The Japan Meteorological Agency has raised the eruption alert level for Mt. Hakone, which straddles the border between Kanagawa and Shizuoka prefectures, warning of a possible phreatic explosion.
The mountain's alert warning was raised on Wednesday from Level 1, indicating a normal situation, to Level 2, which restricts access around the volcanic vent. It was the first Level 2 warning regarding the mountain since the five-level alert system was introduced in 2009.
The municipal government of Hakone, Kanagawa Prefecture, has issued an evacuation directive within a 300-meter radius of the Owakudani district. This resulted in the suspension of services by Hakone Ropeway Co., which operates in the district.
Mt. Hakone has been showing signs of increased volcanic activity recently, such as a series of earthquakes occurring since April 26, and crustal movements have been confirmed through a clinometer.
On Tuesday, 116 volcanic earthquakes were observed, a record for one day. Three earthquakes that could be felt were confirmed, the first in four years.
According to the agency, the focus of one perceptible earthquake, which occurred at 9:13 p.m. on Tuesday, was about five kilometers deep, deeper than the other two noticeable earthquakes.
Sadayuki Kitagawa, head of the agency's volcanology division, said at a press conference Wednesday morning that the earthquake could have been caused by hot gas and other elements deep in the ground.
"This is obviously different from past situations. As hot water that has pooled in shallow areas has become unstable, there is a higher possibility of a phreatic explosion," Kitagawa said in explanation of the raised alert level.
If such an explosion occurs, cinders more than 50 centimeters in diameter could be sent into the air and fall around Owakudani, he added.
As of noon Thursday, 10 volcanic earthquakes had already been observed, and the agency continued calling for caution.
Responding to the raised alert level, all services on the Hakone Ropeway were suspended between Sounzan and Togendai stations, which is inside the area covered by the evacuation directive. An about 1.2-kilometer prefectural road to a parking lot for Owakudani was closed to traffic.
Minor eruption forecast
Mt. Hakone has seen numerous phreatic and phreatomagmatic eruptions over the past 650,000 years.
The Ashinoko lake is said to have been formed due to phreatic explosions about 3,000 years ago, when landslides from the mountain blocked the flow of a river. Mt. Hakone's last eruption is believed to have been sometime in the 12th or 13th century.
Since April 26, the number of earthquakes due to volcanic activity has surged. Steam has been observed belching out of the mountain, whose slope has shown signs of slight deformation.
According to the Japan Meteorological Agency, magma chambers are located deep beneath Mt. Hakone. The magma causes volcanic gases to heat groundwater, creating a large volume of steam. The agency said earthquakes occur when the bedrock cracks due to the resulting pressure.
A Level 2 warning has been issued around Owakudani, meaning that phreatic eruptions and the ejection of cinders within a range of several hundred meters are anticipated. In last year's eruption of Mt. Ontake, which claimed the lives of many tourists, volcanic cinders were spread over about a kilometer.
The town of Hakone has already prohibited entry to Owakudani and surrounding areas. The Hot Springs Research Institute of Kanagawa Prefecture explained that only the area surrounding Owakudani is expected to be affected at this stage.
According to Prof. Setsuya Nakada, a volcanology expert at the University of Tokyo's Earthquake Research Institute, "Even if [Mt. Hakone] does erupt, it will likely be on a smaller scale than that of last year's eruption of Mt. Ontake."