Mount Fuji Overdue for Eruption, Experts Warn

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Mount Fuji looms just 70 miles (112 kilometers) from Tokyo, a metropolitan region home to nearly 30 million people (map of Japan). Many millions more live much closer.

Three years ago a government report estimated that ash, lava, and smoke resulting from a large Fuji eruption could cause the equivalent of 21 billion U.S. dollars in damages.

It's therefore no wonder that Fuji is arguably the world's most closely watched volcano.

The mountain is wired to the hilt with dense global positioning system arrays and seismometers. It is scrutinized with state-of-the-art laser mapping technology and is bombarded with constant investigations.

Although the flurry of low-frequency earthquakes has since calmed down, Fuji is hardly safer now than it was a few years ago, says Chris Newhall of the U.S. Geological Survey Volcano Disaster Assistance Program.

Deep, long-period earthquakes "are thought to represent a supply of basaltic magma from depth into the roots of the volcanic system," Newhall said.

In other words, the quakes are signs that magma is building up within the mountain's bowels.

"A volcano can absorb quite a few of these [buildups] without erupting. But each one adds a little more heat and gas," he said.

"It's a bit like torquing a ratcheted spring: A little bit now, a little bit later, and eventually it's cocked" and ready to blow.

Precisely when Fuji will be fully cocked, nobody knows.

Fake Quakes

Recently scientists detonated a series of explosions around Mount Fuji using 1,102-pound (500-kilogram) charges buried 263 feet (80 meters) into the ground.

Seismic waves created by these simulated earthquakes bounced back to some 400 different monitoring stations. The results will help researchers glean information about the mountain's subsurface structures.

Figuring out the terrain and the amount of magma below can increase the odds of successfully predicting when the volcano will erupt.

(Related National Geographic magazine feature: Predicting the next big quake.)

"Analyzing the seismic wave field is currently our best way of gauging what's down there," said Hiromu Okada of Hokkaido University in Sapporo.

In 2000 Okada helped predict the eruption of another Japanese volcano, Mount Usu on the northern island of Hokkaido.

Okada is quick to clarify that Fuji has not to date offered any evidence of imminent danger and that hazardous, massive-output eruptions like that of Mount St. Helens in 1980 remain a statistically rare occurrence on Earth.

What's more, the 30-year interval idea may be obsolete, he says.

The volcano's 1707 eruption was so large and destructive, it may have altered Fuji's underground structure, throwing the mountain off what was once a more regular cycle of eruption.

On the other hand, the change in interior structure could mean that Fuji is loading up with a different, more explosive type of magma than in previous eruptions.

Quiet Period

Many locals, including the farmer Issei, don't believe the volcano will erupt, but they concede that if or when a major earthquake hits, Fuji could awaken.

And like residents of San Francisco, California, people in Japan have been preparing for their next Big One, known as the great Tokai earthquake, for the past 25 years.

The Tokai segment is an especially active part of a subduction zone—where one tectonic plate is slipping underneath another—that sits just southwest of Tokyo.

Tokai last produced a major quake in 1854, and scientists believe that it is long overdue for another one.

"The expected Tokai or Nankai earthquake has a strong effect on the whole tectonic stress system for nearly all of the Japanese archipelago," Okada said.

"If some magmatic system is ready or nearly due to erupt, an earthquake could be an effective trigger."

In fact, that's probably what happened in 1707, when the Tokai area experienced a huge earthquake just two months before Fuji blew.

While scientists get increasingly nervous about the meaning of 300 years of quiet, it's that same quiet that makes residents, like local farmer Issei, complacent.

"People forget," Aramaki, the volcano expert, said. "Look what happened in 2000 with the news of seismic activity. Everyone was surprised because they thought Fuji was dormant."

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