The agency elevated the warning on Mount Zao, a cluster of volcanoes on the border of Yamagata and Miyagi prefectures whose highest point is 1,841 meters (6,040 feet), to 2 from 1, meaning people should avoid going near the crater.
“There is a possibility of a small-scale eruption,” the agency said in a statement, noting that a number of small earth movements were detected Tuesday, along with a slight bulging of the ground in one area.
It also warned of the possibility that volcanic rocks could be jettisoned as far as 1.2 km in an eruption.
According to the agency, volcanic tremors, caused by movements of hot subterranean water and volcanic gas, were observed over a period of 31 minutes from 2:18 p.m. on Tuesday and were larger than those recorded on Sunday and early Tuesday morning.
The Yamagata Municipal Government has issued an evacuation advisory for the areas around the crater and also advised a nearby ski resort to stay on alert.
The announcement came a week after a member of Japan’s Self-Defense Forces was killed when rocks from the sudden eruption of Mount Moto-Shirane rained down on skiers at a mountain resort in Gunma Prefecture.
Video footage taken by people on the mountain showed black ash spewing into the sky as volcanic debris rained down, some punching holes in the metal roof of a ski gondola. Eleven people were injured and around 100 skiers took refuge in a mountain hut for several hours until rescued.
Between the eruption and Jan. 26, a little more than 20,000 people canceled room reservations in Kusatsu, according to a survey by an organization representing local hotels and inns.
Zao, like Moto-Shirane, is a popular resort area famed for its so called “snow monsters” that are created by water vapor freezing on trees in winter. Its slopes are packed with skiers in winter and hikers in other seasons.
Currently, 50 out of 111 active volcanoes nationwide are under constant watch by the agency. The 50 were selected by a team of experts in coordination with agency officials. New activity is a crucial factor in deciding whether to begin continuous monitoring, an official said.
In September 2014, 63 people were killed on Mount Ontake in the country’s worst volcanic disaster in nearly 90 years.
