On 21 January, JMA reported that a thin ash blanket was seen on the NW crater rim of Asama. According to news articles, JMA raised the Alert Level from 2 to 3 on 1 February after detecting ground deformation and increased seismicity. An eruption the next day produced an ash plume that rose to an altitude of 4.2 km (13,800 ft) a.s.l. Ash fell in nearby communities and was detected as far away as eastern Chiba, 170 km SE. Based on analysis of satellite imagery, pilot observations, and information from JMA, the Tokyo VAAC reported that on 2 February ash plumes rose to altitudes of 3-4.6 km (10,000-15,000 ft) a.s.l. and drifted SE.

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.

Sources: The Japan Times, Associated Press, Tokyo Volcanic Ash Advisory Center (VAAC), Japan Meteorological Agency (JMA)