P wave tomographic imaging of the crust and upper mantle beneath the Japan Islands

More than 50,000 high‐accuracy arrival times from 1200 shallow and deep earthquakes selected from the Japan University Network Earthquake Catalog are used to investigate the three‐dimensional (3‐D) seismic velocity structure beneath Japan. Travel times and ray paths are accurately calculated by using an efficient 3‐D ray‐tracing technique. P wave tomographic images of the crust and upper mantle to a depth of 500 km beneath the Japan Islands are determined with spatial resolutions of 25–50 km. The obtained P wave velocity in the crust and uppermost mantle is slightly higher and relatively homogeneous in western Japan, in contrast with an average lower velocity and a large heterogeneity in eastern Japan where many active volcanoes exist. This feature correlates well with the distributions of heat flow and gravity anomalies. Low‐velocity (low‐V) zones are imaged in the crust and uppermost mantle beneath active volcanoes. The low‐V zones in the mantle wedge dip toward the west and extend continuously to a depth of about 200 km. A high‐velocity (high‐V) zone corresponding to the subducted Pacific plate is delineated. The thickness of the high‐V zone is estimated to be 80–90 km. In southwestern Japan, the descending Philippine Sea plate is also delineated; its thickness is estimated to be 30 km.