栃木県西部の日光・足尾地域における微小地震活動

Abstract

The Nikko-Ashio region, northern Kanto district, Japan is known for its active seismicity of microearthquakes. Although there are some permanent seismic networks around this region, the detectability of those networks isn't enough to clarify detailed characteristics of seismic activity in this region. We have performed temporary seismic observations repeatedly with high-gain short period seismographs in this region to investigate microearthquake activity since 1980s. A continuous seismic observation was started on september 1992 with seismic network of eight telemetered stations.According to seismic observations for more than ten years, we have obtained general properties of the microearthquake activity in the Nikko-Ashio region as follows.There are two active seismic areas; one is the area very close to Mt. Sukai, which is located on the border of Tochigi and Gunma prefectures, extending north-south direction and the other is the area beneath Ashio town, about 10km southeast of Mt. Sukai. The seismic activities of both areas are nearly stationary for more than ten years. The lower limit of focal depths of microearthquakes is about 10km beneath Ashio town and 6km near Mt. Sukai. On the whole, the lower limit of seismicity decreases towards north up to Mt. Nikko-Shirane, an active volcano located about 12km north of Mt. Sukai. This feature forms an aseismic zone in the upper crust beneath Mt. Nikko-Shirane.From November 1992, new microearthquake swarms began to occur by Lake Chuzenji, about 10km northeast of Mt. Sukai, and beneath Mt. Nikko-Shirane. In particular, the focal depths of some events beneath Mt. Nikko-Shirane are less than 2km. These new swarm activities followed several low-frequency microearthquakes near the Moho boundary beneath Ashio town on November 1992. Low-frequency microearthquakes are generally considered to relate to deep magmatic activity. Meanwhile, several studies suggest the existence of a clear seismic reflector which is considered as a partial melting zone in the middle crust.These phenomena suggest that the microearthquake activity in the Nikko-Ashio region may have some relation to the volcanic or magmatic activities in the lower crust. We need to monitor the microearthquake activity in this region with the special concern about its relation to volcanic activity of Mt. Nikko-Shirane.