Fri, 11 Mar 2011 17:13 UTC
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."
Yeats praised the Japanese building codes, public education and scientific initiatives for avoiding more serious damage that could have occurred. West Coast residents have become more prepared in recent years, but more can be done, he said.
Cascadia earthquakes, which slip along the interface between the Juan de Fuca and North America plates, occur at irregular and unpredictable intervals between 200 and 1,000 years apart. Using the low end of that range, we are 100 years overdue for the next great earthquake. At the other extreme, we might have 800 years to go.
Meanwhile, more frequent earthquakes are likely to occur along faults within the plates or faults near the surface. The latter includes the Seattle fault, which cuts across Puget Sound north of Bremerton. During the last great earthquake on the Seattle fault, some 1,100 years ago, the south end of Bainbridge Island was pushed up 22 feet.
An earthquake on the Seattle fault could create a tsunami between 6 and 10 feet high that would reach Bremerton within minutes, according to a 2001 study. Although such an earthquake is unlikely, some experts say people near the water would be wise to get to higher ground as soon as the shaking stops.