High-VP/VS zone accompanying non-volcanic tremors and slow-slip events beneath southwestern Japan

In southwestern Japan, the Philippine Sea plate subducts beneath the Eurasian plate. Non-volcanic tremors and slow slow-slip events are observed beneath southwestern Japan. Seismic tomography based on the data obtained from the dense seismic network revealed the velocity structure in and around the subducting Philippine Sea plate. The oceanic crust of the Philippine Sea plate subducting from the Suruga and Nankai troughs can be imaged clearly beneath the Tokai region and the Kii peninsula. Non-volcanic tremors and short-term slow-slip events may occur in the high-VP/VS zones owing to the fluid dehydrated from the subducting plate where the oceanic crust of the Philippine Sea plate encounters the serpentinized wedge mantle of the Eurasian plate. These tremors and short-term slow-slip events occur in the high-VP/VS zone at depths of 30–40 km beneath the Tokai region, the landward edge of the high-VP/VS zone beneath the Kii peninsula and eastern Shikoku region, and the seaward edge of the high-VP/VS zone beneath the western Shikoku region. Long-term slow-slip events were observed beneath the Tokai region and the Bungo channel and both these events are located on the seaward side of a high-VP/VS zone. These long-term slow-slip events are considered to be caused at the plate boundary between the oceanic crust of the Philippine Sea plate and the lower crust of the Eurasian plate by the high pore-fluid pressure caused by the fluid dehydrated from the subducting plate. The existence of the high-VP/VS lower crust of the Eurasian plate may control the occurrence of the long-term slow-slip events.