Three-dimensional P- and S-wave velocity structures beneath the Ryukyu arc

The three-dimensional (3-D) P- and S-wave velocity structures beneath the Ryukyu arc are determined by seismic wave travel time tomography, using recordings on temporary stations in the sea area as well as those on routine stations. For the inversion, we explicitly define the upper boundary of the Philippine Sea slab. As a result, we obtain 3-D P- and S-wave velocity structures beneath the Ryukyu arc, which are more accurate than previous ones. P- and S-wave low velocity zones and high Vp/ Vs zones exist just beneath active volcanoes at about 10 kin depth, which show the influence of arc magmas beneath the volcanoes. Prominent P- and S-wave low velocity zones exist along the Okinawa trough at about 50 km depth, where Vp/Vs values are high. The low-velocity and high Vp/Vs anomalies will represent some mantle materials flow upward from the depth greater than 50 km along the trough. Low velocity anomalies are visible beneath the Philippine Sea slab, which are possibly associated with small-scale mantle convections. (C) 2003 Elsevier B.V. All rights reserved.