Studies on Three-Dimensional Seismic Velocity and Attenuation Structures in the Chubu and Kinki Regions, and Their Tectonic Implications

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(1) 3-D P-wave velocity structure beneath central Japan has been investigated by applying an inversion technique, using 7,500 P-wave arrival times from 120 shallow and intermediate depth earthquakes. The results clealy reveal prominent low-velocity bodies just beneath active colcanoes in central Japan and above the subducting plates.(2) A new method to detect 3-D velocity anisotropy in addition to lateral heterogeneities has been proposed, and applied to P-wave travel time data in central Japan. No significant P-wave anisotropy was detected where shear wave anisotropy has been revealed.(3) Seisomotectionc features of the Hida region, central Honshu, Japan has benn investigated on the basis of seismicity and focal mechanism of earthquakes, with reference to the geological and tectonic setting. The non-uniform seismicity along the Atotsugawa fault, and its sharp variations of deepest seismicity, and the source of the compressive stress working in this region are discussed in detail.(4) 3 … More -D upper crustal structure in the Hida region has been derived by Thurber's inversion method applying to 2,500 P-wave travel time data obtained from a large number of local shocks. It has been found that high-velocity regions exist in the central part particularly along the western part of the Atotsugawa fault, and that low-velocity zones extend over the middle crust beneath active volvanoes in the Hida mountains.(5) 3-D upper crustal structure in the Hokuriku region has been derived from the avove method, using 278 local shocks that occurred around thei region. The results show that there is high-velocity zone trending in the NE-SW direction down to a depth of 3 km.(6) 3-D upper crustal structure in the central to northern Kinki region has been derived by the inversion technique, and compared with seismicity in this region. The largest variations in the velocity structure are found to be within 8 %, but less than 4 % in most part of the region. The deepest seismicity appears to become deeper from the SW to NE direction. Less