A recent series of eruptions at Mount Asama that began last month is the latest reminder that Japan is a country of volcanoes.

Japan has 108 active volcanoes that have erupted at least once in the last 10,000 years.

While the Meteorological Agency has succeeded in forecasting eruptions — in some cases by monitoring such pre-eruption signs as earthquakes — it is often hard to determine the timing, scale and duration of an eruption because each volcano is different.

Also, many of the volcanoes do not have a fixed pattern of activity, making forecasts difficult, experts say.

Twenty volcanoes are currently being monitored on a 24-hour basis by four regional observation centers operated by the agency. They include 13 volcanoes designated as Rank A because they are most active and seven that are less active but could cause serious damage because of their proximity to residential areas.

Earthquake recorders, cameras, tiltmeters and global positioning systems are installed around these volcanoes, said Hitoshi Yamasato, a senior coordinator for volcanic affairs at the agency.

Data gathered through such equipment are analyzed at the centers, located in Sapporo, Sendai, Tokyo and Fukuoka. Yamasato said that based on the analyses, the centers issue three levels of warnings: volcanic alert, volcanic advisory or volcanic observation report.

Governments in the vicinity of volcanoes take into account the centers’ judgment when deciding whether to issue evacuation orders, he said.

At Mount Asama, which straddles Nagano and Gunma prefectures, small-scale earthquakes caused by rising magma have been observed since 2000.

Yamasato said the number of tremors increased on Aug. 31. But because similar activity had been observed a number of times in recent years, the Tokyo center — about 150 km from the volcano — only issued an observation report that day.

Mount Asama erupted the next day.

Yamasato explained the agency’s error by saying, “We could be crying wolf if we issued volcanic advisories too often.”

An advisory is normally issued when small-scale eruptions are observed or the Meteorological Agency finds imminent signs of an eruption.

Mount Asama is about 10 km from the resort town of Karuizawa, Nagano Prefecture. Its ash was found to have reached the Tokyo metropolitan area in mid-September.

Over the past century, Asama repeatedly had small to medium-scale eruptions. In 1783, a major eruption killed more than 1,000 people. The pyroclastic flow destroyed a nearby village.

Yamasato said he has not seen any sign that a large eruption is imminent at Mount Asama.

“When the volcano is headed for a major explosion, the scale of its activity will become gradually bigger,” he said. “I don’t think we will have a sudden bang.”

While Mount Asama has had various types of eruptions, some of the nation’s other volcanoes have fixed patterns in their activity, making eruption forecasts easier, said Toshitsugu Fujii, a professor at the Earthquake Research Institute of the University of Tokyo.

For example, the Meteorological Agency succeeded in forecasting the eruption of Mount Usu in Hokkaido, which erupted March 31, 2000.

The major eruption damaged houses and roads in nearby towns, but all residents were safe because local governments had issued an evacuation order after receiving an alert from the agency two days before.

A hot spring resort at the foot of the volcano was closed for several months after the eruption and it took more than a year before the evacuation order was entirely lifted.

The volcano erupted four times during the last century. The stickiness of Mount Usu’s magma causes earthquakes ahead of each eruption, making forecasts easier, Fujii said.

Yamasato of the Meteorological Agency said success in preventing human casualties from the 2000 eruption of Mount Usu was partly attributable to a lesson learned from a tragedy at Mount Unzen, Nagasaki Prefecture, which erupted in 1991.

Forty-three people, including reporters who entered the area despite an evacuation order and firefighters who were trying to guard the reporters, were killed by a sudden pyroclastic flow from the volcano.

When Mount Usu erupted four years ago, nearby governments banned people from entering off-limit areas, he said. A law on natural disasters was revised in 1995 to penalize anyone who entered an off-limit area following an evacuation order.

Hazard maps showing areas that are dangerous during eruptions were also distributed to residents living near Mount Usu. This helped smooth the evacuation, Yamasato said.

Fujii of the University of Tokyo said Mount Mihara on Izu Oshima Island, about 120 km southwest of Tokyo, might erupt in the near future.

The volcano erupts about every two decades. It last erupted in 1986.

Mount Fuji might also erupt, he said.

The nation’s highest mountain last erupted in 1707, destroying most of the houses in a nearby village. Records show volcanic ash from Mount Fuji reached the area that is now Tokyo.

Before 1707, Fuji had erupted every several decades or 100 years, he said.

“It’s now nearly 300 years since the last eruption,” Fujii said. “It’s too long and the next eruption could happen anytime soon.”

He said an eruption of Mount Fuji would likely be preceded by an increase in the number of volcanic quakes.

The Meteorological Agency said many low-frequency quakes were detected beneath the mountain in 2000 and 2001. The number of such quakes has since decreased.

Major postwar volcanic eruptions

Following is a chronology of major volcanic eruptions in Japan during the postwar period;

Aug. 14, 1947: Eleven people killed by falling rocks in an eruption of Mount Asama, which straddles Gunma and Nagano prefectures.

Sept. 24, 1952: Thirty-one killed by an eruption of an underwater volcano near Aogashima off Tokyo.

June 24, 1958: Twelve killed by falling rocks in an eruption of Mount Aso, Kumamoto Prefecture.

June 29, 1962: Five killed and 11 injured by an eruption of Mount Tokachi, Hokkaido.

Oct. 24, 1978: Three killed by mud flows following an eruption of Mount Usu, Hokkaido.

Oct. 3, 1983: Some 400 buildings damaged by an eruption of Mount Oyama on Miyake Island in the Izu chain.

Nov. 21, 1986: All residents of Izu Oshima Island forced to evacuate by an eruption of Mount Mihara.

June 3, 1991: Forty-three killed by pyroclastic flow during an eruption of Mount Unzen, Nagasaki Prefecture.

March 31, 2000: A total 15,267 people forced to evacuate by an eruption of Mount Usu, Hokkaido.

Sept. 4, 2000: All residents of Miyake Island forced to evacuate in an eruption of Mount Oyama.

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