updated 4/28/2006 3:50:47 PM ET 2006-04-28T19:50:47

More than 200,000 people perished when a monstrous wave swept the Indian Ocean in 2004. In hopes of avoiding a similar disaster here, a tsunami warning system has now been expanded to both coasts of the United States.

“We take building this warning system very seriously,” Conrad C. Lautenbacher, head of the National Oceanic and Atmospheric Administration, said in an interview.

DART Deep-ocean sensors in the Pacific, Atlantic and Caribbean now listen for earthquakes on the seafloor, sense the pressure of waves passing over them, and radio their findings to scientists at warning centers in Alaska and Hawaii.

Unlike wind-driven surface waves, tsunamis are caused by seismic activity such as undersea earthquakes, landslides or volcanoes.

That means tsunamis are deep, reaching all the way to the seafloor, so that when they reach land they are forced upward into often towering walls of water that can inundate coastal communities.

Occurring relatively rarely, they drew little public attention until the tsunami that originated on the Indonesian coast in December of 2004.

Shaken by that devastation, governments and scientists redoubled their efforts to prepare coastal populations for this hazard and to find ways to issue warnings.

Deadly tsunamis have struck Hawaii and the waves have caused serious damage on the West Coast also, so government scientists have been wary of the waves in the Pacific for years.

A 1946, a Pacific-wide tsunami destroyed the U.S. Coast Guard’s Scotch Cap lighthouse at Unimak, Alaska, killing all five of its occupants. That wave reached the Hawaiian Islands about five hours later, obliterating Hilo’s waterfront and killing 159 people.

And a 1964 quake in Alaska generated waves that caused damage in southeastern Alaska, in Vancouver Island, British Columbia, and in the states of Washington, California and Hawaii. More than 120 died. Hardest hit was Crescent City, Calif., where waves reaching as much as 20 feet destroyed half of the waterfront business district. Eleven people lost their lives there. There was extensive damage in San Francisco Bay.

On the Atlantic side, people have been killed in the last 150 years by these waves striking the Virgin Islands, Panama, Jamaica, Puerto Rico, the Dominican Republic and even Canada.

Lautenbacher’s agency has now expanded its observations in the Pacific and added coverage of the Atlantic Coast, Gulf of Mexico, Puerto Rico, Virgin Islands and eastern Canada, he said.

The administration, which helps communities develop plans to deal with hurricanes, tornadoes and other emergencies, expanded its tsunami planning and now has certified 29 coastal communities as TsunamiReady. That means they have conducted public education programs, established evacuation routes, and set up means to issue warnings

There had been six deep-ocean tsunami sensors in the Pacific previously. Now there are 10 in that ocean and five in the Atlantic and Caribbean, with plans to have a total of 39 in service within two years.

The DART sensors combine a unit on the seafloor that detects seismic activity and measures water pressure with a floating buoy overhead. When the submerged unit feels seafloor shaking or senses changes in pressure overhead that indicate a significant wave it tells the floating unit to radio the information via satellite to the warning centers.

Those centers are now staffed full time and have new computer models that use the information to estimate the danger to a variety of coastal areas.

The center in Ewa Beach, Hawaii issues warnings for Hawaii and foreign countries, while the one in Palmer, Alaska, sends warnings to the continental United States.

Lautenbacher explained that the warnings go to National Weather Service offices and are disseminated through its warning systems including NOAA Weather Radio, weather and other emergency communications systems and direct connections to state and local emergency response officials.

In addition to expanding its network of DART deep-ocean sensors the agency has installed nine of a planned 16 new sea-level stations that track sea level and has upgraded 20 of the 33 stations in use.

And researchers have improved their database of nearly 2,000 tsunamis that have occurred worldwide in the past, extending as far back as 2000 B.C. This information helps identify areas particularly at risk.

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