| Summary: | Abstract We determined the three-dimensional structure of the crust and upper mantle off the Boso Peninsula, Japan, by analyzing seismograms recorded by ocean-bottom seismometers and land stations between 2011 and 2013. We employed seismic tomography to determine the P- and S-wave velocity structures and earthquake locations simultaneously. The tomographic image shows that the mantle parts of the Pacific and the Philippine Sea plates have high-velocity anomalies. The upper boundary of the Philippine Sea plate is delineated as approximately 2–6 km shallower than that previously estimated from land-based data for the area 140.5°E–141.5°E and 35°N–35.5°N. A pronounced low-velocity anomaly in P- and S-waves with low-V p/V s ratio (1.5–1.6) was observed at depths shallower than 20 km in the overriding North American plate. This anomaly may be caused by the presence of rocks with a low-V p/V s ratio, such as quartzite, and the water expelled from the subducted Pacific and Philippine Sea plates. Graphical Abstract .
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