High-resolution image of the subducting Philippine Sea plate beneath western Shikoku, southwest Japan

Abstract:

In the Nankai region, southwest Japan, low-frequency earthquakes (LFEs) are very active at the down-dip limit of the megathrust source region. To reveal the spatial relationship between underground structure and seismic activity, we newly construct high-resolution receiver function (RF) profiles of the subducting Philippine Sea plate (PHS) and overriding continental plate. In this study, we installed 30 short-period seismographs in western Shikoku for one year. The survey line was aligned in the NNW-SSE, perpendicular to iso-depths of the slab, and it was passed through above one of dominant LFE clusters. We apply the RF analysis to not only these temporal stations but also nine NIED Hi-net / F-net stations near by the survey line. Amplitude of converted phase at a dipping interface becomes unclear when a seismic wave comes from up-dip direction. Thus, we excluded the RFs estimated from earthquakes located in the SSE direction, up-dip direction of our survey line. Since transverse RFs have important information for dipping velocity interfaces, we draw both the radial and transverse RF profiles. We clearly confirm the existence of the north dipping oceanic Moho (OM) and the continental Moho (CM) from the profiles. The OM is lying at 30 km depth beneath the Pacific coastline, and inclines to the north with dip of 7°. The dip angle of the OM changes steeper at the middle of the survey line. In the northern part of the survey line, the flat CM is located at 30 km depth. At the area where the OM reaches 40 km depth, the CM becomes shallow to the south. From our new subduction image, the LFEs are active within the oceanic crust between the lifting CM and dipping OM. The LFEs are caused by dehydration of the subducting slab. When dehydration occurs in the oceanic crust, the hydrated mineral transforms its phase. At this time, the density of mineral is increase. We say that this feature corresponds to the slight increase of the dip of the OM.