Volcanic tremors on Mount Asama, which erupted Wednesday night, had begun to subside Thursday morning, the Meteorological Agency said.
But the agency warned that the volcano, which straddles Nagano and Gunma prefectures, could become active again and called on the public to remain alert.
Wednesday’s eruption, the first on such a scale since April 1983, occurred around 8 p.m.
Lava flowed down the western slope of the mountain, apparently triggering a forest fire, but no casualties have been reported.
Chief Cabinet Secretary Hiroyuki Hosoda told reporters Thursday morning that the government will continue to monitor the volcano, but added that there have been no signs of a massive eruption so far.
An agency observation team sent to the eruption site to gauge the range of volcanic cinders and ash in the area found pieces of pumice roughly 4 km northeast of the crater. The largest piece measured up to about 8 cm in diameter. The team also found that about 2 mm to 3 mm of volcanic ash had accumulated in an area 5 km to 6 km northeast of the mountain.
The Meteorological Agency reported that ash from Wednesday’s eruption has been observed in three prefectures — Fukushima, Tochigi and Gunma. The furthest location at which ash from Mount Asama was found was Soma, Fukushima Prefecture, which is about 250 km northeast of the volcano.
About 40 residents and tourists in the village of Tsumagoi, Gunma Prefecture, located at the foot of the volcano, returned home after spending the night at public facilities. An elementary and a junior high school, as well as a kindergarten in the town of Naganohara in the prefecture, were closed as roads to the schools were shut down.
Volcanic tremors, which had been observed since immediately after the eruption, gradually subsided and had virtually ceased by early Thursday, the agency said.
As of 3 p.m. Thursday, the number of volcanic quakes for the day stood at 10, down from 183 on Wednesday. Crustal movements stopped within an hour of the eruption and the forest fire has died out.
“Eruptions of similar or lesser intensity may occur without any warning,” an agency official said.
Municipalities have continued to seal off the area within 4 km from the crater. Areas within 2 km from the crater were hit by showers of volcanic cinders, while volcanic ash falls were observed in Tochigi and Fukushima prefectures and large explosions were heard at an observation site of the agency about 8 km from the crater, the agency said earlier.
The agency had set the activity level of the 2,568-meter mountain at 2 on a 6-point scale during the past month, but raised it to 3 following the eruption, saying small or midsize eruptions had occurred and due precautions were needed.
The mountain has shown signs of volcanic activity since June 2002 and experienced repeated tiny eruptions between February and April last year.
According to a group led by Gunma University volcanology professor Yukio Hayakawa, the estimated volume of ash that spewed from Wednesday’s eruption was about 200,000 tons. That is nearly the same scale as the roughly 160,000 tons released in the 1983 eruption.