1985年大山火山付近に発生した群発地震について

Abstract

An earthquake swarm including the main event (M=4.9) occurred only several kilome-ters east to the Daisen volcano, Tottori Prefecture from the end of June 1985. This swarmstrongly drew our attention, since it could be a precursory activity of the Daisen volcanism, or could be followed by a greater earthquake which would cause a landslide. From the dis-aster prevention point of view, fortunately the number of earthquakes gradually decreased andat the present time (1986 April) this swarm almost died out and no volcanic activity has beenreported until now.We have, however, summarized this swarm here, because the main event was a compara-tively large one that ever occurred in and around the networks of our institute. The mostinteresting nature of the swarm is that we could divide it into three stages, both from timesequence and epicentral distributions. We also studied the focal mechanism solution for themain event. It shows a similar pattern to those for the 1943 Tottori Earthquake and for the1983 event in the midst of Tottori Prefecture, which indicates that the present swarm iscaused by the NWW-SEE compressional force widely predominant in the Inner zone of theSouthwest Japan. The b-value was also calculated to be 0.65 for 168 events. Moreover, weset up 4 temporary seismic stations near the epicentral region, one of which was to clarifythe relation between the earthquake occurrence and the sound that had been reported by theresidents in the epicentral region. But we could not find the relation, because no moresound was heard during the temporary observation.