Tectonic Tremor Modulation by Intraslab Fluid Diffusion During Slow Earthquakes

Abstract:

Observations in different subduction zones suggest that overpressured fluids may be linked to the origin of tectonic tremors (TT) and low frequency earthquakes (LFE). Fluids at nearly lithostatic pressures within the top few kilometers of the oceanic crust have been inferred in several subduction zones, including the state of Guerrero, Mexico. In this work, we report tens of thousands TT locations and mechanisms using the TREP method (Cruz-Atienza et al., JGR, 2015) from a catalog that includes TTs occurred during the two long-term Slow Slip Events of 2006 and 2009-2010 in Guerrero. Unlike earlier investigations, most TT hypocenters are 43 km depth and have rake angles subparallel to the Cocos-North America plate convergence direction. These results are in agreement with independent locations and mechanisms of LFEs and Very Low Frequency (VLF) earthquakes in the region. Our TT locations also reveal source streaking with speeds ranging between 20 and 70 km/h. Poroelastic modeling of fluid diffusion during the SSEs with an experimentally-based permeability function of the effective pressure and a sealed plate interface show that the evolution of Coulomb Failure Stresses within the top 4 km of the slab are consistent with the time-dependent occurrence-rate of LFEs. Besides, solutions of such a non-linear diffusion problem predict the existence of solitary pressure waves (i.e., solitons) propagating at speeds similar to those of the tremor source migration. This suggests the existence of pore-pressure gradients in the oceanic crust probably due to local dehydration processes and temperature changes along the slab. We conclude that fluid diffusion in the slab uppermost layer controls both, the LFEs recurrence rate and the rapid tremor sources migration (i.e., streaking).