North Coast at Greater Quake Risk Than Once Thought : Science: Researchers say a temblor of 7.6 or higher hit around Eureka 300 years ago. A similar jolt could strike again.

SAN FRANCISCO â€”  New evidence indicates that the coast of California from Eureka north is more vulnerable to great earthquakes than had been thought.

The evidence shows an earthquake that ripped along the coast around 300 years ago was “probably larger� than 7.6 magnitude and could have been as great as 8.4, according to Samuel H. Clarke of the U.S. Geological Survey. This means the region could again be hit by a quake of similar size.

Such an earthquake could also generate a tsunami, and waves nearly 100 feet tall could wash ashore and destroy communities that have had no reason to prepare for such a cataclysmic event, Clarke said this week at a joint meeting of the Geological Society of America and the Seismological Society of America.

In recent years, scientists have grown increasingly concerned over the earthquake potential of the Cascadia subduction zone, where the Gorda plate is being pushed under the North American plate along the coast of the Pacific Northwest. The new evidence extends that concern farther south.

The subduction zone has always been known as a seismically active region because of the moderate earthquakes that have occurred there and the volcanoes that dot the area, but only recently have scientists determined that the region is capable of producing great earthquakes.

“If you want to pick a (geological) structure that may generate the largest earthquake in the lower 48 states, this is it,� said Gary A. Carver, a geology professor at Humboldt State University in Arcata. Carver and Clarke have spent the last few years digging trenches and studying the evidence of an earthquake that struck just south of Eureka about 300 years ago.

Carver said experts in California have been obsessed with the San Andreas Fault, yet the Cascadia subduction zone is twice the length of the San Andreas and is now believed capable of generating larger quakes than the notorious fault. People are better prepared for a great quake on the San Andreas than they are for one that could devastate wide areas of the Pacific Northwest, he said.

The evidence is difficult to find because much of it is under water or overgrown with the region’s lush vegetation, but the two scientists said they believe the Eureka area has been hit by five major quakes in the last 2,000 years. The record gives no clear indication of when the next great quake is likely to strike, but if it is anything like the one that hit 300 years ago it could prove catastrophic, they said.

Much of the record for past earthquakes along the Cascadia zone has been compiled by the U.S. Geological Survey’s Brian Atwater. He has found extensive evidence along the Oregon and Washington coast showing that huge areas have repeatedly dropped and then been uplifted by what he believes to have been great earthquakes.

That, in turn, has resulted in some coastal areas being flooded. The best evidence of that, Atwater maintains, is trees that were killed when their roots suddenly became flooded with salt water.

Farther down the coast, Clarke and Carver found similar evidence in Northern California. Digging through the muck at low tide, they have found the stumps of huge trees that were killed about 300 years ago when the land on which they were sitting suddenly dropped below sea level. The amount of displacement of the earth, about 10 feet, told them it was a major quake of at least 7.6 magnitude.

Carver and his wife, Deborah, have also paid attention to oral histories handed down from generation to generation by Indians who lived in the region. One, by a Yurok Indian known only as Ann of Espu, was recorded by a historian around the turn of the century.

“These stories were told before whites arrived,� Carver said, and he finds the Yurok tale particularly intriguing because she talks of how the land along the coast suddenly dropped. The story is called “How Prairie Becomes Ocean.�

Carver said oral histories are not considered reliable evidence, but he finds this particular tale fascinating.

Scientists are studying the growth rings of the dead trees in hopes of resolving a raging debate over just how much of the subduction zone is likely to break at any one time. The amount of rupture determines the size of the quake, and the scientists hope that tree rings will tell them the dates of past quakes precisely enough to determine the exact length of past ruptures.

Scientists have argued among themselves over whether large areas of the subduction zone have ruptured in giant earthquakes in the past, or if the zone tends to break in shorter segments in smaller quakes.

But Carver and Clarke have no doubt that the quake they are studying near Eureka was a big one.

They are convinced it was at least 7.6 magnitude, and that is “less probable than a magnitude 8.4 or larger,� Clarke told scientists attending the meeting.