Intraslab Fluid Migration During Slow Slip Earthquakes and Nonvolcanic Tremors in Guerrero, Mexico.

Abstract:

Observations in different subduction zones have suggested that overpressured fluids nearby the plate interface are closely related to the origin of nonvolcanic tremors (NVT) and low frequency earthquakes (LFE). The physics of the plate interface would strongly differ depending on whether NVT and LFE triggering is produced by fluid migration due to the SSE strain field or, to the long-term fluid emplacement induced by the slip transients. In this work we study the migration of confined fluids in the subducted Cocos plate during two SSE in Guerrero. To this purpose we coupled the evolution of the 3D strain field induced by the silent events of 2006 and 2009-2010 with the 2D poroelastic equation for fluid transport. We considered the SSE slip history determined by previous authors through the inversion of GPS data and used a 3D finite difference code to compute the evolution of the associated confining pressure in the crust. As a result, we obtained the velocity vectors of the fluids within the top 5 km of the Cocos plate (i.e., slab) assuming typical permeability values from 1x10^-9-1x10^-16 m² reported in the literature for the oceanic crust. For the 2006 SSE, two low-pressure zones acting as fluid “attraction poles” arise at 150-175 and 200-230 km from the trench, with diffusion velocities ranging between 10^-3 and 10^-9 km/day. These zones surprisingly agree with the regions where NVTs are systematically observed during the occurrence of the SSEs. For the 2009-2010 SSE, which is composed by two successive slip transients, fluids essentially migrate towards the plate interface (i.e., upward) in its horizontal segment, where must of NVTs and LFEs seem to take place. Since the maximum Darcy’s velocities for both events are of the order of meters per day (i.e., 3 orders of magnitude smaller than the migration speed of the LFE streaks), we concluded that the fluids diffusive transport is not responsible of the NVT and LFE migration during the SSEs. Instead, our results suggest that the strain field periodically induced by the slip transients act as a long-term pumping process that ultimately concentrates the fluids (and thus reduces the effective pressure) in the regions where the seismicity takes place as a consequence of small stress perturbations associated to the propagation of the aseismic slip.