気象庁地震月報データから求めた日本列島下の3次元地震波速度構造

Abstract

We derived three-dimensional P-wave velocity structure beneath the Japanese Islands by a tomographic inversion of travel time data. We used 37, 617 arrival time data from 581 earthquakes observed at 99 Japan Meteorological Agency stations by high-sensitive type seismometers (EMT, EMT76 and OBS) and estimated the velocity anomalies in a number of blocks and corrections to the source parameters simultaneously. In this study we used a block size of 0.5°×0.°×50km (for the uppermost three layers, 0.5°×0.5°×33km).Down to a depth of about 300km, we obtained detailed features. High velocity anomalies corresponding to the Pacific slab from Hokkaido to Kanto district and the Philippine Sea slab from Aichi prefecture to Shikoku are well delineated. Especially, we found aseismic extension of the Philippine Sea slab beneath the Japan Sea coast of Chugoku district. Low velocity anomalies corresponding to the volcanic front and the wedge portion of the upper mantle are found. We also see low velocity anomalies on the oceanic side of the Pacific slab below the depth of 250km.In comparison with the result from ISC data obtained by Kamiya et al. in 1989, we got clearer images in this study in the shallow regions down to a depth of about 300km, because of accurate data used in this study. The efficiency of this data set for travel time inversions like as this study is found to be comparable to the Japan University Network Earthquake Catalog data.