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Increased seismicity at Mount Ontake, Alert Level raised, Japan

increased-seismicity-at-mount-ontake-alert-level-raised-japan

Increased seismicity detected under Mount Ontake this week forced the Japan Meteorological Agency (JMA) to raise the volcano's Alert Level from 1 to 2 — the highest since August 21, 2017. The last eruption of this volcano took place in 2014. It was a powerful and sudden eruption in which 63 people lost their lives. The volcano's first historical eruption took place in 1979 from fissures near the summit.

JMA said 97 earthquakes were detected under the volcano in 4 hours to 18:00 LT on February 23, 2022.

The agency is urging residents and tourists to refrain from entering the danger zone in Otaki Village and Kiso Town in Nagano Prefecture and Gero City in Gifu.1

A sudden eruption of this volcano at 03:00 UTC on September 27, 2014, ejected volcanic ash up to 11 km (37 000 feet) above sea level, killed 58 people and left 5 others missing. At least 40 were severely injured.2

One witness told Japanese public broadcaster NHK that the eruption started with large booms that sounded like thunder.

Mikio Oguro, an NHK journalist who was on the slope on an unrelated assignment, told the station that he saw massive smoke coming out of the crater, blocking sunlight and reducing visibility to zero.

"Massive ash suddenly fell and the entire area was totally covered with ash," he said.

Most people were killed by heart and respiratory failure after inhaling ash when caught in the outer areas of a pyroclastic flow. 

Geological summary

The massive Ontakesan stratovolcano, the second-highest volcano in Japan, lies at the southern end of the Northern Japan Alps. Ascending this volcano is one of the major objects of religious pilgrimage in central Japan.

It is constructed within a largely buried 4 x 5 km (2.5 x 3.1 miles) caldera and occupies the southern end of the Norikura volcanic zone, which extends northward to Yakedake volcano.

The older volcanic complex consisted of at least four major stratovolcanoes constructed from about 680 000 to about 420 000 years ago, after which Ontakesan was inactive for more than 300 000 years.

The broad, elongated summit of the younger edifice is cut by a series of small explosion craters along a NNE-trending line. Several phreatic eruptions post-date the roughly 7 300-year-old Akahoya tephra from Kikai caldera.

The first historical eruption took place in 1979 from fissures near the summit.

A non-eruptive landslide in 1984 produced a debris avalanche and lahar that swept down valleys south and east of the volcano.

Very minor phreatic activity caused a dusting of ash near the summit in 1991 and 2007.

A significant phreatic explosion in September 2014, when a large number of hikers were at or near the summit, resulted in many fatalities.3

References:

1 Ontakesan Volcanic Warning – JMA

2 Deadly eruption at Mount Ontake, Japan – The Watchers

3 Ontakesan – Geological summary – GVP

Featured image credit: Luka Peternel

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