A minor eruption of Mt. Asama, which sits on the border of Gunma and Nagano prefectures, occurred at 1:51 a.m. Monday, according to the Meteorological Agency.
Smoke billowed about 2,000 meters into the air above the 2,568-meter volcano in the first activity there since August.
The agency has issued a warning that the activity could be followed by a medium-sized eruption.
It also said that the volcano had spewed rocks for the first time since November 2004.
According to the agency’s volcano section, the rock was scattered up to one kilometer from the mouth of the volcano.
At the volcano’s observation point, there were reports of the earth shaking and rumbling noises as a result of the eruption. Smoke was said to be billowing about 200 meters into the air from the volcano as of 11:30 a.m. on Monday. The agency said the volcanic gases were likely released as a result of magma activity.
The volcanic ash was spotted not only in Nagano Prefecture’s Karuizawamachi, a town near the mountain, but as far away as the center of Tokyo, Yokohama and Kimitsu, Chiba Prefecture, among other places. The latest eruption marked the first time since September 2004 that volcanic ash has fallen in the center of the capital.
The agency issued a crater proximity warning Sunday after detecting swelling at the top of the volcano, which is seen as a precursor of an eruption. It also raised the alert level from level 2 (do not approach the crater) to level 3 (do not go within four kilometers of the crater).
In 2004, the volcano was active for more than three months, including four medium-sized eruptions between September and November. Although there were no casualties, the eruptions did cause damage to crops as volcanic ash was dispersed as far away as Tokyo and Kanagawa Prefecture.
“Since the minor eruption in August, there has been ongoing swelling at the peak,” said Setsuya Nakada, a professor at Tokyo University’s Earthquake Research Institute.
“A lot of magma has accumulated in shallow areas near the peak, meaning an eruption could occur anytime,” he said.
“In September 2004, the eruption started with medium intensity. This time, it started with a small eruption. We can’t predict whether the activity will settle down or increase, so we need to stay alert,” Nakada added.