東北日本弧の三次元地震波速度構造とVp/Vs構造

概要

論文の詳細を見る
Precise estimations both for P- and S-wave velocity structures are essential for understanding the nature of the Earth's interior. We attempted to estimate P- and S-wave velocity structures beneath the northeastern (NE) Japan arc by travel time tomography. We used 169,712 P-wave and 103,993 S-wave arrival times read from waveform data of 4,338 events recorded at more than 200 seismic stations. We estimated the velocity structure in the whole area of NE Japan by setting grids with a horizontal spacing of about 25 km and a vertical spacing of 10-30 km. Low-velocity zones (LVZs) almost parallel to the subducted high-velocity Pacific plate are identified for both P- and S-waves. These LVZs have larger amplitudes for S-wave compared to P-wave. Estimated Vp/Vs ratio is 1.69 on average in the upper crust, 1.75 in the lower crust, and 1.77 in the uppermost mantle. This difference in Vp/Vs ratios among layers are probably caused by differences in the material composing each layer. Our results clearly show LVZs for both P- and S-waves in the crust just beneath active volcanoes. In the uppermost mantle, LVZs are distributed continuously along the volcanic front. These LVZs have smaller Vp/Vs values in the upper crust, while they have larger values in the lower crust and uppermost mantle.