Collaborative Research: Seismic and Aseismic Slip Interactions on a Subduction Megathrust, Guerrero, Mexico

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A transient event in Guerrero, Mexico, propagated from east to west along-strike, near the locking transition on the megathrust, and it affected the entire length of the Guerrero seismic gap over a period of months during early 1998. Campaign GPS data suggest the possibility that the 1998 Guerrero event is causally related to afterslip following the 1995 Mw=7.3 Copala earthquake. The Copala earthquake was located about 120 km east of the Guerrero gap, but afterslip appears to have propagated westward along-strike as much as 100 km during the first six months following the seismic rupture. Curiously, background seismic activity on the Guerrero megathrust unusually quiescent during the several months before, during and after the aseismic slip event, but returns to normal or above-normal levels several months after the transient ended. These observations raise several fundamental questions. First, did the Guerrero aseismic slip event really begin as afterslip following the Copala earthquake? Or is the occurrence of aseismic slip east of Acapulco in 1995-96 and west of Acapulco in 1998 merely coincidental? Secondly, how did the stress changes generated by these aseismic slip events influence seismic activity on the megathrust?This research project is examining these questions using JERS-1 InSAR data collected during the years 1994.2 to 1997.3 to estimate coseismic slip of the Copala event and postseismic afterslip in the region east of Acapulco. Also, strong ground motion seismic data from the Guerrero accelerograph network and broadband data from the Mexican regional network are being used to estimate earthquake source parameters from events with magnitude Mw7 that had never been examined previously. The events are being relocated using modern network differencing techniques, and the focal mechanisms, moment release, stress drops and other source parameters are also being characterized. The research team is combining the geodetic and seismic source to the slip history on the subduction megathrust during the past decade, and the associated change in Coulomb failure criteria as a function of time. The researchers are also assessing the space-time connection between Copala earthquake afterslip and the Guerrero slip transient, as well as the relationship between aseismic slip and seismic activity, using a model of nonlinear dynamic friction on the megathrust.