Genesis of Intermediate-Depth and Deep Intraslab Earthquakes beneath Japan Constrained by Seismic Tomography, Seismicity, and Thermal Modeling

The distribution of intermediate-depth and deep intraslab earthquakes with respect to subducting slabs offers a unique insight into seismogenesis at high pressures and temperatures that should inhibit brittle failure. This study constrains the surface of the subducting Pacific Plate beneath Japan at depths between 100 and 380 km based on a previous continental-scale adjoint tomography model. Earthquake distributions relative to the slab surface reveal double seismic zones located within the top 60 km of the Pacific Plate. Thermal modeling suggests that the lower-plane seismicity corresponds to temperatures between 400 and 900 degrees C. The seismogenic pressure and temperature conditions correlate approximately with the conditions of dehydration reactions of several hydrous minerals, that is, antigorite (serpentine) and chlorite at depths between 100 and 200 km and phase A at greater depths between 200 and 380 km. These correlations indicate that at these depths water released from dehydration processes may facilitate triggering slab mantle earthquakes.