Interplate coupling and a recent aseismic slow slip event in the Guerrero seismic gap of the Mexican subduction zone, as deduced from GPS data inversion using a Bayesian information criterion

Abstract

A large-scale slow slip event with a magnitude equivalent to Mw = 7.4 has been observed at seven continuous GPS stations located in the Guerrero–Oaxaca region, southern Mexico, for several months from late 2001 to middle 2002 [Geophys. Res. Lett. (2003) 30: doi: 10.1029/2003GL017219]. We re-analyzed the GPS time series data to estimate the displacements associated with the slow slip event and also the displacement rates during the interseismic period prior to this event, by applying an inversion technique using a Bayesian information criterion (ABIC). We then carried out GPS data inversion again with ABIC and some detailed forward modeling to obtain spatial distributions of the back-slip rates and the slow slip on a 3-D curved plate interface between the subducting Cocos and the overriding North America plates. The results show that the average direction of the back-slip rates is N31.3°E ± 5.6°, which is congruent with that of plate convergence in this region. Interplate coupling is found to be very strong down to a depth of about 45 km, indicating a coupling ratio of 0.83–0.86 except for a couple of segments on the model region, and decreases dramatically down below. The average direction of the slow slip is oriented about 20°–35° counterclockwise from the opposite direction of the plate convergence. The slow slip detected on the mid-depth segment to the upper rim of the model region reaches about 9–18 cm in the landward region of the Guerrero seismic gap from inversion and forward modeling. From these results, we conclude it is possible that the slow slip may have invaded a deeper part of the strongly coupled, seismogenic zone at least up to a depth of about 25 km. We also estimate stress changes due to the slow slip event. Some decrease in shear stress on the plate interface at these depths suggests that if such a slow slip event occurred episodically, it is possible that the time of occurrence of a forthcoming large earthquake in the Guerrero gap would be delayed to some extent.