第1回, 第2回倉吉爆破および花房爆破観測より得られた西部日本の地殻構造

Abstract

By using data of the first and second Kurayosi and the Hanabusa explosions presented in Part 1, the crustal structure in the western part of Japan was derived. The velocity structure of the second layer with velocity 6.1km/s was considered and the existence of the layer with velocity 6.4-6.6km/s was ascertained. On the travel time graph of the Kurayosi explosions, the phase with velocity 6.4-6.6km/s gives clear first arrivals. Neglecting fluctuation of travel times due to inhomogeneity of the surface or within the crust, the trial was made to obtain the range of depth of boundaries as well as that of velocity whenever possible. From the interpretation of travel time graphs, two possibilities for the deeper part of crust, Model I and Model II, are given in Figs. 3 and 4. In Model I there is a gap of the Mohorovicic discontinuity west of Biwa Lake by about 4km and in Model II there is another layer with pretty high velocity 7.4-7.5km/s above the layer with velocity 8.0-8.2km/s. The thickness of crust is quite different between the two models. The apparent velocity of 8.0km/s was obtained pretty clearly.