烏山-菅生沼断層南部の第四紀活動の可能性

抄録

In order to investigate the detailed underground structure and fault displacement, we carried out geophysical explorations such as observation of explosion seismic waves, electrical sounding and resistivity well log around the estimated basement fault called the Karasuyama-Sugaonuma fault. Synthetic analysis of these data revealed that the underground structures from the pre-Tertiary basement to shallower layers have the following characteristics. (1) scale basement fault that has a gap more than 1000-1500m, extends SSE-NNW from Sugaonuma A large to Sashima-cho, (2) the eastern side of this fault, the basement rises at a gentle slope toward Mt. Tsukuba, accompanying a small scale basement fault and (3) shallow layers above the basement are much disturbed around these faults.
By investigating resistivity well log data at 33 shallow wells, we found steep inclination of overriding Quaternary layers that is harmonious with the offset of the large scale basement fault. The slope decreases monotonously from deeper to shallower layers belonging to the Shimosa Group. Those facts strongly suggest that the fault activity has continued through the Quaternary period. Rate of the vertical displacement is roughly estimated to be 0.04mm/year.