Subduction structure beneath the western part of Kii Peninsula, southwestern Japan，revealed by active and passive seismic experiments

The Nankai trough region, where the Philippine Sea Plate (PHS) subducts beneath SW Japan arc, is a well-known seismogenic zone of interplate earthquakes. A narrow zone of nonvolcanic tremor has been found in the SW Japan fore-arc [Obara, 2002]. The spatial distribution of the tremor is not homogeneous in a narrow belt. Obara [2002] suggests a dominating factor of tremor is fluids. Seismic structural images in and around the active tremor zone beneath the eastern part of the Kii Peninsula have been reported [e.g., Iwasaki et al., 2017]. However, little is known about the seismic structure where tremors are inacitive. To reveal the subduction structure in and around the inactive tremor zone, we conducted active and passive seismic experiments in the western part of the Kii Peninsula. 90 offline recorders were installed on a 90-km-long line nearly parallel to the direction of the subduction of the PHS during a six-month period from June, 2017. In September of 2017, we also conducted a deep seismic profiling. We deployed 302 seismometers on a 60-km-long line in the NW-SE direction with about 200 m spacing, on which 4 explosives with a charge size of 200 kg were fired. To investigate the earthquake locations and 3D Vp and Vp/Vs structures, the double-difference tomography method [Zhang and Thurber, 2003] was applied to the P- and S-wave arrival time data obtained from 649 local earthquakes and 4 shots．To reveal the geometry of the subducting PHS and the overriding island arc crustal structure, we applied the seismic reflection method to explosion data. Seismic reflection image shows several features of the deeper part of the crust and the northward dipping plate boundary at a depth of about 30-40km. The depth section of Vp/Vs structure shows the northward dipping high-Vp/Vs zone, which is interpreted to be the subducting PHS. The Vp/Vs value of the inactive tremor zone is smaller than that of the active tremor zone beneath the eastern part of the Kii Peninsula. A thickness of the reflection band corresponding to the inactive tremor zone is thinner than that of reflection band corresponding to the active tremor zone beneath the eastern part of Kii Peninsula. These results suggest that fluids in the active tremor zone are more abundant as compared with the inactive tremor zone. Fluids dehydrated from the PHS may control the degree of plate coupling.