Outer‐rise earthquakes and seismic coupling

Abstract

Outer‐rise earthquakes can be separated into two basic categories based on focal mechanism: compressional and tensional events reflecting compressional and tensional outer‐rise stress regimes. We propose a model that connects the stress regime in the outer‐rise to the behavior of large subduction earthquakes and explains the focal mechanisms and both the temporal and spatial occurrence of outer‐rise earthquakes.We find that compressional outer‐rise events tend to occur oceanward of seismic gaps in seismically coupled subduction zones, or equivalently just prior to large subduction zone earthquakes. The compressional stress regime is probably caused by slip in adjacent subduction zone segments. There are two modes of occurrence for tensional outer‐rise earthquakes depending on whether they occur in coupled or uncoupled subduction zones. In coupled subduction zones, tensional events tend to occur after large subduction events as tensional stress from slab pull is temporarily transmitted to the outer‐rise. In subduction zones that are inherently uncoupled, tensional events can occur at any time. Therefore, in uncoupled subduction zones the outer‐rise is always in tension, and only tensional outer‐rise events occur there; while in coupled subduction zones, the outer‐rise is in a compressional state prior to large subduction earthquakes and a tensional state afterwards due to the large displacements.