Low-frequency earthquakes and tomography in western Japan: Insight into fluid and magmatic activity

Low-frequency (LF) microearthquakes are detected in the lower crust and uppermost mantle beneath western Japan, which may reflect fluid and magmatic activity in the subduction zone. In this work we combine seismic tomography and the LF earthquakes to study the crustal and upper-mantle structure and fluid and magmatic activity in western Japan. High-resolution tomographic images under this region are determined using high-quality arrival-time data. In Kyushu the subducting Philippine Sea slab is detected clearly as a high-velocity zone, and the arc-magma related low-velocity (low-V) anomalies under the active arc and back-arc volcanoes are imaged clearly. Prominent low-V zones are visible in the crust and uppermost mantle beneath the Quaternary volcanoes along the Japan Sea coast, and LF events occur actively under some of those volcanoes. LF events are also detected in the source areas of the 1995 Kobe earthquake (M 7.2) and the 2000 Tottori earthquake (M 7.3), supporting the hypothesis that the large crustal earthquakes were triggered by the crustal fluids. These results indicate that fluid and magmatic activities exist widely in western Japan, which are caused by dehydration of the subducting Philippine Sea slab and corner flow in the mantle wedge.