Crustal tomography under the Median Tectonic Line in Southwest Japan using P and PmP data

The Median Tectonic Line (MTL) is the largest active fault system on the Japan Islands and underlies many densely populated areas. To understand the seismotectonics and to reduce seismic hazard in the MTL region, we determined a detailed 3-D crustal structure under the region using a large number of arrival times of the first P-waves and Moho-reflected waves (PmP). Results of detailed resolution tests show that the addition of PmP data can significantly improve the resolution of the lower crustal structure and the entire crustal structure can be imaged better than that by using P-wave data alone. The obtained tomographic images show that prominent low-velocity (low-V) anomalies are visible in the upper crust along the MTL. In addition, some low-V anomalies are visible in the lower crust on either side of the MTL. These low-V anomalies can be the weaker sections of the seismogenic crust and can be imprint of the existence and migration of aqueous fluids released by the dehydration process of the subducting Philippine Sea plate under Southwest Japan.