Lateral variation of the uppermost oceanic plate in the outer-rise region of the Northwest Pacific Ocean inferred from Po-to-s converted waves

The uppermost structures of incoming oceanic plates have been investigated by seismic exploration surveys, primarily based on two-dimensional profiles. However, their regional-scale lateral variations and shear wave velocity structures with higher-frequency components remain elusive. This study, using passive seismic records, attempted to retrieve Po-to-s converted waves (Pos) by cross-correlating Po wave coda from the radial and vertical components in the 2-6 Hz frequency band and to show lateral variations of the shear wave velocity within the oceanic crust, including crust-related seismic interfaces and layer-dependent anisotropic structures. Po waves were collected from continuous records acquired from active seismic surveys performed across wide areas of the Northwestern Pacific over shorter observation periods and passive seismic surveys performed near the Japan Trench over longer observation periods. As a result, this study obtained clear Pos waves converted at the basement and oceanic Moho of the seaward region, whereas weakened or no Pos waves were observed at ocean bottom seismometers near the trench. The primary reasons for Pos wave weakening or absence were considered to be structural changes, including normal faults due to the plate flexures in the outer-rise region, hydration at the uppermost oceanic plate, and fractures associated with volcanic activities that can be seen on the oceanic plate (petit spots). Furthermore, layer-dependent shear wave anisotropies were estimated for the sediment and crust. Fast polarization directions were oriented in trench-parallel directions near the trench and in the NNW-SSE directions in the seaward region. The pattern change to near-trench polarization directions would correspond to stress field-induced aligned fractures, including cracks and normal faults, created by stress fields induced by plate bending in the outer-rise region.