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Dynamic rupture of asperities and stress change during a sequence of large interplate earthquakes in the Mexican subduction zone

Instituto de Geofisica Universidad Nacional Autonoma de Mexico, Mexico 04510 D.F., Mexicomikumo{at}ollin.igeofcu.unam.mx
Earthquake Research Institute University of Tokyo, Tokyo 113, Japan

Abstract

We investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the Mexican subduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr. For this purpose, we develop 3D dynamic rupture models incorporating a shallowly dipping fault located above the subducting plate. The spatial distribution of dynamic stress drop over the fault has been estimated for each of the events, through an inversion procedure using some of the previously derived kinematic fault parameters as observational constraints.

The results revealed quite heterogeneous stress changes during these earthquakes coming from medium to high dynamic stress drop due to the rupture of a few patch-like asperities and from stress increase in between and around them. Two weak asperities located southeast of the Michoacan segment were ruptured first by the 1979 Petatlan event. The 1981 Playa Azul event ruptured two asperities in the central zone with a stress drop higher than 80 bars. The largest 1985 Michoacan earthquake resulted from the rupture of two large-size, strong asperities located at both sides of the 1981 fault zone with high stress drop of 80 to 100 bars and from another two asperities at depth. Two days after this largest event, two asperities were broken during the Zihuatanejo aftershock in the southeastern adjacent zone. Many aftershocks of these large events tend to be distributed in the zones of stress increase outside the asperities, while only small numbers of aftershocks have been observed within these asperity zones. It appears that several major asperities that existed in this extensive segment have been ruptured successively so as to fill unbroken gaps on the plate interface. Thus, the stress change left over from the previous earthquake has dominant effects on the next event in this subduction zone.

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