Around midnight on January 27, 1700, Japan’s Pacific coast had an unwelcome visitor. The wintry seas suddenly rose seven feet and more and stayed that way for many hours, washing through houses and flooding rice paddies up and down the coast. The ocean’s long heave was a tsunami--a wave triggered by a seafloor earthquake--but it was modest compared with others in the Japanese records, and the quake itself wasn’t felt in Japan. This past year Kenji Satake of the Geological Survey of Japan reported that he’d figured out why: the Japanese tsunami of 1700, he said, was but a faint signal from a giant earthquake that occurred ten hours earlier and 5,000 miles away in the Pacific Northwest. The farflung wave implies that the quake was one of the largest ever--a magnitude 9 event that must have rocked the coast all the way from northern California to Vancouver Island.
The quake, centered on a fault just offshore called the Cascadia Subduction Zone--where Pacific seafloor dives under the continent-- generated a much larger tsunami along the North American coast. In marshes as much as half a mile inland, geologists Jody Bourgeois and Mary Ann Reinhart of the University of Washington had traced a thin, unbroken sheet of sand the wave left behind. They calculated that it must have been at least 30 feet high at the coast.
That told them and Satake, who was working with Bourgeois at the time, that the quake itself must have been large. But it didn’t tell them how large; if the quake struck at high tide, it could have been no more than magnitude 8. It was then that Satake decided to try to resolve the issue by looking farther afield--that is, toward home. I was just curious how big the tsunami would have been in Japan, he recalls.
A magnitude 8 quake, his computer model told him, would have raised the sea at Japan by no more than a foot. But the tsunami triggered by a magnitude 9 quake would have been seven feet high, enough to cause damage. When Satake consulted a colleague in Japan, he found that historical records there included just the damage reports he was looking for--from a tsunami in 1700 that geologists had never been able to link to a specific quake.
More important than the solution of this bygone Japanese mystery is the bearing of Satake’s work on an American one. Geologists have been debating just how large an earthquake the Cascadia Subduction Zone--which runs near such cities as Seattle and Portland--can generate. Satake’s results suggest it may be capable of a catastrophe.
