Vp and Vs tomography of Kyushu, Japan: New insight into arc magmatism and forearc seismotectonics

Abstract

High-resolution 3D tomographic images of the crust and upper mantle are determined under the entire-arc region of Kyushu by inverting a large number of arrival time data of P-wave, S-wave and sP depth phases. The hypocenters of the offshore earthquakes are accurately located by applying a combined method that makes use of sP depth phases and double-difference of the P-wave and S-wave arrival times. Our tomographic results confirmed the major features delineated by previous studies and revealed some new features of structural heterogeneity related to arc magmatism beneath the Kyushu arc. High-velocity anomalies correspond to the subducting Philippine Sea slab whilst low-velocity zones in the mantle wedge are clearly imaged, coinciding well with the velocity heterogeneities determined by previous studies. Strong low-velocity anomalies are distributed extensively along the volcanic front and extend to the back-arc side in the crust and upper mantle. Our present seismic images together with the previous results suggest that the fluids supplied by the dehydration processes of the descending Philippine Sea slab and the hot upwelling materials related to the opening of the Okinawa Trough might have contributed to the Unzen volcanism. Under the forearc region of the Kyushu subduction zone, prominent low-velocity anomalies are visible at depths of 10–40 km along the upper boundary of the subducting Philippine Sea slab and large interplate earthquakes are located around the low-velocity zones, which is similar to the features revealed in the Northeast Japan subduction zone. The spatial distribution of large thrust earthquakes below the offshore region reflects the existence of weak coupled or decoupled portions on the interface of the subducting Philippine Sea slab.