抄録
On September 1, 2004, a middle-scale eruption occurred at Mt. Asama. Before the eruption, long-period volcanic earthquakes were observed with the broadband seismographs located at the summit of Mt. Asama since October 17, 2003. The signals are so feeble that we can hardly recognize them even at the second nearest station from the summit crater. The long-period earthquakes had been occurring at least before September 5, 2002. The earthquakes have very unique waveforms and can be categorized in 3 types (type 1, type 2, type 3). The sources of these long-period events are located relatively shallow at a depth of a few hundreds meters, just beneath the crater. The depth of the type 1 event is just beneath the vent, the types 2 and 3 being 100～200 meters deep. The activity of these long-period events had been synchronized with the activity of volcanic earthquakes until the last ten days of July 2004. However, the activity had dropped gradually, and no event has been observed since August 24, 2004, just 8 days before the eruption. An earthquake swarm started on August 31, lasting just before the eruption. The hypocenters of swarm, which are determined using the Double Difference method, lie just beneath the vent ranging from 300m to 800m deep, suggesting that the shallowest part of the vent is broken during the swarm activity.
