Earth Planets Space, Vol. 58 (No. 8), pp. e37-e40, 2006E-LETTER
Kei Katsumata, Naoto Wada, and Minoru Kasahara
Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
(Received May 12, 2006; Revised June 30, 2006; Accepted July 18, 2006; Online published September 16, 2006)
We applied an inverse method developed by Zhao et al. (J. Geophys. Res., 97, 19909-19928, 1992) to 42,834 P and 18,263 S wave arrival time data observed at 152 seismographic stations for 1143 local earthquakes at depths between 0 and 200 km in order to estimate three-dimensional P and S wave velocity structures beneath the Hokkaido corner, Japan-Kurile arc-arc junction. High- and low-velocity zones were clearly imaged in the Hidaka Mountain Range at depths shallower than 35 km. The low-velocity anomalies of P and S waves were found to be distributed in the mantle wedge at depths between 35 and 100 km beneath the volcanic front, as also observed in the Tohoku region. Another low-velocity zone was found to exist in the fore-arc region at depths of 50-70 km above the plate boundary; this zone was not detected in Tohoku, suggesting that the dehydration process in the fore-arc region is different from that in the Tohoku region.
Key words: Hokkaido, P and S wave velocity structures, Hidaka Mountain Range, volcanic front, mantle wedge, magma, dehydration process.