Small eruptions on 4 and 28 September, the first since October 1998

The last reported volcanic activity at Komaga-take was a small-scale phreatic eruption on 25 October 1998 (BGVN 23:09). After 1998 there were no reports of volcanism until September 2000 when small eruptions occurred on the 4th and 28th.

Komaga-take, about 120 km SW of the prefectural capital Sapporo, erupted at 2214 on 4 September. According to a Bernama news article, Hokkaido University Professor Hiromu Okada stated that he did not see any fresh cracks or crevices in the volcano when he observed it by helicopter the day after the eruption. He added that "there were many cinders popping out in a wide area ." The same day the Sapporo District Meteorological Observatory reported that a column of "smoke" rose up to 500 m above the volcano. According to the Bernama article, local authorities issued volcano warnings to the 60,000 residents of Mori, ~10 km NW of the volcano, and four other nearby towns. An Associated Press article reported that according to the observatory, a small amount of ash was observed on the ground in a town NW of the volcano. The Tokyo VAAC reported that no ash cloud was visible on GMS-5 imagery.

The volcano erupted for a second time on 28 September, but inclement weather inhibited observations. According to an Associated Press article, Hisao Hashimoto stated that a light coating of ash covered cars in a town at the base of the volcano. There were no reports of injuries or damage.

Information Contacts: Tokyo VAAC (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Bernama-dpa (URL: http://www.asiapac.net/bernama/prwire/); Associated Press (URL: http://www.ap.org/).

Information is preliminary and subject to change. All times are local (unless otherwise noted)

May 1990 (BGVN 15:05) Cite this Report

Weak vapor emission; earthquake swarm

Seismicity has generally remained low since 1966 when JMA installed a sensitive seismometer (2,000x magnification) 4.1 km WSW of the summit crater. This low level was punctuated during recent years by several swarms of small earthquakes. On 13 June 1983, 19 earthquakes were recorded from 1842 until 1900. At 1416 on 3 December 1989, a shock (M 3.4) felt at the base of the volcano was located 7 km N of the summit at 4 km depth by the seismic network. Twenty small earthquakes were recorded in the next 4 hours and roughly 50 from 0034 to 0144 on 7 April 1990. The wave form and character of the 7 April swarm was very similar to that of the 1983 swarm, but different from that of December 1989. Despite the increased seismicity, steady weak steam emission from the fumarolic area on the summit continued unchanged as of early June.

Information Contacts: JMA.

February 1996 (BGVN 21:02) Cite this Report

Tremor and extension precede early March phreatic eruption

In the early evening of 5 March, >=6 minutes of large amplitude volcanic tremor registered and during the night an eruption began. The monthly record of earthquakes from 1966 until the eruption showed little in the way of a diagnostic rise prior to the eruption (figure 1). The above mentioned tremor was detected at the local Japan Meteorological Agency (JMA) seismic station (4.1 km WSW from the crater). Tremor was also noted by Usu Volcano Observatory (UVO), which maintains five seismic stations. On 5 March UVO recorded 5 minutes of premonitory tremor and an abnormally high number (> 10) of small volcanic earthquakes. Prior to the heightened seismicity, UVO researchers found continuous extension of Komaga-take's 1929 crater area begining in 1989, and their leveling survey in November 1995 showed a reversal from subsidence to uplift.

Figure 1. Monthly number of earthquakes of Komaga-take recorded at JMA's local seismic station in the years 1966-96. An eruption took place in early March 1996. Courtesy of JMA.

On the morning of 6 March JMA reported that a white plume rose to 150 m above the summit and ashfall was visible for >10 km from the summit on the SSE flank. According to Tad Ui, who made observations from a helicopter during the morning of 6 March, steam-dominated eruption clouds rose from inside the craters of the 1929 eruption and also from new, 100-m-long, N-S trending fissures S of the craters. He estimated the cloud height at around 400 m. Ashfall covered new snow; no mudflows were observed. Videos taken around this time by the press and local residents showed violent, gray, ash-laden clouds jetting from newly formed fissures.

On 8 March, a white-colored plume was 900 m above the summit. By 12 March, eruptive activity declined. Post-eruption seismicity was weak: tremor was not observed from 5 March to as late as 10 March, and after 6 March small earthquakes occurred several times a day.

An aviation notice on 7 March stated that the top of the ash cloud was at ~1,500-m altitude (~300 m above the summit) and drifting SE; a notice the next day reported ash to ~1,700 m altitude drifting W. Notices were again issued on 13 and 19 March for ash clouds to ~1,300-m altitude.

Ui and other scientists from Hokkaido University will analyze new products; preliminary analysis suggested that the initially erupted tephra contained little fresh glass, and other magmatic materials appeared absent. The mass of tephra erupted in this event was estimated at ~25,000 tons.

Residents evacuated the night of 5 March were permitted to return on 8 March. Small phreatic eruptions of the kind witnessed beginning on 5 March 1996 could be precursors to larger explosions. Phreatic eruptions were observed during June 1919, and in June 1929, prior to larger events.

Information Contacts: Tadahide Ui, Hokkaido University, Grad School Sci., Kita-ku, Sapporo 060 Japan; Usu Volcano Observatory (UVO), Faculty of Science, Hokkaido University, Sohbetsu-cho, Usu-gun, Hokkaido 052-01 Japan; Volcanological Division, Seismological and Volcanological Dept, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan; Setsuya Nakada, Volcano Research Center, Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113, Japan (URL: http://www.eri.u-tokyo.ac.jp/VRC/index_E.html); Bureau of Meteorology, Darwin, Australia.

