Crustal structure in Izu Peninsula, Central Japan, as derived from explosion seismic observations. 1. Mishima-Shimoda profile.

Abstract

In December, 1979, a detailed explosion seismic experiment was conducted by Research Group for Explosion Seismology in the Mishima-Shimoda profile in Izu Peninsula, Central Japan, where activities such as large earthquakes with damage, earthquake swarms, anomalous crustal movement have recently occurred. The analysis of travel time data of good quality was made mainly by the time term method. The crustal structure derived revealed significant lateral heterogeneity especially around the middle of the profile. In the northern part of the profile, there are two or three layers with a total thickness of about 2km above the homogeneous granitic layers, while in the southern part one or two layers with a total thickness of about 1km exist above the granitic layer. In the granitic layer in the southern part a velocity-depth function which gives 5.4km/sec at the top and 6.0km/sec around a depth of 4km could be accurately obtained.The comparison of the crustal structure with seismicity, gravity anomaly, crustal movement, and geology along the profile gives important results. Most earthquakes occurred in the granitic layer, especially in the southern part of the profile where a velocity-depth function was found. The distribution of Bouguer gravity anomaly is concordant with the crustal structure. The area of anomalous ground uplift coincides with the southern half of the profile where the structure of surface layers becomes complicated and a velocity-depth function was found in the granitic layer. Significant lateral changes of the crustal structure take place near the area where several active faults such as Sano, Himenoyu are present and the 4.1-4.2km/sec layer is identified with the Yugashima group from geological data.