When both Shizuoka and Yamanashi prefectures held disaster drills earlier this year, they were not rehearsing for an earthquake.
The 15,000 Yamanashi participants included officials and residents from a number of towns around Mount Fuji, Japan’s highest peak rising to 3,776 meters. The drill focused on evacuation from the threat of lava flows, falling ash and mudslides. Shizuoka’s drill in May projected a scenario of quakes sparked by an eruption. Both drills were the the first of their kind in the prefectures.
Mount Fuji is one of 86 active volcanoes in Japan, which account for 10 percent of the world’s total. It has not shown major activity for almost 300 years, since a 1707 eruption sent ash spewing as far as Tokyo.
Beginning late last year, however, an increase in low-intensity quakes beneath the mountain has been recorded by the Japan Meteorological Agency. In October, 133 tremors were monitored, 221 in November and 143 in December, the JMA said. After a quieter period at the start of 2001, in April and May the number of tremors increased again to 132 and 164, respectively. In previous years, the number has ranged from three to 30 per month.
Tomoyuki Kanno of the JMA’s volcanology division said the agency will step up monitoring: “If anything should happen to Mount Fuji, the impact would be huge. It’s an active volcano, so we are using this opportunity to consider disaster prevention.”
However, both Kanno and officials of the Shizuoka Prefectural Government were anxious to play down fears that the current spate of rumbles points to a full-scale eruption in the near future.
“The quakes are so low-frequency that they cannot be felt by humans,” said Takayoshi Iwata of Shizuoka’s disaster prevention division. “There’s no reason to think it will blow.”
Yoshimitsu Okada of the National Research Institute of Earth Science and Earthquake Prevention agrees with Iwata, saying the recent spate of low-frequency quakes was recorded at depths of around 15 km, too deep to be a serious threat.
But he did point to other abnormalities that “may indicate some signal of the anticipated [Tokai] earthquake.”
One of those is a change in the pattern of low-frequency quakes in the Tokai region at the same time as Mount Fuji’s quake pattern increased. Another may be the sudden elevation of land in Omaezaki, Shizuoka Prefecture, that previously had been subsiding at a rate of 5 mm a year due to tectonic plate activity below the city.
Masato Koyama, a volcanologist at Shizuoka University and expert on Mount Fuji, suggests the coming Tokai quake may well trigger an eruption of Mount Fuji. His theory is based on historical records showing that an eruption in 1707 followed a large quake in Tokai 49 days earlier.
However, Kanno of the JMA insists there may be no connection: “If there were, it’s likely Mount Fuji would have blown after the 1923 Great Kanto Earthquake.”