Along-Fault Fluid Migration Involved in Strong Slow-Slip Events in Guerrero, Mexico

Fluids have long been suggested to play an important role in slow earthquake generation. High pore-pressures that lower the effective normal stress where slow earthquakes occur are due to a build-up of metamorphic fluids that are trapped at the plate interface. We analyze a low-frequency earthquake (LFE) catalog of unprecedented density to study the response of the Guerrero subduction to a M_w 7.5 slow-slip event (SSE). About half-way through the SSE, we observe a significant deceleration of LFE activity in the deep part of the subduction that then propagates towards the trench over a series of several weeks. We also investigate the recurrence of LFE bursts as a function of distance from the trench and find that the LFE recurrence decreases with increasing distance from the trench before the SSE. During the six month-long SSE, however, the LFE burst recurrence is uniform across ~80 km. Taking into consideration these two observations, we suggest that plate interface permeability is significantly increased during the SSE and a large-scale fluid migration towards the trench takes place along the subduction interface. Sustained fluid migration over the first half of the SSE eventually drains the plate interface in the downdip slow earthquakes source region. The reduced pore-pressure of the drained plate interface, and the consequent increase of effective normal stress, significantly reduces the LFE activity. After the SSE, fluid pressure is then recharged from the surrounding area over several months. This proposed large-scale fluid migration mechanism explains our observations and also provides a source for episodic fluid discharges during megathrust events updip in the seismogenic zone.