Asama, located near the resort town of Karuizawa ~150 km W of Tokyo, has been seismically active since 18 September 2000. Heightened seismicity occurred in June 2002, when the daily number of volcanic earthquakes exceeded 300 (BGVN 27:06). The Asama Volcano Observatory (ERI, University of Tokyo) and JMA reported that a new episode of elevated seismicity started around 0620 on 18 September 2002. A relatively large amount of volcanic gas trailed from the summit. The seismicity increased after 0800, 18 September, such that 243 volcanic earthquakes took place on 18 September and another 128 on the 19th, after which the seismic activity decreased. However, the temperature of the crater bottom remained at the elevated levels observed since May 2002. No change was observed in ground deformation.

According to the Japan Meteorological Agency (JMA), seismicity had been at background levels for several months, and the temperature of the crater had been rather low prior to four minor eruptions between 6 February and 18 April 2003. The first eruption occurred at about noon on 6 February as an ash cloud was seen rising to 300 m above the summit crater, with minor ashfall around the summit. Seismic tremor related to the emission started at around 1201 and lasted about 40 seconds. On 30 March at 0154 hours, a gray ash cloud rose 300 m, with minor ashfall around the summit. Then, on 7 April at 0924, an ash cloud rose 200 m. On 18 April at 0732 the volcano spewed a mixture of black smoke and pale ash ~300 m high. There were no reports of injuries or damage from these eruptions, and the JMA reported that more such activity is expected. All of the eruptions were brief, none having durations of more than 10 minutes. No unusual precursory seismic activity preceded these events, but plume activity has increased since the beginning of February.

Geological Summary. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asamayama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has observed activity dating back at least to the 11th century CE. Maekake has had several major Plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

Information Contacts: Hitoshi Yamasato and Tomoyuki Kanno, Japan Meteorological Agency (JMA), Volcanological Division, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan (URL: http://www.jma.go.jp/jma/index.html); Hidefumi Watanabe and Setysuya Nakada, Volcano Research Center-Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo, Tokyo, 113-0032 Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html).