Hokkaido closes Mount Tokachi after the first Level 3 alert

Japan's Meteorological Agency raised the eruption alert for Mount Tokachi, a 2,077-metre volcano in central Hokkaido, from Level 2 to Level 3 at 09:30 on Saturday, 12 September. Level 3 means the mountain is closed. It is the first time Tokachi has been placed at that level since the alert system began in 2008, and the first Level 3 assigned to any Hokkaido volcano in that period.

The Sapporo Regional Headquarters said large-amplitude volcanic earthquakes near the edifice have increased since 17 August. At the Iwozawa station north-west of crater 62-2, two events with a maximum amplitude of 2 micrometres or more were recorded in the 08:00 hour on Saturday. That brought the 30-day count to five and crossed the written threshold for a step up.

What the instruments have been saying since March

Heat at the summit has been high for months. From March the agency has recorded expansion at a somewhat deep part of the mountain, a signal that magma is moving. From April, sulphur dioxide output has risen. Those two trends, plus the recent large earthquakes, are why the agency now says a crater-to-3-kilometre eruption is more likely.

People inside about 3 kilometres of crater 62-2 are warned against ballistic blocks. Downwind areas may see ash and small stones. The alert covers Biei, Kamifurano, Furano, Minamifurano and Shintoku. Kamifurano ordered an evacuation of the Fukiage Onsen district and advised older and vulnerable residents in the Tokachidake Onsen district to leave.

By 15:00 on Saturday the agency said activity after 09:00 had not changed in a sharp way. Steam from 62-2 looked ordinary. The Level 3 stays in force because the month-long earthquake count, not a single plume, is the trigger.

A mountain with a 30-to-40-year rhythm

Tokachi sits in Daisetsuzan National Park and is one of Japan's 100 famous mountains. It erupted in 1926 and again in 1962. Local commentary this weekend noted a 30-to-40-year interval in the modern record. Inns near the trailheads have already taken cancellations. The mountain is a summer and autumn walking draw. Level 3 ends that season at a stroke.

Level 2 had already kept people away from the crater rim. Level 3 keeps them off the peak. Level 4 would move vulnerable residents. Level 5 would empty dangerous settlements. The agency has not stepped that far. It has, for the first time on this mountain under the present scale, decided that a walk to the summit is no longer an acceptable risk.

Hokkaido's next decision is practical. How long Fukiage stays empty, and whether the autumn bus routes to the onsen reopen, depends on whether the 2-micrometre quakes keep arriving.