Tsunami Alert Stations Proposed : Tidal waves: U.S. system would rapidly relay warnings to threatened areas. Seven sites are planned from Alaska to Humboldt County.

Federal officials are proposing to place the country’s first tsunami warning stations on the Pacific Ocean floor to convey within seconds the precise movement of seismic sea waves, popularly known as tidal waves, that periodically threaten the U.S. West Coast.

According to the plan prepared by West Coast scientists of the National Oceanic and Atmospheric Administration, four of the seven planned stations would be off mainland Alaska or the Aleutian Islands and three off the northwest United States.

The station farthest south would be north of Cape Mendocino off the Humboldt County coast. NOAA scientists and officials of the California Seismic Safety Commission have expressed concern that a prospective mammoth earthquake off the coast in what is known as the Cascadia Subduction Zone could send huge waves into Eureka and Crescent City.

Edward N. Bernard, director of the NOAA’s Pacific Marine Environmental Laboratory in Seattle, said a $100,000 prototype station could be at work by 1995 and the rest in place later in the decade. He estimated the yearly cost of maintaining all seven stations at $250,000.

Richard McCarthy, senior engineering geologist with the Seismic Safety Commission, pointed out, however, that relaying information quickly about threatening tsunamis is not enough by itself to afford much additional security to coastal communities.

“The public has to know what to do if they get the warning. Each city has to have inundation maps--showing likely paths of tsunamis once they hit land--and identified evacuation routes, so people can move quickly if the waves are about to strike,” McCarthy said.

Only San Mateo County, more than 200 miles south of the most endangered area in California, has so far gotten a good start at these secondary preparations, he said, although he noted that inundation maps are soon to be released for Crescent City and Eureka.

The scientists note that in case of a giant quake just off the coast, devastating waves can strike the nearest cities in just 10 or 20 minutes, so rapid warnings can sometimes be essential in saving lives, as was the case with waves generated last summer by an earthquake off Japan’s Hokkaido Island.

Studies have indicated that a mammoth quake off the Northwest U.S. coast, ranging from magnitude 8.5 to 9.2, might occur about once every 500 years.

But McCarthy and Bernard noted that California has sustained some damage from lesser tsunamis striking from Alaskan, Japanese or Chilean waters five times in the last 150 years. Locally generated tsunamis have been noted about 15 times during that period.

The most deadly tsunami to strike a California city in modern times followed the Alaskan earthquake of March 27, 1964. Twelve-foot waves that hit Crescent City several hours later killed 10 people, injured 35 and destroyed 150 buildings.

Despite this, McCarthy said tsunamis are mainly “the forgotten hazard” in California geology. “But my view is it is a hazard, and we should do what we can to educate the public to it.”

According to the plans, each tsunami warning station would be placed on the sea bottom about 2 1/2 miles under the ocean’s surface. Each would be designed to immediately send a signal to a buoy on the surface indicating the amplitude, speed and direction of the seismic sea waves.

These vital characteristics have been mostly guesswork until the waves actually struck. But with the proposed warning stations, the buoy on the surface would send a signal by way of a satellite, or perhaps through a cellular telephone hookup, direct to NOAA land stations in Alaska and Hawaii, which would relay warnings to the threatened communities.

The plan is that within seconds, cities in Alaska, western Canada, Oregon, Washington and California would be notified of the danger. Japan already has an elaborate, land-based warning system of municipal warnings that includes sirens, and radio and television broadcasts.

Another value of such a system, the scientists said, is that it would also pinpoint areas where the waves are not a danger. At present, warnings are occasionally issued after big Pacific quakes that are not followed by dangerous waves. Somewhere in the Pacific, there is usually a noticeable tsunami about three or four times a year.

A prototype similar to the one to be constructed by the NOAA is in operation by Chilean scientists off Valparaiso in South American waters.

Warning Stations

Seven tsunami warning stations are planned for the Pacific floor to convey within seconds a precise measure of the amplitude, speed and direction of seismic sea waves that periodically threaten the West Coast. Four stations would be in Alaskan and Aleutian waters, where many of the waves are generated, and three off the Northwestern United States.

HOW THE SYSTEM WILL WORK

1. An undersea station, about 2 miles below the surface, measures the characteristics of the tsunami and, through acoustic telemetry, sends a signal to a floating buoy.

2. The buoy sends the signal via satellite or cellular telephone hookup to land stations.

3. Within a minute or two, threatened communities could be told of tsunami danger, much more quickly and precisely than is now possible.

Source: National Oceanic and Atmospheric Administration