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

Abstract:

Observations in different subduction zones have suggested that overpressured fluids close to the plate interface may be related to the origin of nonvolcanic tremors (NVT) and low frequency earthquakes (LFE). One condition for a causal relationship to exist between fluids and such seismicity is their spatial collocation. Fluids at nearly lithostatic pressures within the top few kilometers of the oceanic crust have been inferred in several subduction zones, including the province of Guerrero, Mexico. However, NVT hypocentral depths in this region have been poorly resolved so that casualty could not even be tested as a hypothesis. In this work, we report NVT relocations from a 3-year-catalog in Guerrero that includes the period of the Slow Slip Earthquake (SSE) of 2006. We used a new location technique called the “Tremor Energy and Polarization” (TREP) method (Cruz-Atienza et al., JGR, 2014), which jointly determines the source location and focal mechanism of sustained tremor signals by simultaneously inverting (1) the energy spatial distribution, (2) the energy spatial derivatives, and (3) the azimuthal direction of the particle motion polarization ellipsoid. In agreement with previous works, NVT epicentral locations concentrate between 200 and 230 km from the trench and then migrate ~40 km trenchward during the occurrence of the SSE. However, unlike earlier investigations, most NVT hypocenters lie at 43 km depth near the plate interface and have subparallel rake angles to the Cocos plate convergence direction. These results are consistent with independent locations and mechanisms of LFE in the region and allow us to examine the casualty hypothesis mentioned above. Poroelastic modeling of fluid transport during two SSEs in Guerrero (Villafuerte and Cruz-Atienza, AGU, 2014) show that fluids migrate towards the spots where this seismicity occurs with maximum velocities between 10^-3 and 10^-9 km/day, which are more than 3 orders of magnitude smaller than the LFE migration speeds. We conclude that fluid diffusive transport is not responsible of the NVT and LFE migration during the SSEs but instead that slip transients act as a long-term pumping process decreasing the effective pressure where the seismicity takes place as a consequence of small stress perturbations associated to the propagation of the aseismic slip.