CALCULATING CALIFORNIA'S TSUNAMI RISK

The threat that Pacific Ocean tsunamis pose to California's coastline just got its first comprehensive study, a milestone in understanding and preparing for the rare, but devastating, waves.

Following the devastating 2004 tsunami in southeast Asia, scientists scrambled to provide county and municipal emergency planners with an estimate of how vulnerable their coastlines were to tsunamis.

Some 350,000 people live in the path of a worst-case scenario, though overall, the risk to life and property is less than previously thought, state officials reported.

"We used one elevation for the whole state. We just drew a line at 30 feet up from sea level," Rick Wilson of the California Geological Survey said.

With the new effort, Wilson and a team of researchers developed detailed maps of what parts of the coast would flood during a range of disaster scenarios.

Overall they found that most surges wouldn't come close to the 30-foot line they had previously assumed.

"Even if we don't show higher inundation, this really helps emergency planners," Wilson said.

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This is particularly true on the outskirts of San Diego, where the city of Coronado sits on a low-lying peninsula. Old tsunami maps predicted the entire town would be submerged, and local officials feared that they would be forced into a desperate attempt to airlift 24,000 people out of harm's way.

New maps predict the worst waves would reach elevations of 15 to 18 feet.

Large earthquakes can generate tsunamis just about anywhere along the Pacific "Ring of Fire," a 40,000-kilometer-long chain of tectonic unrest that includes Japan, Kamchatka, the Alaska's Aleutian Islands, the Andes in South America and California.

The team considered threats from all of these regions, and found that a magnitude 9.0 or greater earthquake in the eastern Aleutians would be the most dangerous for California.

In particular, Santa Cruz could see waves of 20 to 25 feet. Even the interior of the San Francisco Bay would not be spared, with 10-12 foot waves reaching Alameda, and eight- to 10-foot surges spilling into the towns of Sausalito and San Rafael.

"Any emergency responder can look at these maps and see what it means for them and their neighbors." Dale Cox of the United States Geological Survey in Sacramento said. "It's intellectually accessible to the public, too. If people are asking 'gee, where do I go?' these maps will tell you. People can look at these and easily tell what's in it for them."

Source: Discovery News

Facts about Tsunami

A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.

These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

Most tsunamis, about 80 percent, happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.

Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.

A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.

A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.

Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly surging tide that inundates coastal areas.

The best defense against any tsunami is early warning that allows people to seek higher ground. The Pacific Tsunami Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a web of seismic equipment and water level gauges to identify tsunamis at sea. Similar systems are proposed to protect coastal areas worldwide.

Source: National Geographic