The Cascadia subduction zone
While Japan struggles to recover from one of the greatest earthquakes in world history, West Coast seismologists are warning that a quake just like it could occur at any time off the Washington and Oregon coasts.
Friday's subduction earthquake in Japan has been calculated at magnitude 8.9, although experts say the number may be increased slightly. The last subduction quake off the West Coast, which occurred on Jan. 26, 1700, has been judged at between magnitude 8.7 and 9.2.
The size of the 1700 earthquake was determined from evidence buried in sediments along the coast. The date is known, because it created a tsunami that washed up in Japan, where observers recorded the date.
In broad-brush terms, "the two earthquakes are very similar," said John Vidale, director of the Pacific Northwest Seismograph Network at the University of Washington. "As a first guess, what might happen here is what happened there."
Subduction earthquakes are caused by the slippage between two tectonic plates. Plates on opposite sides of the Pacific Ocean are different, but in either case a tsunami would travel a short distance before hitting land. As in Japan, people living in Washington's coastal communities might have less than half an hour to escape from a surge of water from a subduction earthquake.
Robert Yeats, professor emeritus of geology at Oregon State University, said West Coast residents should take the Japanese experience to heart.
"What you are seeing in Japan today is what you will see in our future,"he said, "except that they are better prepared than we are."