Major Quake in BC Possible

(February 03, 2007): Seismologists from the Pacific Geoscience Centre in Sidney, B.C., have warned there is an increased probability of a major earthquake hitting B.C.'s south coast during the next week.

Geophysicists are tracking a series of tremors that started in Puget Sound along the U.S. West Coast three days ago and are working their way up Vancouver Island. The background seismic activity, which can be detected by sensitive equipment installed up the coast by U.S. and Canadian governments, was recorded through southern Vancouver Island Thursday, said seismologist Garry Rogers.

The tremors are caused by a rip or tear in the bottom plate of the Cascadia subduction zone. This zone is the region where the denser oceanic plate subducts or plunges under the less dense continental plate.

While the probability of a major quake is still low, all of the energy from this plate movement is stored like a giant spring in the area directly under Vancouver Island. Scientists discovered several years ago that the slippage along the Juan de Fuca (oceanic) & North American (continential) plates occurs like clockwork every 14 months, and last about a week or two.

When the crust under Vancouver Island can no longer store anymore of this energy, there will be a catastrophic release - with devastating consequences.

Because of the very large fault area, the Cascadia subduction zone can produce very large earthquakes, magnitude 9.0 or greater, if rupture occurred over its whole area. When the "locked" zone stores up energy for an earthquake, the "transition" zone, although somewhat plastic, can rupture. Thermal and deformation studies indicate that the locked zone is fully locked for 60 kilometers downdip from the deformation front. Further downdip, there is a transition from fully locked to aseismic sliding. (Nedimovic, et al., 2003)

In 1999, a group of Continuous Global Positioning System sites registered a brief reversal of motion of approximately 2 centimeters over a 50 kilometer by 300 kilometer area. The movement was the equivalent of a 6.7 magnitude earthquake. (Dragert, et al., 2001) The motion did not trigger an earthquake and was only detectable as silent, non-earthquake seismic signatures. (Rogers & Dragert, 2003)

The last known great earthquake in the northwest was in January of 1700, the Cascadia Earthquake. Geological evidence indicates that great earthquakes may have occurred at least seven times in the last 3,500 years, suggesting a return time of 400 to 600 years. There is also evidence of accompanying tsunamis.

A future rupture of the Cascadia Subduction Zone would cause widespread destruction throughout the Pacific Northwest.

Other similar subduction zones in the world usually have such earthquakes every 100–200 years; the longer interval here may indicate unusually large stress buildup and subsequent unusually large earthquake slip.

• Cascadia Subduction Zone

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