跡津川断層系周辺における稠密地震観測

Abstract

跡津川断層系周辺では, 稠密観測網を構築して微小地震観測を行っている。その結果, とりわけ深さ分布については, 地震活動の下限が明瞭になってきた。また, 断層中央部のクリープしていると指摘されている場所では, 深さ5㎞までの浅いところで, 地震が殆ど発生していないことも明らかになった。さらに, 従来震源決定出来なかったような小さい地震まで決定出来るようになり, 検知能力が格段に向上し, 発震機構解も小さな地震について決定できるようになった。

New observation stations of earthquakes have been installed to study detailed seismicity and related problems. Seven stations were set near the fault system and the obtained data greatly improved the accuracy of hypocenters, specially focal depths along the fault system. This report describes the details of the system and brief results of the observation. The change in the cutoff depths of seismicity is clearly defined as a concave shape along the fault. Furthermore, very few events are revealed to occur down to 5km below the creep section of the fault. Besides small earthquake swarms of magnitude 0 can be located in the central part of the fault. Earthquake mechanisms can be also determined as small event as Ml.0 from the P-wave first motions.