Abstract
In the active volcano field in Japan, the investigations are not many on the underground structure of volcanoes by seismic prospecting. In March, 1968 and 1969, the explosions (explosives ; about 500kg) by the Geological Survey of Japan were executed at Chigasaki, the northern extremity of Izu-Oshima. We observed the seismic waves by these explosions at the permanent and temporary observation points, and obtained the underground structure of Oshima using travel time. In case of the structure as a parallel layer over a half space, the velocities of P and S wave were 2.2km/sec and 1.1km/sec in the upper layer, and 4.6km/sec and 1.9km/sec in the lower part. The thickness of the layer was estimated to be about 800m. Presuming as two parallel layers over a half space, the velocities of P waves were 2.0km/sec in the first layer, 3.1km/sec in the second layer and 4.6km/sec in the lowest part. In this case, the thicknesses of the first and the second layer were about 400m and 600m, respectively. We obtained the arrival direction of P wave at each point using the orbital analysis. These directions are systematically deviated from the direction of wave source (shot point) as Fig. 10, and such a result seemd to be caused by the heterogeneous effect of the volcanic structure.
