A Lady's Ruminations

"Jane was firm where she felt herself to be right." -Jane Austen, Pride and Prejudice


Wednesday, September 14, 2005

Oh, goody

We had some earthquakes about 4 summers ago. We don't usually have things like this up here.

LiveScience: Slow Seismic Slip Event Underway in Pacific Northwest---

An important seismic event imperceptible to humans has begun in the Pacific Northwest as predicted, according to the government agency Geological Survey of Canada.

The chance of a major earthquake is 30 times higher now for a roughly two-week period, but the odds are still remote, scientists say.

The event is called episodic tremor and slip (ETS). It involves a slow movement of the Juan de Fuca and North America tectonic plates along the Cascadia margin of southern British Columbia. Faults associated with the plates have been the sites of major earthquakes -- akin to the colossal tsumani-causing quake last December in Indonesia -- every 500 years or so, the geologic record shows. The last such temblor in the area struck on Jan. 26 in the year 1700.

Slow creep

The movement is slower than a traditional earthquake but more rapid than the normal creep associated with the fault. It runs in the reverse direction of the normal creep.

The movement was predicted. Scientists recently learned that these ETS events recur about every 14 months. It has been detected by Global Positioning System instruments.
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The slip began Sept. 3 on the Olympic Peninsula of Washington State and has migrated north to the Vancouver Island area, Cassidy wrote. Victoria moved 0.12 inches (3 millimeters) to the West over the course of two days. The events are thought to last six to 15 days.

Cassidy's colleague, Stephane Mazzotti, has done some calculations on the odds of a large temblor.

"The probability of occurrence of a megathrust earthquake is about 30 times higher during this approximately two-week window, than during the rest of the 14.5 month cycle," Cassidy told LiveScience. "Having said that, 30 times a small number is still a small number."

Geologists simply don't know when one of these events will trigger a major quake, Cassidy said.

The immediate importance of the event is that it occurred as predicted and can now be used to improve understanding of the region's seismology.

"By better understanding these events, we will be able to better predict the effects (and perhaps timing) of future magnitude 9 earthquakes along the West Coast," Cassidy and his colleague write.

(Curtsy to Drudge)