東北地方の地殻・上部マントルのP波及びS波3次元速度構造

概要

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We obtained a new model of three-dimensional (3-D) P- and S-wave velocity structures in the central part of Tohoku District, northeastern Japan, using data obtained from temporary seismic observations with a satellite communication telemeter system. The observations were part of the Joint Seismic Observations in the Tohoku area during the period from October 1997 to June 2000. We used the arrival time data to estimate 3-D P-wave velocity (Vp) and S-wave velocity (Vs) structures. We inverted 23,590 P- and 13,313 S-wave arrival times from 396 local and regional earthquakes recorded by 108 stations. Then we determined Vp, Vs, and Vp/Vs models in and around the backbone range of the Tohoku district by applying a seismic tomography method to the arrival time data. The velocity distribution obtained shows strong lateral heterogeneities in the crust and the uppermost mantle, which vary with a wavelength of 10 to 20 km in the east-west direction. The structure has a north-south strike in general. Low Vp and Vs anomalies are visible under the Quaternary volcanoes. High Vp/Vs zones are also revealed in the lower crust beneath the volcanoes. A high Vs and low Vp/Vs anomaly was found in the Kitakami Massif.