Abstract:

We used 231918 Pwave arrival times from 3218 earthquakes recorded by the seismic networks operated by Japan Meteorological Agency and Japanese national universities to determine the threedimensional Pwave velocity structure in the depth range of 0160 km beneath Southwest Japan. In Kyushu, the subducting Philippine Sea slab was imaged clearly as a highvelocity zone. The forearc mantle wedge was serpentinized due to the dehydration of the subducting Philippine Sea slab in the depth range of 3060 km. The further dehydration of the Philippine Sea slab at about 120 km depth caused partial melting in the mantle wedge, and the upwelling of melting materials finally led to active volcanoes on the surface. A pronounced lowvelocity anomaly and high seismic activity exist beneath Daisen volcano in western Honshu, suggesting that Daisen is a potentially active volcano and so it has a danger to erupt in the future.

Key words: seismic tomography, Philippine Sea slab, lowfrequency microearthquakes