreshocks than in aftershocks in simulations that include a high-density fault zone. This effect is not captured by the ETAS model of fault interactions. It follows that a foreshock acceleration stronger than expected from ETAS statistics does not necessarily require an aseismic slip. It can be a manifestation of a cascading process enhanced by the topological properties of the fault network.

Key Words
friction, discrete fault, simulation

Citation
Im, K., & Avouac, J. (2022, 09). Cascading Foreshocks and Aftershocks in a Discrete Fault Network. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)