Dynamic modelling of the subduction zone of central Mexico

In central Mexico some significant normal faulting events have occurred wit hin the subducted oceanic Cocos plate, just below or near the down-dip edge of the strongly coupled interface. These normal faulting shocks followed l arge shallow thrust earthquakes. in other subduction zones such events gene rally precede the up-dip thrust events. A vertical 2-D finite element model ling has been used to simulate the subduction of the Cocos plate beneath th e North American plate when the slab is driven by an active convergence vel ocity or slab pull. We find that the latter mechanism plays only a minor ro le due to shallow subduction. The modelling results show that the stress pa ttern is very sensitive to the geometry of the plates. In particular, norma l faulting earthquakes that follow large thrust events can be explained on the basis of the flexural response of the overriding and subducting plates to the peculiar geometry of this subduction zone, where the subducting slab becomes horizontal at about 100 km from the trench. This horizontal part o f the subducting plate, down-dip with respect to the main thrust zone, is u nder an extensional stress field. This provides an alternative explanation to the slab pull for the occurrence of normal faulting intraplate earthquak es. In order for normal faulting earthquakes to occur in the early part of the seismic cycle, it is necessary that the large up-dip thrust events have a partial stress drop. We find that for small fractional stress drop, a wi de region of extension remains below the down-dip edge of the main fault pl ane following a large thrust earthquake. Thus, the main thrust earthquakes do not invert the polarity of the active stress field, which is compression al and extensional up-dip and down-dip, respectively, with respect to the m ain thrust fault. Larger fractional stress drops result in larger delays in the occurrence of normal faulting events after the main thrust events.