By M. Alex Johnson
A massive earthquake like the one that unleashed a giant tsunami and killed nearly 16,000 people in Japan a year ago not only could happen here in the U.S., but probably will — and relatively soon in terms of seismological history.
The Tohoku earthquake was the most closely monitored in history, yielding an unprecedented breadth of data, geophysicists and seismologists say. And for residents of the Pacific Northwest, the new data should be worrisome. “It’s just like Japan, only a mirror image,” said Gerard Fryer, a geophysicist at the University of Hawaii and the Pacific Tsunami Warning Center.
The disaster in Japan occurred because of stress from the Pacific tectonic plate sliding below Japan, according to new research discussed last month at the annual conference of the American Association for the Advancement of Science in Vancouver, British Columbia.
The lead researcher, John Anderson, a geophysicist at the University of Nevada-Reno, said the plates locked together, slowly pushing Japan westward. The plates released catastrophically on March 11, 2011, creating a magnitude-9.0 earthquake and tsunami waves that topped 100 feet, said Anderson, who spent most of the past year in Japan as a visiting research professor in Tokyo.
While most Americans probably think the San Andreas fault running through California poses the greatest threat of unleashing a killer mega-quake, data from the Japanese quake indicate that the distinction actually belongs to the Cascadia fault line, which runs through southern Canada, Washington and Oregon to Northern California, Anderson said at the conference.
Like Fryer, he called the Pacific Northwest trench a “mirror image” of the Japanese trench — except potentially even more dangerous. “In this mirror image, one can see that if the same earthquake occurred in Cascadia, the fault would rupture to a significant distance inland, since the Cascadia trench sits much closer to the coastline than the trench off the coast of Japan,” Anderson said.
While some probability models predict that a Cascadia earthquake wouldn’t rupture so far under the land, “if it does, the data from the Tohoku earthquake predict stronger ground motions along our West Coast than those seen in Japan,” he said.