The Meteorological Agency on Sunday issued a volcanic alert for the area around the crater of Mount Io, which straddles Miyazaki and Kagoshima prefectures in southern Kyushu.
Seismic activity is increasing around the 1,317-meter-high mountain, which is part of the Kirishima volcano group, the agency said.
Local government officials in the city of Ebino declared a 1-km no-entry zone around the crater of Mt. Io, part of the Kirishima mountains, while the weather agency said that large stones could be ejected within a 1-km radius in the event of an eruption.
The agency had detected 53 volcanic temblors as of 7 p.m. Sunday at the volcano, the highest daily total since seismometers were installed in March 2013.
Volcanic activity has intensified since last year, the agency said. In addition to volcanic quakes, fumarolic gas was recorded in December and areas with high ground temperatures have been increasing.
Mt. Io is known to have erupted between 1500 to 1700, the agency said.
The agency launched an on-site investigation while authorities working with the Miyazaki Prefectural Government called for climbers to descend from the mountain.
A similar warning for the crater area was issued in October 2014 and lifted in May last year. But after the warning was lifted, tremors resumed in July, with five observed as of Feb. 10, the agency said. Then, on Sunday, the shallow volcanic quakes began growing to the point that 36 had been recorded by 11 a.m.