Stress transfer and seismic phenomena in coupled subduction zones during the earthquake cycle (original) (raw)

1988, Journal of Geophysical Research

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 spaceand 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. 1. Ruiz, 1986; Drnowska et al., 1986; Astiz and Kanarnori, 1986; Christensen and Ruff, 1983; Korrat and Madariaga, 1986; Spence, 1987]. The distinction is that the notion of a seismic gap involves, essentially, only the area of main thrust earthquake. By comparison, that of an earthquake cycle encompasses space-and time-dependent phenomena occurring not only in the zone of the main subduction event but also downdip from that zone, including intraplate seismicity in the descending and overriding slabs [e.g., Shirnazaki, 1978] and phenomena in the nearby oceanic plate, including outer-rise events. It includes the time variation of stress and hence seismicity in all such regions, especially complex seismicity patterns in the area of the future main shock, including eventual quiescence. All these phenomena are linked together by the underlying mechanism of subduction of the descending slab, moderated and opposed by frictional forces arising from contact with the overriding continental plate and from motion relative to the mantle.