North Coast Plans for <i> Other</i> Big One : Disasters: Geologists draw up scenario for deadly tsunamis that could hit now or in 300 years. Residents are urged to plan for quick move to higher ground.

It is a daunting challenge for disaster planners, a catastrophe that could strike tomorrow, or not for 300 years or longer--and one that would happen so quickly, there probably would not be adequate warning for those most exposed to losing their lives.

In the first scenario state geologists have drawn up for a tsunami, a towering wall of water often mistakenly called a tidal wave, Humboldt and Del Norte counties on California’s North Coast would be struck by a two-fold blow.

The geologists calculated the effects of a magnitude 8.4 earthquake centered offshore in the Cascadia Subduction Zone, where three great continental plates--the Pacific, the Gorda and the North American--come together.

Within 20 minutes, according to the scenario, the first of a series of quake-triggered waves as high as 30 feet would strike the North Coast, sweeping over the Samoa Peninsula, where 2,000 people now live, across Humboldt Bay to the edge of downtown Eureka.

Farther north along the coast, the run-up of the waves would go several blocks deeper into downtown Crescent City than did the tsunami of 1964, generated by the Alaska earthquake. The Crescent City tsunami killed 10 people.

Japanese tsunami records indicate that such a gigantic quake may well have occurred off North America on Jan. 25, 1700, when few people were living on the North Coast. Studies of paleoseismographic evidence indicate that such events recur every 200 to 600 years.

“We are aware of the general characteristics of the danger,” said California State Geologist James F. Davis. He said the probability at any given time is not sufficiently high to inspire authorities to institute a formal warning system--such as the sirens used in Japan.

But, he added, the purpose of the scenario is to encourage people who live in the threatened areas to make plans for moving quickly to higher ground, and for local agencies to consider how they would try to cope with such a disaster. According to the scenario, such a quake would also rupture many highways and isolate some parts of the North Coast for days.

On the long, narrow Samoa Peninsula there would not be enough time between the shaking and the arrival of the first wave to leave the area. There might not even be enough time for broadcasts to tell residents where the epicenter of the shaking was, or whether it was at sea, where tsunamis are generated.

So residents would have to act, without knowing the dimensions of the threat. A Pacific-wide tsunami warning system is set up to give notice of seismic waves caused by big earthquakes or landslides in Alaska or other distant points, when there would be hours to get ready. But there is no such warning system for a locally generated tsunami.

Edward N. Bernard, a West Coast official of the National Oceanic and Atmospheric Administration, who helped provide the details of the scenario, said there are 50-foot-high sand dunes and forested portions of the Samoa Peninsula that are high enough above sea level to provide some protection against the projected waves.

For longer-term planning purposes, Bernard suggested that Northern Californians might follow the example of the Japanese by planting “tsunami forests” that serve as barriers to the giant waves. “The trees dampen the energy of the waves,” he said.

“Every year, the trees get bigger and thicker and pose a greater barrier. This is much more effective than a seawall.”

Since the scenario has been released, there have been two community meetings on the peninsula at which some of the lessons have been discussed by residents.

One of the most important facts to absorb, scientists say, is that the first wave may not be the highest or most destructive and that waves may recur for hours. So people who reach a haven must stay there until authorities give an all-clear sign. In Crescent City, most of the casualties in 1964 occurred because of a later wave, when residents had returned to the port area.

John Woolley, chairman of a community services district, said communities must establish trails leading to the highest dunes, and educate residents on where they are. He said it is also important to pass the knowledge of what to do from one generation to the next.

“In the meantime, I’m taking steps to teach my family what to do,” he said. “If it happens tomorrow, we know where to go.”

Linda Nellist, the state Office of Emergency Services coordinator in Humboldt County, said she considers the scenario a worst-case event, but said there will probably be lesser, but still serious, events before it happens.

“People will finally realize these things are possible, but this may not be the first major event they feel,” she said. “They need to prepare for the lesser events too. . . . We already know that we have earthquakes here regularly that don’t result in huge waves. If you want to be a survivor, then a personal plan is essential.”

The main lesson is clear, Nellist said. “When there’s a long period of vigorous ground shaking, move to high ground as soon as possible. Each individual will have to ask the question: Where is the safest place? The top of a structure? A sand dune? A high parking lot? There are many calculations to be done.”

Lori Dengler, a seismologist who teaches at Humboldt State University in nearby Arcata, noted that although powerful earthquakes are frequent in the area, the loss of life has been minimal. Not a single life was lost in the 1992 quakes of magnitude 7.1, 6.5 and 6.2, although 356 people were injured.

“In the scenario, the shaking damage is not that great,” Dengler said. “The greatest danger we face is the potential loss of life from the tsunamis, rare as they are.”

Tsunami Threat

State geologists have prepared a scenario for a magnitude 8.4 earthquake off California’s North Coast, which could be followed by tsunami with a series of 30-foot waves.

Where: The tsunami would pass over the Samoa Peninsula, where 2,000 people live, into Humboldt Bay.

When: Such a scenario is believed likely between now and the year 2300.

Source: Dept. of Conservation, Division of Mines and Geology