Megaquake, similar to Japan’s, predicted for Pacific Northwest

VANCOUVER, British Columbia >> Ground movements in coastal British Columbia, Washington state and Oregon suggest the Cascade region is ripe for a giant quake similar to the one that struck Japan in March, scientists said Sunday.

Such a quake would almost certainly trigger another Pacific-wide tsunami, they said at the annual meeting of the American Association for the Advancement of Science.

The most recent megaquake in the region was a magnitude-9.0 temblor southwest of Seattle in 1700, an event deduced from the resulting tsunami that hit Japan.

Another giant quake is possible where two tectonic plates are colliding in the northeastern Pacific, said Kelin Wang of the Geological Survey of Canada. Just offshore, the Juan de Fuca plate is grinding under the North American plate at the Cascadia subduction zone, an area that stretches from Vancouver Island to Northern California.

"It will produce a pretty big tsunami," Wang said in a symposium at the Vancouver Convention Center. "We know that because the tsunami generated by the 1700 event hit Japan and caused pretty large damage, comparable to the damage caused by the 1960 earthquake off Chile. So we know Cascadia is capable of producing a pretty large tsunami."

The clues to the future quake are ground stations equipped with GPS, Wang said. All of the coastal stations east of the Cascadia zone are moving east, being pushed inland as the plates collide. That indicates the fault is stuck and pressure is building, he said.

Eventually the pressure must be released, resulting in "elastic rebound" — an earthquake.

The magnitude is difficult to forecast, but the same tectonic process was at work in Japan before last year’s devastating 9.0-magnitude Tohoku quake hit March 11, he said. Before the quake, all the GPS stations in Japan were moving west, a mirror image of what’s happening at Cascadia.

"Great earthquakes occur in a subduction zone typically every few hundred years," Wang said. "The problem is we have been recording earthquakes for only about 100 years."

But comparing subduction zones around the world can help bridge the gap in that information, he added.

"The deformation cycles of giant earthquakes like Tohoku and Cascadia can be understood by piecing together snapshots from different subduction zones," Wang said.

The quake and resulting tsunami left 15,844 people dead, 3,450 injured and 5,891 missing and crippled the Fuku­shima Dai­ichi nuclear power plant. The Pacific-wide tsunami also caused widespread damage in Hawaii, particularly to Keehi Lagoon and along the Kona Coast of Hawaii island.

"I don’t want to be forecasting a huge disaster with the Cascadia earthquake," said University of Nevada seismologist John Anderson. "I think that for Vancouver or Portland or Seattle, the level of ground shaking could be comparable to the levels that were seen along the coastal regions of Japan."

Although earthquakes are common around Japan, the size of the March 11 temblor was "largely unanticipated by seismologists," said James Mori of the Disaster Prevention Research Institute at Kyoto University. "The fact that tens of thousands of people were killed was really a shock."

The scientists spoke Sunday in a symposium called "The Magnitude 9.0 Tohoku Earthquake and Tsunami: Significance for Japan and the World."

Panelist Stuart Nishenko of Pacific Gas and Electric Co. in San Francisco said the Fuku­shima disaster caused a global "crisis of confidence" that has many nations reassessing their nuclear power programs. Germany, for instance, announced last year that it would shutter its plants by 2022.