Japanese scientists studying the huge sea waves spawned by violent coastal earthquakes have concluded that a single giant quake -- one of the largest ever known -- ruptured more than 500 miles of the Pacific Coast from Northern California to Puget Sound almost exactly 300 years ago.

And vitally important for today's densely populated towns and cities of the region, a repetition of any such quake could generate immense tsunamis, 60 feet high or higher, that would threaten countless lives, drown coastal communities and devastate the entire region's economy, American seismologists say.

With intriguing precision, the Japanese researchers pinpointed the date, time and magnitude of an ancient event so powerful that some seismologists have insisted that another one like it in the same region would be highly improbable. The unique quake, said the Japanese scientists, struck with a rarely equalled magnitude of 9 at just about 9 p.m. on January 26, in the year 1700.


For more than a decade, American researchers have analyzed scattered hints of ancient coastal quakes by determining the ages of drowned tree stumps, by analyzing beach sands swept far inland and by dating sunken layers of peat and mud that all indicate something seismically big must have struck the Pacific Northwest coast roughly 300 years ago.

But they could not tell whether their evidence showed that many large quakes in succession had occurred in the past, or whether they were looking at the remains of one truly enormous one.

Now, in a unique feat of detective work, Japanese seismologists have pinned the evidence down by studying ancient records of quake- spawned tsunamis that rolled all the way across the Pacific to Japan and by simulating the behavior of the waves with sophisticated computer programs.

In a report being published today in the scientific journal Nature, the Japanese scientists described how they found in old Japanese manuscripts five detailed accounts of tsunamis three centuries ago that inundated villages and rice paddies along the Pacific coast of Honshu with waves as high as 15 feet. The surging waves hit five coastal villages simultaneously along a 500-mile stretch of the island Jan. 26, 1700, according to Kenji Satake of the Geological Survey of Japan, and his colleagues.

Satake's team then used its computers to simulate the action of typical "far field" tsunamis and to study the speeds at which they can travel across the Pacific. And because it takes just about 10 hours for such waves to reach Japan from the American west coast, the Japanese researchers were able to determine that the tsunami records they examined must have originated at 9 p.m. on that day three centuries ago.

They also examined historic records of "giant" earthquakes in Chile that have been carefully preserved for centuries, as well as other records from the quake-prone peninsula of volcanic Kamchatka, and from Russian settlements in Alaska and the Aleutians. Those three regions produce the same kind of "subduction" quakes that mark the Pacific Northwest region known as the Cascadia Subduction Zone, where massive plates of the Earth's crust thrust themselves under the North American continental plate.

Only three such giant quakes are known to have occurred anywhere in this century: in Alaska in 1964, Chile in 1960 and on Siberia's Kamchatka Peninsula in 1952.


But the Cascadia zone has not produced any earthquakes with magnitudes greater than 6 during the past 70 years, so many scientists have insisted that because the region is relatively inactive and because sediments from the Columbia River must constantly "lubricate" the downward-thrusting oceanic plates so they never build up significant strain, a true giant earthquake of magnitude 9 there is highly unlikely in the future.

Now, the evidence from the Japanese researchers is strong enough to call for serious studies of how best to prepare for another giant earthquake in the Cascadia region and for the tsunamis that would inevitably ensue, say seismologists Hiroo Kanamori and Thomas Heaton of Caltech in a commentary to be published in Nature today with the report from Satake and his colleagues.

Despite the opinions of some other seismologists, the possibility of another really "giant" quake that could rupture the ground along a line from Eureka to Seattle and Vancouver can no longer be ignored, they say. It would shake the Earth's surface for hundreds of miles around and generate tsunamis higher than 60 feet all along the region's coast with "devastating effects," the California seismologists say.