The huge Jan. 26, 1700, Pacific temblor was comparable to the biggest quakes of the 20th century, geologists told the annual conference of the Geological Society of America on Tuesday.
"The 1700 earthquake tells us about our present earthquake hazards and we use the past as a guide to the future," U.S. Geological Survey geologist Brian Atwater told The Associated Press.
Scientists didn't know about the quake off the Washington coast until 20 years ago, when a newly authenticated record of a fatal shipwreck in Japan added a major clue.
"I'm gratified by the convergence of the information from a variety of different sources, from historical Japanese rice-warehouse records to tree ring data to the dates that trees are killed along the coast by drowning in salt water," said Ray Wells of the USGS office in Menlo Park, Calif.
Written records collected from villages on the Japanese island of Honshu show the coast was hit by a series of waves Jan. 28, 1700. Because no Japanese earthquake had occurred to generate the waves, it's likely they came from somewhere else, Atwater said.
In a village that now is part of the city of Miyako, 300 miles northeast of Tokyo, the tsunami is believed to have crested at about 10 feet. That suggests a quake with a magnitude in the range of 8.7 to 9.2, he said - probably about 9.0.
Atwater, an affiliate professor of earth and space sciences at the University of Washington, presented evidence of the Japanese shipwreck.
The vessel carried 470 bales of rice, nearly 30 tons in all, bound for Edo, now Tokyo. By Jan. 26, 1700, it had sailed about 100 miles down the Japanese coast and was about to enter a river when the tsunami hit.
It lasted 18 hours. By the time the waves subsided, a large storm drove the ship into coastal rocks, killing two crew members and destroying the cargo, Atwater said.
Wells told reporters the fault at issue in that quake, the Cascadia Subduction Zone, appears to be closer to the coasts of Washington state and Oregon than the 50 to 100 kilometers previously believed.
"It's within a few tens of kilometers of the shoreline," Wells told The AP. "It varies along the coast. It's a little closer in north-westernmost Washington, beneath the unpopulated coast of the Olympic Park, and it's just offshore off southwest Washington and Oregon."
Awareness of that long-ago quake has prompted revision of building codes and emergency plans in the Northwest.
Two other 20th century quakes of comparable scale were the 9.0 earthquake on Russia's Kamchatka peninsula in 1952, and the 9.5 Chilean quake in 1960.
By Jim Cour