JMA cited a model by Nakamichi and others (2009) regarding the intrusion of a magmatic body and the subsequent 2007 eruption, to describe phreatic eruptions at Ontakesan (figure 12). The exact date of the eruption was undeterminable. This finding of fresh ash was believed to indicate that the 2007 eruption was not merely phreatic but involved some escape of juvenile components to the surface. Based on the data collected, JMA inferred that the earthquake had resulted from a magmatic intrusion that had advanced toward the surface reaching ~4 km below the volcano's summit.Ī field study two months later, on, noted fresh volcanic ash from Ontakesan's 79-7 crater. The specific dates of the seismicity and earthquake were not specified in the available reporting. Seismic and other data considered by JMA showed that an earthquake had occurred during late March, originating directly below Ontakesan. On 16 March 2007, fumarolic activity increased fumes at the summit were occasionally detected by a surveillance camera (at "Mitake Kurozawa," but the exact location was not found in English on maps in Ontakesan reports). Furthermore, GPS observations detected a small amount of crustal deformation preceding the seismic activity, indicating a slight inflation at Ontakesan. The January 2007 tremor was described as very low frequency, containing a 15- to 20-second-long component. Tremor had, according to a JMA plot, remained near zero during all of 2006. On 25 January 2007, tremor occurred with the largest recorded amplitude in at least a year. During January 2007, instruments recorded 90 earthquakes on the 16th and 164 earthquakes on the 17th. During mid- to late December 2006, instruments detected inflation of the volcanic edifice and an increase in shallow seismicity directly below the summit. The March 2007 eruption was preceded by minor inflation and an increase in seismic activity (JMA, date unknown). The period of activity after 1979 represents what appears to be Ontakesan's most active during the past 250 years (Oikawa, 2008).Ģ006–2007. Although there are no reported records of historical eruptions before 1979, fumarolic activity was noted as ongoing for several hundred years near to Ontakesan's summit (Jigokudani and Hachotarumi). However, recent research cited in JMA (date unknown) indicates that over the past 10,000 years, four magmatic eruptions have also occurred. Historically, activity at Ontakesan has consisted mainly of phreatic explosions every several hundred years. ![]() The data for this report was collected chiefly from online reports by the Japanese Meteorological Agency (JMA), the Geological Survey of Japan (GSJ), and the National Institute of Advanced Industrial Science and Technology (AIST). Our last Bulletin report ( BGVN 28:11) noted occasional white plumes during 2000 to 2003. The 2014 impulsive eruption was documented by an outstanding number of close-up photographs and videos taken by eyewitnesses.īetween the 20 eruptions, activity receded to background levels. ![]() The eruption killed ~57 people and an additional 6 were still missing as of 27 October 2014 (Kyodo, 2014). The 2014 eruption included ashfall, pyroclastic flows, and related density currents. Hundreds of people were on the mountain at the time. The 2014 eruption took place during the autumn color season on a Saturday. The volcano is a famous tourist area to see color changes in autumn foliage, it also contains considerable alpine touristic infrastructure, including lodges. The 2014 eruption took place with a sudden onset and with few if any precursory warnings. The first eruption was on an unknown date in late March 2007, and the second eruption was on 27 September 2014. ![]() During this reporting interval, two eruptions occurred, both broadly described as phreatic, yet containing a minor component of identified juvenile magmatic material. This Bulletin report on Ontakesan (Kiso-Ontakesan, Ontake) covers activity from November 2003 to November 2014. Eruptions: (a) March 2007 and (b) 27 September 2014 (~60 deaths and ~70 injured)
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