Convergence of aqueous fluid at the corner of the mantle wedge: Implications for a generation mechanism of deep low-frequency earthquakes

Abstract

Recent seismic observations have shown that low-frequency earthquakes occur in nonvolcanic regions in subduction zones in southwest Japan and Cascadia, and it is suggested that an aqueous fluid released from the subducting lithosphere is necessary for the occurrence of these earthquakes. In a mantle wedge under a subduction zone, a permeable network is likely created by hydrofracturing due to continuous fluid release from the subducting lithosphere. The migration of aqueous fluid through a permeable network is governed by the pressure gradient resulting from the viscous mantle flow that is induced by the subducting oceanic lithosphere. Calculations of the streamlines of the aqueous fluid in the mantle wedge under the subduction zones show that the fluid released from the subducting crust migrates toward the corner of the mantle wedge. The convergence of the aqueous fluid toward the corner of the mantle wedge results in the accumulation of the aqueous fluid near the wedge corner. Due to the propagation of fluid-filled cracks under the effect of buoyancy, the accumulated fluid may easily migrate upward in the overlying lithosphere and along the plate interface. The propagation of fluid-filled cracks from the region containing the accumulated fluid is likely to be the cause of deep low-frequency earthquakes occurring near the corner of the mantle wedge in these subduction zones.