Double seismic zone within the ridge-shaped slab beneath southwest Japan

We have investigated the intra-slab seismicity and focal mechanisms in the subducting Philippine Sea plate, beneath the region spanning Ise Bay and Lake Biwa, southwest Japan. Based on the hypocentral distribution of earthquakes, the slab geometry shows a gently dipping trend and ridge-shaped features, with the intra-slab seismicity forming a distinct double-plane seismic zone. The seismicity of the lower plane is much less active than that of the upper plane and is located about 10 km beneath the upper plane seismicity. Examination of the focal mechanism solutions revealed that the upper plane earthquakes occurred basically in a tension field, as is the case with most intra-slab events beneath southwest Japan. In contrast, we found that the lower plane earthquakes near the axis of the ridge-shaped slab occurred in a compression field. The seismogenic stress of the lower seismic plane is probably caused by the bending responsible for the formation of the ridge-shaped slab. Our analysis of earthquake waveforms confirmed that the upper and lower plane seismic zones correspond to the oceanic crust and mantle, respectively. Lateral stretching and bending of the slab could be the main driving factors that control the seismogenic stress field of the double seismic zone in this region.