LAUSR

Abstract The region off the Boso Peninsula, Japan, is a tectonically complex area where the Pacific plate is subducting beneath both the landward plate and the Philippine Sea plate (PHS) from the Japan trench and the Izu-Bonin trench as the PHS is subducting under the landward plate from the Sagami trough. It is important to better determine the structure of this region to deepen our understanding of its seismicity. Previous seismic reflection studies have shown that reflections from the upper surface of the PHS vary with depth, being stronger in the main slip area of the slow slip events beneath the Boso Peninsula (Boso SSEs). However, the spatial relationship between the reflective area and the SSEs is poorly constrained. This study mapped the distribution of the reflective area using data recorded by ocean bottom seismometers during an active-source seismic experiment. We constructed a 3D P-wave velocity structure by using traveltimes of first arrivals from 18 ocean bottom seismometer records. We also adapted the traveltime mapping method to reflection traveltimes, projecting them to the depth–distance domain, to map the 2D distribution of strong reflections from the top of the PHS. These reflections were concentrated in two areas, one near the main slip area of the Boso SSEs and the other about 60 km to the east. In the first area, the absence of strong velocity contrasts near the top of the PHS suggests that the reflections were generated by a thin low-velocity layer. In contrast, the structure of the second area has a convex shape of high velocity with a high velocity gradient near the top of the PHS. This structure may represent boninitic material of the outer-arc high, partially serpentinized peridotite, or gabbro displaced by intraoceanic reverse faults.