Tomographic image of crust and upper mantle off the Boso Peninsula using data from an ocean-bottom seismograph array

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 Pand 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 highvelocity 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 Pand S-waves with low-Vp/Vs 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-Vp/Vs ratio, such as quartzite, and the water expelled from the subducted Pacific and Philippine Sea plates.