烏山-菅生沼断層南部付近における爆破地震動の観測

抄録

In order to obtain information on the underground structure around the Karasuyama-Sugaonuma fault, we carried out a temporary observation for artificial explosions which were blasted at four sites in Saitama and Ibaraki Prefectures in March 1982. Crossing the southern part of the fault, seven seismometers were deployed on an east-west striking observation line 30km long. For three of the explosions, good records were obtained at all the stations. Travel time analyses provided three layers; apparent p-wave velocities in the layers are 1.7km/sec, 2.8-3.1km/sec and 6.1km/sec from top to bottom. Travel times for the 6.1km/sec layer jump near Sashima-cho, Ibaraki Prefecture, where the western branch of the fault is supposed to lie. For the wave propagating through the topmost layer between the eastern and western sides of the north-south striking fault, a systematic difference in the wave form was observed. Those observed features strongly suggest that not only the basement layer (V_P=6.1km/sec) but also upper layers are displaced or fractured by the Karasuyama-Sugaonuma fault.