飛騨山脈焼岳火山周辺における東北地方太平洋沖地震後の群発地震活動

Abstract

Seismic activity near the Yake-dake (Mt. Yake) volcano in the Hida mountain range that took place immediate after the 2011 off the Pacific Coast of Tohoku earthquake was investigated. It initiated about ten minutes after the mainshock of the Tohoku earthquake and lasted for about one month. At the beginning, two active swarms were observed. One is at the northern flank of the Yake-dake volcano and the other is located between Yake-dake and Mt. Norikura volcanoes. The latter activity decreased by March 20, and the former activity lasted until early April. It includes two M≥4.5 earthquakes and we could locate more than 9,600 events in the study area during March and April. We mainly focused on the activity near the Yake-dake volcano in this paper. Near the Yake-dake volcano, seismic activity began with M4.7 (JMA) earthquake at 14:57 JST on March 11. This M4.7 event is located 3 km north to the volcano and seismicity increased between the summit of the Yake-dake volcano and the hypocenter. On March 21, an M4.8 (JMA) event took place at 13:15 JST also at 3 km north to the volcano. After this second M≥4.5 earthquake, seismic activity migrated to the north about 1 km. Focal mechanism solutions of these swarm earthquakes show NW-SE compression stress field, which coincides with regional stress field indicated by previous studies. No temporal changes of focal mechanisms are shown during March and April, which probably indicates no magmatic activity such as dyke intrusion related to the Yake-dake volcano took place in this time period.