March 1996 (BGVN 21:03) Cite this Report

Additional information about the 5 March eruption

Volcanic tremor was registered for six minutes starting at 1810 on 5 March by the JMA station 4.1 km WSW of the crater. During this activity, two main vents opened on and near the S side of Showa 4-nen (1929) crater. A line of vents extending ~200 m N-S formed on the S part of the crater floor. Strong eruptive activity was observed until 7 March and then decreased. Volcanic earthquakes had increased somewhat prior to the eruption, but seismicity remained low afterwards through mid-April.

Information Contacts: Volcanological Division, Seismological and Volcanological Department, Japan Meteorological Agency (JMA), 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100 Japan.

May 1996 (BGVN 21:05) Cite this Report

Steaming activity continues

Activity has declined since the eruptive events of March when two vents opened on and near the S side of Showa 4-nen (1929) crater, and a line of vents extending ~200 m N-S formed on the S part of the crater floor. The height of the gas plume remained at 100-200 m. A volcanic earthquake occurred on 15 May. No volcanic tremor was observed.

Komaga-take sits 30 km N of Hakodate City (population 320,000). The andesitic stratovolcano has a 2-km-wide horseshoe-shaped caldera open to the E. The volcano has generated large pyroclastic eruptions, including major historical eruptions in 1640, 1856, and 1929. In the 1640 eruption, debris from a partial summit collapse entered the sea resulting in a tsunami that killed 700 people. Although the 1929 eruption was one of the largest 20th century eruptions in Japan, it may not have had clear geophysical precursors.

Information Contacts: Japan Meteorological Agency (JMA), Volcanological Division, Seismological and Volcanological Department, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan.

September 1998 (BGVN 23:09) Cite this Report

Phreatic eruption spreads ash 25 October

The Japan Meteorological Agency (JMA) issued an advisory and three observation reports concerning Hokkaido-Komaga-take volcano on 25 October following a small-scale phreatic eruption that began at 0912 the same day. Ash rose in a column to a height of ~1,200 m above the crater. The eruptive activity soon declined. There were no report of injuries or damage caused by the eruption, and no evacuation order was issued.

Volcanologists surveyed the activity from a helicopter the afternoon of 25 October (figure 2). They reported that the eruption originated from the same crater that opened during the 1929 eruption, which was also the site of the March 1996 eruption. Ash covered a significant area around and to the E of the crater. The scale of this eruption apparently was smaller than that of the March 1996 eruption.

Figure 2. An aerial view of Komaga-take showing fuming activity from the 1929 Crater about 6 hours after the 25 October 1998 eruption. View is from the SE looking towards the Komanose Rim (back) and the Sawaradake Rim (back right). The 1942 Large Fissure (middle, diagonal) and the 1996 Southern Fissure Crater (middle center) can also be seen. Hyoutan Crater (front center) is adjacent to the 1929 Crater. Photograph by Bousai Heli; courtesy of Hiromu Okada, Usu Volcano Observatory.

Volcanic tremor lasting six minutes was associated with this eruption. In addition, five volcanic earthquakes were recorded in the 12 hours following the first eruption signs.

Komaga-take is located 30 km N of Hakodate City (population 320,000). The andesitic stratovolcano has a 2-km-wide horseshoe-shaped caldera open to the E. The volcano has generated large pyroclastic eruptions, including major historical eruptions in 1640, 1856, and 1929. In the 1640 eruption, debris from a partial summit collapse entered the sea resulting in a tsunami that killed 700 people. Although the 1929 eruption was one of the largest 20th-century eruptions in Japan, it may not have had clear geophysical precursors.

Information Contacts: J. Miyamura, Sapporo District, Japan Meteorological Agency, 1-3-4 Ote-machi, Chiyoda-ku, Tokyo 100, Japan; Hiromu Okada, Usu Volcano Observatory, Institute of Seismology and Volcanology, Hokkaido University, Sohbetsu-cho, Hokkaido 052-0103, Japan.

August 2000 (BGVN 25:08) Cite this Report

Small eruptions on 4 and 28 September, the first since October 1998

The last reported volcanic activity at Komaga-take was a small-scale phreatic eruption on 25 October 1998 (BGVN 23:09). After 1998 there were no reports of volcanism until September 2000 when small eruptions occurred on the 4th and 28th.

Komaga-take, about 120 km SW of the prefectural capital Sapporo, erupted at 2214 on 4 September. According to a Bernama news article, Hokkaido University Professor Hiromu Okada stated that he did not see any fresh cracks or crevices in the volcano when he observed it by helicopter the day after the eruption. He added that "there were many cinders popping out in a wide area ." The same day the Sapporo District Meteorological Observatory reported that a column of "smoke" rose up to 500 m above the volcano. According to the Bernama article, local authorities issued volcano warnings to the 60,000 residents of Mori, ~10 km NW of the volcano, and four other nearby towns. An Associated Press article reported that according to the observatory, a small amount of ash was observed on the ground in a town NW of the volcano. The Tokyo VAAC reported that no ash cloud was visible on GMS-5 imagery.

The volcano erupted for a second time on 28 September, but inclement weather inhibited observations. According to an Associated Press article, Hisao Hashimoto stated that a light coating of ash covered cars in a town at the base of the volcano. There were no reports of injuries or damage.

Information Contacts: Tokyo VAAC (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Bernama-dpa (URL: http://www.asiapac.net/bernama/prwire/); Associated Press (URL: http://www.ap.org/).