West Coast Quake Zone Is Locked and Loaded, Study Says
By JULIE SATOW Staff Reporter of the Sun
The western coast of North America may be the site of one of the most devastating earthquakes ever.And it could happen sometime soon, researchers at Columbia University say. The area off the coast of Vancouver and Seattle, called northern Cascadia, is the site of two tectonic plates bumping against each other. Along this region where the northern Pacific seafloor is being pushed under the edge of the continent are locked zones — zones where the plates are tightly locked against each other by mounting friction. When the friction grows too great, it erupts in an earthquake. The larger the locked zones, the greater the possible tension that can build between the plates.The more tension produced, the more energy released, and the larger the earthquake. The study in Nature conducted by researchers at the Columbia University’s Earth Institute, has found the locked zone in northern Cascadia is 20 miles wider than had been thought, making it a 56-mile swath rather than the 36 miles that had been previously estimated. This width extends farther east toward the land, creating additional hazard for communities along the coast. “This locked zone could reach a point where so much energy is being stored it would create one of the largest earthquakes ever,”said Thomas Pratt,a research geophysicist at the United States Geological Survey. The Geological Survey’s National Earthquake Hazards Reduction Program and the Geological Survey of Canada funded the study. An earthquake in northern Cascadia would release over 1,000 times more energy than the earthquake that shook Seattle in February 2001, said the research scientist at the Lamont-Doherty Earth Observatory at Columbia who led the study, Mladen Nedimovic. That earthquake had a magnitude of 6.8. An earthquake in northern Cascadia would have a magnitude of at least 9. The area Mr. Nedimovic studied runs from Washington State to British Columbia, but the locked zone extends farther south to northern California. Mr. Nedimovic believes the extra width he discovered in the northern Cascadia segment of the locked zone is true for the zone’s entire length. This would make the locked zone even larger,
increasing the risk of an even more serious earthquake in the region. Mr. Nedimovic told The New York Sun he hopes to receive a grant to study the southern Cascadia region to confirm his theory. An earthquake in northern Cascadia could occur at any time, say researchers. Earthquakes in the region occur every 200 to 800 years. The last earthquake in this zone was thought to have been around the year 1700. “We are currently within the time frame where another large earthquake is expected,” Mr. Nedimovic said. “It is a definite possibility,” Mr. Pratt added. “It is no secret that this is an area that has great potential for an earthquake,” said Marianne Bichsel, a spokeswoman for Seattle Mayor Greg Nickels. She said the city is prepared for man-made and natural disasters, and received nearly $30 million in the past few months from the Department of Homeland Security to beef up its emergency preparedness. To uncover details of the locked zone in northern Cascadia, Mr. Nedimovic used seismic reflection data. Traditionally used for the exploration of gas and oil, seismic reflection uses vibrations that reflect off the earth’s surface to take a picture,similar to how a sonogram takes a picture of an infant in the womb. Until now, scientists have largely depended on guesswork to determine the boundaries of a locked zone. Thermal modeling, where researchers use temperatures to determine the zone, and dislocation modeling,where global positioning systems and other methods are applied, are based on vague estimates. The seismic reflection data, on the other hand, can show with more certainty what the locked zone looks like and where it ends. “This is the first time we have evidence of where the locked zone is, and for the first time we can actually see the boundaries,” Mr. Pratt said. Seismic reflection “may be a globally important predictive tool for determining the maximum expected rupture area” for all earthquakes, Mr. Nedimovic said. His next steps will be to perfect the use of seismic reflection off the coast of Alaska.
DEVASTATION Workers check the damage to Interstate 880 in Oakland, Ca. in October 1989 after an earthquake measuring 7.1 on the Richter scale hit the Bay area. AP/PAUL SAKUMA