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Tuesday, June 14, 2011

FYI

TOKAI EARTHQUAKE

Tokai Big One still tops in speculation


Staff writer

Seismologists have warned of the likelihood of a Tokai region earthquake for years.

News photo
Dry run: Residents of Omaezaki in Shizuoka Prefecture participate in a combined earthquake and tsunami drill on May 21 to prepare for the widely expected Tokai earthquake. KYODO PHOTO

The chances of the quake, whose epicenter, they speculate, could be in or off the coast of Shizuoka Prefecture, are so high that Prime Minister Naoto Kan last month ordered the shutdown of the Hamaoka nuclear plant, which sits in one of the projected epicenter zones.

However, the penchant for Japanese seismologists to focus so much on a potential temblor when there effectively is nowhere in the country free of earthquake risk seems hard to fathom.

Following are questions and answers about the Tokai quake scenario.

What is the likelihood of a major Tokai temblor?

According to the Headquarters for Earthquake Research Promotion, a governmental body under the Education, Culture, Sports, Science and Technology Ministry, the chance of an 8-magnitude Tokai quake in the next 30 years as of Jan. 1 was 87 percent, the figure Kan used for ordering the Hamaoka power plant to halt its reactors.

Is the figure higher than other locations?

The probability is much higher, but there are other places with higher probability.

For example, the chance of a magnitude 7.1 quake is about 90 percent off Shikotan and Etorofu, two of the disputed, Russian-held islands off Hokkaido, over the next 30 years. And the chance of a magnitude 7.1 quake is about 80 percent off Tokachi and Nemuro, also in Hokkaido.

Is the official estimate reliable?

That's hard to tell. For example, there was only a 20 percent chance forecast of a quake occurring off the Pacific coast over the next 30 years from Jan. 1. But farther north from Tokai was where the historic, midafternoon magnitude-9 Great East Japan Earthquake hit on March 11, leaving some 15,300 dead and 8,300 missing. Most were claimed by tsunami.

Shortly before the 7.3-magnitude Great Hanshin Earthquake hit in January 1995, resulting in the loss of 6,434 lives, the 30-year chance of a quake there was between 0.02 and 8 percent.

"Predicting an earthquake is impossible, but we came up with those probabilities because we must try as best as possible to alert the public to the quake threat," said Naoki Satake, an official of the headquarters. "We should assume quakes can occur anywhere."

Why is so much attention focused on the Tokai quake threat?

Seismologists have for decades offered several reasons for their sense of alarm.

One is that the projected epicenters sit on the boundary between the Eurasia plate and the Philippine Sea plate, where the latter is descending toward to the mantle, forcing the Eurasia plate down with it and the energy for it to spring back upward at any time.

Another reason is because an 8-magnitude temblor could hit below a heavily populated area, which would mean casualties and physical damage would be greater than elsewhere, said Mikio Satomura, a seismologist at Shizuoka University.

Also, because a Tokai quake would probably be plate-caused as opposed to fault-caused, it would be expected to create tsunami. And because the epicenter would be close to the coast, the waves would hit quickly, which means having a prompt warning system is crucial, he said.

Experts also note that Tokai quake predictions are likely to be more accurate than other quake predictions on a scale that narrows down the potential time line to days or weeks, instead of years or decades.

If the Tokai quake hits under land, it will make it easier to obtain soil samples and other measurements that can be used to study tectonic movements and other signs of an impending megaquake, Satomura said.

Also, there are more historical data in Tokai — including documents and geological evidence of past quakes — than in other areas, which aids prediction.

When was the last huge Tokai earthquake?

In 1854, the Ansei Tokai Earthquake hit what was then known as the Ansei region.

Tokai quakes have occurred in intervals of 100 to 150 years since 1498, according to Shizuoka Prefecture's Earthquakes Disaster Measures Center.

Seismologists , however, see a cycle of 100 to 200 years dating back to 684.

A key point in the debate is that Tokai quakes often seem to coincide somewhat with temblors in Tonankai, in present-day southern Aichi and Mie prefectures, and off those prefectures, and in Nankai, or southern Wakayama and Kochi prefectures, just offshore.

1944 experienced a Tonankai quake and Nankai did in 1946.

The two-year gap is short enough for the quakes to be deemed sequential.

The fact that there were no Tokai quakes at those times "is one of reasons we believe a Tokai quake will come next," Satomura said, adding it is possible a Tokai temblor could trigger Tonankai and Nankai quakes, multiplying the damage.

Is there a Tokai quake damage scenario?

According to a Cabinet Office study, the death toll from a Tokai megatemblor would range from 7,900 to 9,200 under the premise that the first main quake wiill hit at 5 a.m. and 6,700 will be killed from tsunami.

Tsunami are expected to kill 400 to 1,400, depending on how well coastal residents are prepared. The rest will be killed by landslides and fires.

However, the death toll could be reduced to around 2,300 if the public is warned in advance, the Cabinet Office study speculated.

But no one knows if such a warning would come within minutes, hours or months prior to the Big One.

The same study also estimates damage from Tonankai or Nankai quakes under the assumption that the initial shock occurs at 5 a.m., roughly the same time the Hanshin quake occurred in 1995.

An early-morning megaquake scenario paints a greater casualty toll than a temblor hitting in the afternoon or evening because most people would be in bed and most deaths would be due to collapsed homes.

Under this scenario, about 12,000 to 18,000 would be killed, plus 3,300 to 8,600 claimed by tsunami, depending on how prepared local residents are.

The Weekly FYI appears Tuesdays. Readers are encouraged to send ideas, questions and opinions to hodobu@japantimes.co.jp


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