Modeling Pore Pressure Changes due to Slow Earthquakes: Implications for Tectonic Tremor Generation in Guerrero, Mexico.

Abstract:

Several studies claim that the origin of tectonic tremors (TT) and slow slip events (SSE) is strongly related to the presence of overpressure fluids. However, in order to determine the role of fluids in the generation of slow phenomena, quantitative models should explain the observations. In the Mexican subduction zones, SSEs are accompanied with tremor, though the spatial-time correlation of these phenomena is not as clear as the Episodic Tremor and Slip (ETS) observed in Cascadia and Japan. The complex behavior of TT and SSE exhibited in Guerrero makes it an interesting region to study and prove new hypothesis about the origin of these phenomena. In Guerrero, TTs occur along the horizontal segment of the slab over two regions denominated the Transient zone and Sweet Spot. These regions are located where near-lithostatic pore pressure (Pp) has been inferred by different studies in the upper oceanic crust. By computing the stress field induced by slow slip transients, we calculate the normal and shear stresses acting on the TT regions and solve the poroelastic diffusion equation considering both the permeability as a function of the effective pressure (Pe) (Evans et al., 1997) and a sealed plate interface. Such a nonlinear problem is solved in two-dimensions by means of a finite volume approach. Our modeling results show that the evolution of the Coulomb Failure Stress (CFS) within the slab, and at the plate interface, mimics the long-term occurrence-rate of the LFEs in the Sweet Spot. In addition, local gradients and Gaussian-like Pp distributions, that may be due to local dehydration processes and lateral temperature changes along the slab, yield solitary pore-pressure waves exhibiting two main characteristics: 1) a fast Pp perturbation front at speeds of some km/h (i.e.. 10-25 km/h) and 2) a slower Pp pulse with velocities on the order of km/day (i.e. 0.5-10 km/day). Both propagation velocities are comparable with the rapid (i.e., streaking) and slow migration of the TT sources recently observed in Guerrero (Cruz-Atienza et al., 2015). Further analysis is necessary to compare the lower speed with the slip front propagation of long term SSE and to assess if such a slow pulse may act as an external source leading the initiation of slow rupture fronts.