The Japan Meteorological Agency (JMA) has issued an alert for a possible eruption of Mount Zaosan, Zaozan volcano group in northeastern Japan citing a large amount of volcanic activity observed there this month, The Asahi Shimbun reports. The last eruption in this volcanic group occurred on May 18, 1940 (Okama crater lake, VEI 1).
Local governments urged climbers not to enter a 1.2-km area from the crater due to the possibility of large rocks being ejected in an eruption
According to JMA's Sendai Regional Headquarters, the number of volcanic tremors, whose epicenter is believed to be located near Okama crater lake has surged since April 7.
On April 13, 30 such earthquakes were recorded by 20:00 local time.
The Okama crater lake, north side view.
184 tremors were recorded since the start of this month, the highest monthly total since the agency's Sendai Regional Headquarters began monitoring the mountain in September 2010. If phreatic eruptions were to occur, the volcano will likely spew rocks measuring 50 centimeters or more.
The agency designated an area 2 km from Okama as a precautionary range and up to 3.5 km on the east side.
Mount Zaosan has had greater activity since last August. The Sendai center has observed 18 tremors associated with movements of magma and hot liquids since. One such tremor was observed on April 9.
So far, no irregularities, such as release of gases, were spotted around the lake.
Sakura and Mount Zaō seen with its inverted image in the river Shiroishi-gawa. Author: ChampagneFight
There are no entries for this volcano in GVP's weekly volcanic reports.
According to GVP, the last eruption in this volcanic group occurred on May 18, 1940 (Okama crater lake, VEI 1).
The Zaozan volcano group, the most active of northern Honshu, consists of a complex cluster of stratovolcanoes straddling the Pacific Ocean-Japan Sea divide. The Pleistocene Ryuzan volcano forms the western group (Nishi-Zao), and Byobu and Fubo volcanoes form the southern group (Minami-Zao). The complex was constructed over granitic basement rocks as high as 1500 m and thus has a relatively small volume.
The 7 cu km Zaozan volcano proper forms the central group (Chuo-Zao), a volcanic complex topped by several lava domes and the Goshikidake tuff cone, aligned along a NW-SE trend. Several episodes of edifice collapse produced debris avalanches during the Pleistocene. Goshikidake contains the active Okama crater, filled with a colorful, strongly acidic crater lake (pH 1.3). It has been the source of most of the frequent historical eruptions, which date back to the 8th century CE. (GVP)
Featured image: The Okama crater lake in 2006.