Waves in the night: Clues to a quake.

Are some of the world's greatest earthquakes lurking in the future for the Pacific Northwest? Geologic evidence tells scientists that large tremors have struck the region repeatedly, most recently about 300 years ago. But the lack of written records before 1800 makes it difficult to judge whether such shocks measured magnitude 8 or magnitude 9 in size. That's a profound difference, since the larger quake packs 30 times the energy of the smaller and would shake the entire region from northern California to southern British Columbia.

Japanese seismologists can help resolve the question, they claimed last month at a meeting of the Seismological Society of America in El Paso, Texas. Kenji Satake of the University of Michigan in Ann Arbor and his colleagues at the University of Tokyo reasoned that great quakes on the west coast of North America would have spawned large tsunamis that crossed the Pacific. Any mention of damage from such waves in Japan's long historical records could then shed some light on the timing and magnitude of North American quakes.

The researchers found that a tsunami did indeed buffet Japan's coast in A.D. 1700, about the time geologists give for the last Pacific Northwest quake. Satake and his colleagues like the match. Although earthquakes in many parts of the Pacific can trigger tsunamis that reach Japan, the scientists ruled out these other sites. Written records in South America and Kamchatka do not mention any great jolts in 1700. And although Alaska lacks records that far back, geologic evidence in the region shows no sign of a giant tremor in that year.

According to Satake, the timing of the tsunami also points to an origin in the Pacific Northwest. The wave hit Japan between midnight and dawn on Jan. 28, which means it would have started its 10-hour journey across the ocean after 9:00 p.m. in the Pacific Northwest. That fits with local tales that describe a great shaking in the middle of a winter night.

From the size of the tsunami, the quake must have been a whopper. Satake calculates that a tremor along the Pacific Northwest coast would have had to reach magnitude 9 to produce the 2-meter- to 3-meter-high waves that pounded Japanese shores. A magnitude 8 quake would generate a tsunami there only about 40 centimeters in height.

The new report has intrigued researchers because it offers information otherwise unavailable about ancient quakes. But they wonder how well Satake and his colleagues can rule out other parts of the Pacific as the source for the tsunami of 1700. If the Japanese scientists are right, then residents across the Pacific Northwest have to face the prospect of a megaquake sometime in their future.