抄録
In the southern Kanto district, the experiments to detect precursary change in velocity of seismic waves by means of explosion seismic method have been repeated under the earthquake prediction research project since 1968. These explosion seismic data supply good materials to obtain information on the crustal structure in the area although the distribution of observation sites is not suitable for the crustal studies. The shot points are Izu Oshima and Tateyama in the Boso Peninsula. Because of charge size and the distance of observation stations from the shot points, the information on the depth of the Mohorovicic discontinuity and Pn velocity could not be obtained.
The features of crustal structure derived from these data in addition to the existing explosion seismic data are as follows:
1) In the Boso Peninsula, thick surface layers with low velocities such as 2.5, 4.5km/s are present above the layer with a velocity of 5.6km/s.
2) Between Izu Oshima and the Boso Peninsula, there is a possibility to have a lateral change of structure. In this paper, it is assumed that this change takes place beneath the fault as indicated in the sea bottom topography, that is, the layer with a velocity of 5.6km/s on the east side contacts with the layer with a velocity of 6.0km/s beneath this place.
3) Beneath the Boso Peninsula, the layer with a velocity of 5.6km/s exists directly above the layer with a velocity of 6.9km/s, that is, the layer with a velocity of 6.0km/s is missing or very thin if exists.
4) Near Nagasa in the central part of southern Boso Peninsula, there is an offset in the upper boundaries of layers with a velocity of 5.6 and 6.9km/s.
5) In the area of Izu Oshima and the Izu Peninsula, the layer with a velocity of 6.0km/s is shallow.
6) From both shot points, Izu Oshima and Tateyama, the layer with a velocity of 6.9km/s becomes shallower toward the Boso Peninsula and Dodaira, while it becomes deeper toward the Izu Peninsula.
7) Near Izu Oshima, the layer with a velocity of 6.9km/s becomes fairly shallower almost independent of the direction of profiles.
