The eruption at Asama that began on 21 January 2009 (BGVN 33:12) was followed by stronger activity on 2 February, and then by crater incandescence and small ash eruptions through 2 May (BGVN 34:04). The last colored plume reported by the Japan Meteorological Agency (JMA), likely containing ash, was on 27 May 2009. Periods of increased seismicity were detected throughout the year.

JMA monthly reports, available through December 2009, noted observations of white plumes in May 2009, with a grayish-white plume rising to 600 m above the Kamayama Crater on 27 May. White plumes in June remained below 500 m height. In July, white plumes rising no higher than 300 m were seen on nine days. Similar plumes were seen during August on 19 days and continued to be observed almost daily through the remainder of 2009.

Intermittent tremor episodes continued to be detected in 2009, along with volcanic earthquakes at a rate of approximately 2,300-3,000 events/month (average ~100 per day) from May through August. Only 1,450 volcanic earthquakes were detected in September (average of less than 50/day). Seismicity increased again after 25 October, rising to 143 events on the 30th, and a monthly total of 1,796. Volcanic earthquakes subsided after 3 November, remaining at less than 100/day through the end of the month (1,311 total). Overall December seismicity was back to the level recorded earlier in the year with 2,347 events, with an increase after the 25th, and a high of 135 earthquakes on the 29th.

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: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).