Seismic Velocity Discontinuities beneath The Japan Islands Revealed by Receiver Function Analysis

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Seismic velocity structure including seismic discontinuities beneath the Japan Islands is investigated using receiver function array analysis method We use a large number of broad band (FREESIA. J-array) and short-period high density (J-array, Hi-net) stations installed all over the Japan Islands. First, we obtained receiver functions (RFs) in a traditional way which has been so fer employed. Then we applied SVD (Singular Value Decomposition) filtering to the original RFs for the short period ones, and to the depth transformed RFs for the broadband ones.Using short period data with high spatial resolutions, we clearly recognized lateral variation of the Conrad, and the Moho discontinuities in the crust Especially in southwest Japan where the station are closely distributed, we detected the inclined Conrad and the downwarping ofthe Moho around the Lake Biwa. For the upper mantle seismic discontinuities, we detected the inclined upper and lower boundaries of the descending Pacific slab in the Tohoku region. For the Philippine Sea slab, it seems to be rather difficult to see Hie boundaries. Also we detected 410 km and 660 km discontinuities. 410 km discontinuity is especially clear in comparison to 660 km one, and seems to be upwarping within the descending slab. 660 km discontinuity seems to be divided into three discontinuities.At last, we added the data of Hi-net, which has been only recently installed. They have a sampling rate of 100 Hz, which enables us to produce higher frequency RFs for imaging the far detailed structure. Though this is only preliminary, there is a possibility that we can image the reflective layering structure in the lower crust, which provides us an important information relating to the initiation of large inland earthquakes.