Tomographic image of crust and upper mantle off the Boso Peninsula using data from an ocean-bottom seismograph array 4. Seismology

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.[Figure not available: see fulltext.]