DYNAMIC RUPTURE OF ASPERITIES AND STRESS CHANGE DURING A SEQUENCE OF LARGE INTERPLATE EARTHQUAKES IN THE MEXICAN SUBDUCTION ZONE

We investigate the spatial and temporal variations of shear stress due to the successive failures over an extensive segment of the Mexican s ubduction zone during a sequence of large interplate earthquakes that occurred over a period of 13 yr. For this purpose, we develop 3D dynam ic rupture models incorporating a shallowly dipping fault located abov e the subducting plate. The spatial distribution of dynamic stress dro p over the fault has been estimated for each of the events, through an inversion procedure using some of the previously derived kinematic fa ult parameters as observational constraints. The results revealed quit e heterogeneous stress changes during these earthquakes coming from me dium to high dynamic stress drop due to the rupture of a few patchlike asperities and from stress increase in between and around them. Two w eak asperities located southeast of the Michoacan segment were rupture d 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 XO to 100 bars and from another t wo asperities at depth. Two days after this largest event, two asperit ies were broken during the Zihuatanejo aftershock in the southeastern adjacent zone. Many aftershocks of these large events tend to be distr ibuted in the zones of stress increase outside the asperities, while o nly small numbers of aftershocks have been observed within these asper ity zones, It appears that several major asperities that existed in th is extensive segment have been ruptured successively so as to fill unb roken gaps on the plate interface. Thus, the stress change left over f rom the previous earthquake has dominant effects on the next event in this subduction zone.