Earthquakes on Gorda Plate a common event


CORVALLIS - The strong earthquake that struck yesterday about 80 miles off the coast of Northern California was a type that's fairly common for that geologically active region and unlikely to produce a major tsunami, said researchers at Oregon State University.

However, that information couldn't be obtained immediately, and the tsunami warning that was sent out for an hour or two was appropriate - and from the nature of the tectonic activity in this area, may occur again in the future with some regularity, OSU scientists said.

OSU experts have done some of the most extensive analysis of faults and tectonic activity in this region of the Gorda plate, and said there have been about 25 similar earthquakes of moderate to large magnitude in this area since 1980, none of them producing a major tsunami.

"These earthquakes, and the one on Tuesday evening, are strike-slip quakes that generally do not produce tsunami events," said Jason Chaytor, a marine geologist with OSU and colleague of Chris Goldfinger, one of the world's leading experts on the Cascadia Subduction Zone. "It's not so much a reflection of the magnitude of the earthquake, which was at first thought to be 7.4, but the nature of the fault movement."

"But it takes a little time to pin down exactly where an earthquake originated and what type it is, so to be safe we have to send out tsunami warnings until we know more," Chaytor said. "And those warnings should be taken seriously."

Last year, Goldfinger and Chaytor published a research paper outlining the intense stresses that the Gorda plate is under and the large number of strike-slip faults that are produced as a result in this area of very active tectonic movement. More information on this research can be found on the web at www.activetectonics.coas.oregonstate.edu

This region is just north of what's called the "Mendocino Triple Junction" of the Pacific plate, the North American plate, and the Gorda plate - which is getting squeezed by the movement of the Pacific plate and undergoing intense deformation.

Also near here, Chaytor said, is the southern boundary of the Cascadia Subduction Zone - a large subduction fault that runs from the triple junction hundreds of miles north to the northern tip of Vancouver Island. It is this fault, experts say, which will some day be the source of a massive earthquake that may shake much of the Pacific Northwest and quite likely will cause major tsunami events.

The Cascadia Subduction Zone is quite similar geologically to the area that had a major earthquake and tsunami last year near Sumatra. Subduction zones are characterized by land movements that have more of an "up and down" movement to them, while in strike-slip earthquakes the fault movement is more side-to-side, and far less likely to cause a tsunami.

There's no known correlation between strike-slip earthquakes on the faults in the Gorda plate, Chaytor said, and activity on the subduction zone.

"We're studying whether there may be any mechanism by which strike-slip earthquakes of this type relieve some of the pressures on the subduction zone," Chaytor said. "That's an important question, but at this point we don't have an answer to it."