Variations of fluid pressure within the subducting oceanic crust and slow earthquakes

Abstract

[1] We show fine-scale variations of seismic velocities and converted teleseismic waves that reveal the presence of zones of high-pressure fluids released by progressive metamorphic dehydration reactions in the subducting Philippine Sea plate in Tokai district, Japan. These zones have a strong correlation with the distribution of slow earthquakes, including long-term slow slip (LTSS) and low-frequency earthquakes (LFEs). Overpressured fluids in the LTSS region appear to be trapped within the oceanic crust by an impermeable cap rock in the fore-arc, and impede intraslab earthquakes therein. In contrast, fluid pressures are reduced in the LFE zone, which is deeper than the centroid of the LTSS, because there fluids are able to infiltrate into the narrow corner of the mantle wedge, leading to mantle serpentinization. The combination of fluids released from the subducting oceanic crust with heterogeneous fluid transport properties in the hanging wall generates variations of fluid pressures along the downgoing plate boundary, which in turn control the occurrence of slow earthquakes. Citation: Kato, A., et al. (2010), Variations of fluid pressure within the subducting oceanic crust and slow earthquakes, Geophys.

Distribution of hydrous minerals in the subduction system beneath Mexico

“…Such large amount of fluid near the plate interface due to subduction related dehydration could also be an explanation for bursts of the nonvolcanic tremor within the subducted oceanic crust and lower continental crust (Payero et al, 2008) and slow slip events (Larson et al, 2007;Kostoglodov et al, 2010;Vergnolle et al, 2010;Radiguet et al, 2011). Recent studies suggest strong correlations between episodic tremor and slip (ETS) events and a low-velocity layer at the top of the subducting oceanic crust (Abers et al, 2009;Song et al, 2009;Audet et al, 2010;Kato et al, 2010).…”

Gravity changes observed between 2004 and 2009 near the Tokai slow-slip area and prospects for detecting fluid flow during future slow-slip events

“…At depths shallower than the Moho, the plate boundary forms a low-permeability barrier (Audet et al, 2009). Recent seismic tomography measurements in the Tokai district (Kato et al, 2010) also indicate the presence of an impermeable cap rock covering the subducting plate boundary from above. These observations imply that fluids released by the dehydration of the oceanic crust are subjected to increased pressure because they are confined within a space bordered from above by the overlying barrier and by seals near the upper and lower ends of the slow-slip area along the plate boundary (Fig.…”

“…In the Tokai area, high-pressure fluids are distributed along the subducting plate boundary and are centered around the depth of the Moho (∼30 km) (Kodaira et al, 2004;Kato et al, 2010). At depths shallower than the Moho, the plate boundary forms a low-permeability barrier (Audet et al, 2009).…”

“…Geodetic and seismological observations have revealed that a majority of SSEs occur repeatedly on the plate boundary at depths of around 20-40 km (Schwartz and Rokosky, 2007;Delahaye et al, 2009), just beneath the seismogenic zone for megathrust earthquakes. Seismic studies have detected high-Poisson-ratio anomalies that indicate the presence of pore fluids in the source areas of these events (Audet et al, 2009;Kato et al, 2010). Deep non-volcanic tremors have also been detected in some of these areas, and their rapid migrations can be explained by fluid flow along the subducting plate (Vidale et al, 2009).…”

“…Slow-slip events (SSEs) in plate-subduction zones are related to the presence of high-pressure pore fluids that are generated by the dehydration of hydrous minerals within the subducted oceanic plate (Schwartz and Rokosky, 2007;Audet et al, 2009;Song et al, 2009;Kato et al, 2010). Geodetic and seismological observations have revealed that a majority of SSEs occur repeatedly on the plate boundary at depths of around 20-40 km (Schwartz and Rokosky, 2007;Delahaye et al, 2009), just beneath the seismogenic zone for megathrust earthquakes.…”