Keishi Okazaki

Permeability anisotropy of serpentinite and fluid pathways in a subduction zone

The results of fluid-flow experiments revealed a significant permeability anisotropy in highly sheared serpentinite. Fluid flow parallel to serpentinite foliation is an order of magnitude or more higher than that normal to the foliation at confining pressures as high as ∼50 MPa. Although buoyancy is the driving force of upward fluid flow in the mantle, the strong anisotropy in permeability results in preferential fluid migration along the subducting plate interface, where extensive plastic deformation and a strong crystal-preferred orientation are expected to occur. The development of a relatively thin hydrous layer in the mantle wedge is consistent with the occurrence of highly anisotropic fluid migration rather than vertical flow.

Slow stick slip of antigorite serpentinite under hydrothermal conditions as a possible mechanism for slow earthquakes

Slow earthquakes, characterized by a different scaling law to regular earthquakes, have been detected at the hydrated plate interface in the subduction zones, but the generating mechanism of them remains almost unexplored. Frictional experiments on antigorite serpentinite under hydrothermal conditions are conducted to assess the distinct scaling law of slow earthquakes. Slow stick-slip was observed at temperatures that were close to the dehydration temperature of antigorite, which is resulted by the localized dehydration of serpentine in the shear zone. The occurrence of slow stick-slip is consistent with the temperature range found in the corner of the mantle wedge in SW Japan and Cascadia, where slow earthquakes occur. The laboratory slow stick-slip shows a similar scaling law of slow earthquakes, but distinct from that of regular earthquakes. We propose that the shear-induced dehydration of the serpentine play an important role for the generation of slow earthquakes.