立山カルデラにおける微小地震臨時観測

Abstract

跡津川断層の西半分に発生する微小地震活動の特徴が，臨時観測網，Hi-net 観測網の展開による非常に高精度のデータによって明らかになっている。しかし，断層東部では地形的な問題，観測条件等が問題となり，観測網の空白地帯であった。我々は，関係機関の協力により，立山カルデラで現地収録による臨時観測を行った。その結果，従来跡津川断層の活動によると考えられていた活動が，茂住―祐延断層の活動による可能性が考えられる。また，深さは，跡津川断層系の活動と飛騨山脈の活動に明らかな違いが見られ，跡津川断層系の東端を示唆しているのかもしれない。

Temporary observations of earthquakes have been carried out in the Tateyama Caldera from 2002 during summer seasons. This observations are aiming at revealing the detailed hypocenter distribution of earthquakes at the end of the Atotsugawa fault system and as well as surveying the seismicity in the Tateyama volcano area. Continuous observations of earthquakes have been done by deploying four individual high-gain stations. The data of the stations have been analyzed with routine observation station network data. The observation periods were limited only 3-4 months during summer because of the difficult accessibility by heavy snow fall. Preliminary results, however, show that seismicity is relatively low in the middle of the caldera and that many micro-earthquakes occur in the rim of the caldera. Focal depths on the east of caldera are less than 5km and significantly shallower than those of 10km in the Atotsugawa fault system on the west of the caldera. This suggests that the Atotsugawa fault system end at the west of the Tateyama volcano region. Besides, in detailed, the earthquakes in the eastern end of the Atotsugawa fault system seem to occur on the Mosumi-Sukenobu fault not on the Atotsugawa fault.