Stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle

Abstract

An elementary mechanical model of a coupled subduction zone is presented. It is used to study the transfer of extensional stresses along the underthrusting plate, and shear stress interactions between the plate itself and its thrust contact zone with the overriding plate, during the earthquake cycle. The model assumes an elastic plate which deforms one dimensionally, has shear drag interactions (treated approximately by the Elsasser procedure) with a Maxwellian viscoelastic mantle, and is subject to steady gravitational sinking forces. Great thrust earthquakes are simulated as sudden stress‐relieving displacements at the contact zone between the slab and overriding plate. The model allows analysis of the space‐ and time‐variable stresses in the main thrust zone, the adjacent oceanic plate, and the subducting slab over each earthquake cycle. Stress variations leading to outer‐rise events are discussed, as well as stress changes responsible for intraplate seismicity downdip from the main thrust zone, based on simulations of periodic great earthquakes. The results of simulated slow progression of deformation (slip) from below into the thrust contact zone are also presented. Such preslip, occurring over a time scale that is small in comparison with the whole seismic cycle (e.g., of the order of a tenth or so), is shown to transiently slow the rate of buildup of, or even relieve, extensional stresses in the slab. In the absence of preslip those stresses build up steadily in time during the latter part of the cycle. The results have implications for induction of sesimic quiescence as well as for explaining patterns of seismicity in relation to spatial position along the underthrusting plate and to time throughout the cycle. Correlations are reported between the model simulations and seismicity observations from coupled subduction zones around the world. The observations include tensional outer‐rise events after large subduction earthquakes and compressional ones in the latter parts of the cycle, normal (tensional) intraplate earthquakes preceding the main thrust events and positioned downdip from the thrust zones, and interlaced periods of seismic activity in the lower part of the main thrust zone and in the descending slab.