Outline of the 1 and 16-17 eruptions at Asama

[1 September eruption]
     Asama Volcano erupted as a Vulcanian explosion at 20:02 of 1st of September 2004 (JST). Lava blocks including pumice bombs were issued about 2 km away from the crater, and trees on the upper slope were burnt. Volcanic ash was drifted to the NE and fell at the Pacific coast about 250 km away from Asama Volcano.
The volume of erupted materials of this eruption was estimated less than 100 thousand cubic meters, and it was largest in scale since the 1983 vulcanian eruption at Asama Volcano. However, involvement of pumiceous bombs in this eruption was different from the latter.
     Asama had repeated small eruptions in 2003, and the seismic activity had been relatively high in level in 2004. From the afternoon of the previous day of this eruption, swarms of earthquakes occurred just beneath the summit. However, the seismic level was not highest among several episodes in 2004. Small inflation began in late July 2004; for example, S-ward movement as much as 5 cm was at the GPS station of ERI (Earthquake Research Institute, University of Tokyo) about 6 km south of the crater. Glowing of the summit in night was withnessed in middle August.
     The activity of Asama Volcano is monitored seismologically by JMA and ERI, and geodetically by Geographycial Survey Institute, JMA and ERI. Thermal activity was monitored with a crater camera by Gumma Prefectural Government and ERI.  As low-frequency earthquake events had been recognized since May of this year, ERI seismologists carried out the seismic experiment by settling the array of about 20 seismometers at the summit since 25 August. There were two permanent seismological stations on the eastern and western side of crater. Signals were cut when eruption started on the night of September 1. The summit inspection after the eruption showed that seismometers of the both side of crater rim were brown out and most of seismometers of the seismic experiments were destroyed by falling of cinders and burnt by their heat.
    Bomb is composed of glassy crust and vesiculated core. Most of them were broken into pieces when they landed on the ground. They are andesite with the bulk composition similar to the 1783 eruption, but matrix glass is high silica rhyolite, much evolved than the latter. Volume of bombs is less than 10 % of cinders of this eruption.

[16-17 September eruptions]
     Though seismicity declined after the 1 September explosion, it had been activated since 13 September and small ash emission began in the early morning of 14 September. Frequencies of ash emission and earthquake became larger with time and strombolian eruptive activity occurred during 16 and 17 September. Ash fell over the town of Karuizawa SE to E of the volcano and ash falling was also observed in Tokyo on 16 September. Intermittent eruptions of these days were similar to those of the 1973 eruption, which included three times of intermediate vulcanian explosions during 4 months. Eruption products of the 16 September contain large amounts of drops of light gray to brown, vesiculated juvenile glassy particles, contrasting to their minor fraction among the 1 September eruption products.
     Aircraft SAR image taken on the 16 September morning by Geogrphycal Survey Institute clearly showed formation of a lava cake of about 150 m across and several tens of meter thick on the floor of the summit crater. Photographs taken on 17 September, just after the intemittent eruptions, by the Yomiuri Shinbun caught 5 to 6 folds of concentric winkles developed on the surface of the lava cake on which light-colored lava blocks were scattered. Based on comparison of photographs of 13 and 17 September, the volume of lava filling the crater unitl this day can be roughly estimated about 1.4 million cubic meter.