抄録
The eruptive activity at Asama that started on 1 September 2004 was characterized by several vulcanian small explosions, including a continuous stromblian-explosion period. About 2×106m3 of magma (DRE) was issued in this eruption, most of which filled the summit crater floor. The sequence of eruptive activity was similar to that of the 1973 eruption among previous eruptions at Asama Volcano. However, the two eruptions are different in the following points. 1) Both magnitude of seismic activity and scale of eruption products are small in the 2004 eruption; about half or less than in the 1973 eruption. 2) A-type earthquakes occurred in the later stage of the 2004 eruption, while in the early stage in 1973. 3) Partial melted country rock (rhyolite tuff) is found among the 2004 eruption products but not in the 1973 ones. As the swarm of A-type earthquakes at Asama occurred in the period of inflation detected by GPS in this decade, intrusion of magma in the deep-seated magma reservoir (or dike) was associated with A-type earthquakes. It is likely that the difference of occurrence of the A-type earthquakes in the both eruptions reflect different magma supply rates; larger in the 1973 eruption and smaller but long-lasting in the 2004 eruption. Continuous inflation and occurrence of A-type earthquakes even in 2005 suggest not declining of the eruptive activity but renewal near future.
