Wednesday, October 28, 2009

Megaquake in Seattle?

Megaquake Looms Over Seattle from Discovery News

Southern California isnt the only part of the US that lives in fear of a major earthquake. This new study suggests that a major earthquake in the Pacific Northwest might occur much closer to major population centers in the Seattle area than previously thought. As Southern Californians, we all know what kind of dangers earthquakes can pose near major metropolises.

The Cascadia fault in Washington is a different type than the faults we have in Southern California. It's a thrust fault - where two continental plates are converging, and one slides underneath the other. As they grind along each other, they can produce earthquakes. In Southern California, our major faults are transform faults - where two plates are sliding past each other. Thrust faults typically generate deeper and more powerful earthquakes than transform faults - while the study estimates a 9.0 magnitude quake on the Cascadia fault, the largest that the San Andreas fault can support in this area is about 8.6, about 2.5x less energetic.

The Northridge earthquake of 1994 was also triggered along a much smaller thrust fault that ran underneath the Los Angeles area. Although that quake was only of moderate intensity (6.7 magnitude), it exemplifies the type of damage that an earthquake along a thrust fault can wreak near a major city.

This might actually be good news, though, for those not in the immediate earthquake area - the new "hot zone" for the fault has moved from offshore to under the Olympic peninsula. A suboceanic earthquake would pose a higher risk of generating devastating tsunamis that would propagate up and down the Pacific seaboard; the last major earthquake on the fault, around the year 1700, caused destructive tsunami waves that traveled as far away as Japan.

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