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Tuesday, April 20, 2010

FYI

Tsunami

Japan's vulnerability to tsunami

Preparedness is vital because oceanic terror can strike with very little warning


Staff writer

Rollers from the giant earthquake in Chile in February and the catastrophic Indian Ocean tsunami in 2004 are still fresh in the world's memories, but in Japan giant tidal waves have never been far from thought.

News photo
Cleanup time: Workers retrieve plastic containers from a fish market in Kesennuma, Miyagi Prefecture, on Feb. 28 following a tsunami caused by a massive earthquake in Chile. KYODO PHOTO

The big tidal waves have killed so many people here that the Japanese word for it is known around the world.

Here are some basic questions and answers on tsunami and how they are regarded in Japan:

What are tsunami?

When a major earthquake occurs under the ocean, it often causes the seabed to rise or sink. The subsequent vertical displacement of water shows up on the ocean's surface, sending big waves in every direction. These are called tsunami, or tidal waves.

Tsunami propagate faster in deep water. According to the book "Shitte Sonaeyo Jishin to Tsunami Namazu Hakasega Oshieru Shikumito Kowasa" ("Know and Be Prepared for Earthquakes and Tsunami — the Mechanisms and Horrors Taught by Doctor Catfish"), written with expert Yoshinobu Tsuji, tsunami can move as fast as 720 kph — the same as a jetliner — in depths of around 4,000 meters, which is the average depth of the Pacific Ocean.

Even at just 10 meters, tsunami can travel at 36 kph, more than fast enough to outrun the typical cyclist.

Tsunami can even hit from the opposite side of the Earth. At 4:11 a.m. on May 23, 1960, for example, an earthquake off Chile spawned tsunami that hit the Pacific coast of Japan at around 2:20 a.m. the following day. Heights reached 5 to 6 meters in some areas, Tsuji's book says.

The more shallow the water, the higher the tsunami — especially at the end of V-shaped and cross-shaped bays and at promontories or shallow beaches. Even inland areas are at risk, because the waves can travel up rivers and through sewage pipes, the book says.

Because tsunami usually hit in series, the first wave is not necessarily the largest. "Know and Be Prepared for Earthquakes and Tsunami" warns that geographical features of the sea can often play a key role. In November 2006, an earthquake near the Kuril Islands caused tsunami to hit undersea mountains and rebound to Japan.

What kind of damage have tidal waves inflicted on Japan?

On July 12, 1993, a tsunami was spawned by an earthquake off Hokkaido. The wave hit nearby Okushiri Island at an estimated height of 15 meters, sending seawater rushing through narrow valleys that raised its height to 30.6 meters, Tsuji's book says.

The tsunami ravaged part of the island in just three to five minutes, washing away several houses and killing more than 200 people.

On June 15, 1896, the Meiji Sanriku Tsunami rung up a death toll of more than 22,000 even though the earthquake registered only 2 or 3 on the Japanese seismic intensity scale, which maxes out at 7. Around 30 minutes afterward, the tsunami attacked the Sanriku coast, killing people in various parts of eastern Japan, the book says.

On May 26, 1983, an earthquake in the Sea of Japan sent waves more than 10 meters high rolling into parts of Akita and Aomori prefectures, killing 13 elementary school students, among others.

What kind of alerts does the Meteorological Agency issue?

The agency says that as soon as an earthquake strikes, it tries to determine whether a tsunami has been generated. If a disastrous wave is expected, a warning or advisory is issued to the endangered regions. If a minor tsunami is predicted, it merely issues a forecast.

The warnings are divided into three levels: tsunami advisory, tsunami warning and major tsunami warning, based on estimated height. The agency also issues information on height and estimated arrival time.

The agency says it tries to get a warning or advisory out within three minutes of a earthquake but can theoretically do it in one or two minutes.

What should you do when you are near the shore when a tsunami alert is issued or you observe some abnormality on the ocean's surface after an earthquake?

Get out of there. Experts warn that you should immediately evacuate to high ground, or the upper floor of a tall building, whenever you detect an earthquake in a coastal area.

Following an earthquake off the Kuril Islands in November 2006, according to Tsuji's book, it was discovered that more than half of the area's parents rejected their children's suggestions to flee even though an evacuation order had been issued. Even a wave less than a meter in height can end up killing someone, the book warns.

Tsunami run at breakneck speed, according to Fumio Yamashita, a relative of tidal wave victims who has written a book on the modern history of Japan's tsunami.

"Be aware that you will win if you swiftly stand up and flee at full speed," Yamashita says.

Most importantly, evacuees should forget about their belongings because "there is nothing more important than your life in this world," Yamashita stresses.

Tsuji's book urges people to get accurate information on a tidal wave situation by radio or TV so they will not be influenced by false rumors, and they should follow the evacuation signs in coastal towns.

Yamashita urges people to prepare for the threat by holding evacuation drills.

How does Japan detect tsunami?

Observation devices are used to monitor changes in sea level around the country. Some use supersonic waves while others are based on the global positioning system. According to the Meteorological Agency, there were 173 tsunami observation points as of end of March.

The agency says it has conducted about 100,000 different tsunami simulations using based on varying epicenters and magnitudes and stores them in its database. When a quake occurs, the agency checks its database for correlations with the actual location and magnitude and issues warnings and advisories.

Which areas of Japan are most prone to tsunami?

Experts suggest anywhere is prone to a tsunami, but the Pacific coast is seen at higher risk because major earthquakes are expected along a path stretching from Kochi to Shizuoka prefectures.

The agency says major earthquakes in the area tend to repeat from every 100 to 150 years. Shizuoka Prefecture in particular hasn't experienced an magnitude 8 earthquake for more than 150 years and is seen as overdue.

Are there any advanced countermeasures being used against tsunami?

In 1966, the Taro district of Miyako, Iwate Prefecture, erected 2,433 meters of coastal levees that rise about 10 meters above the ocean. The sheer scale of the levees prompted some to dub them the "Great Walls of Taro."

The area has a history of tidal wave problems. In addition to the Meiji Sanriku Tsunami in 1896, which killed 1,859 villagers among the nationwide death toll of more than 22,000 people, the Showa Sanriku Tsunami in the same area on March 3, 1933, left 911 people dead.

The area is also fully equipped with a disaster prevention radio system to inform residents of tsunami.

The town of Taiki, Mie Prefecture, also has an early tsunami warning system. If a strong earthquake lasts more than 20 seconds, town officials said they will unconditionally sound a siren and issue an evacuation recommendation. They will not even wait for the Meteorological Agency.

During the Tonankai Earthquake in 1944, a tidal wave hit the area just 27 minutes after, killing 64 people. Officials said that precious evacuation time can be lost waiting for a tsunami warning from the Meteorological Agency.

Is there any place where you can experience a simulated tsunami?

The Karakuwa Hanto Visitor Center in Kesennuma, Miyagi Prefecture, said it has a facility where you can see what a tsunami is like, including sounds, images, vibration and wind.

In Shizuoka, the prefectural earthquake preparedness education center allows you to experience a virtual tsunami within a big, dome-shaped screen and water tank.

The Weekly FYI appears Tuesdays (Wednesday in some areas). Readers are encouraged to send ideas, questions and opinions to National News Desk


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