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"7.8 Canada Earthquake 1623 4949 12555", from what I know, this has not happened yet.

Residents of southern British Columbia, Washington, Oregon, and northern California know that great (magnitude 8 or 9) earthquakes have occurred every few hundred years on the west coast. These earthquakes take place offshore on the shallow part of a major fault where the ocean floor slides under, or subducts beneath, western North America.

A team of scientists at the Geological Survey of Canada of the federal Department of Natural Resources have observed unexpected behavior of the deeper parts of the same fault underlying southern Vancouver Island and the Olympic Peninsula. Their study showed that instead of slipping steadily, as was generally assumed, this deeper fault interface exhibits episodes of slip of several centimetres over periods of one to two weeks. The discovery of this behaviour by GSC scientists at the Pacific Geoscience Centre, Sidney, B.C., was published in the journal Science in 2001.

In this publication, the authors suggest that an episode of slip adds stress to the shallow, locked part of the fault bringing it slightly closer to failing, or giving way, resulting in the next great earthquake. There's no need for alarm, however. These episodes of slip occur every 13 to 16 months and therefore represent "business as usual" for the fault. Instead of steadily building up stress on the locked zone, as was formerly thought, it now looks as though stress is added episodically over a period of a few weeks.

The orginal slip event was detected with a network of sensitive Global Positioning System (GPS) sites established to monitor the crustal stretching and squeezing of this earthquake-prone subduction zone. This is the same satellite technology that is used for navigation and general positioning but much greater precision is achieved by using specialized GPS receivers, stable antenna mounts and careful data processing. It is thus possible to measure horizontal positions to about a millimetre over a few hundred kilometres.

GPS and other geodetic observations over the past two decades have established that the western margin of North America, from southern British Columbia to northern California, is being slowly compressed eastward (black arrows) because the subducting oceanic plate drags the North America plate margin with it.

The 1999 slip episode was detected from GPS observations that showed that in the summer of 1999, sites located in Puget Sound and southern Vancouver Island briefly reversed their direction of motion (red arrows). These motions of up to 6 mm occurred over about 10 days at any given GPS site, and took about 35 days to travel from Puget Sound to central Vancouver Island. These motions can be modelled by slip of ~ 2 cm occurring on the fault over an area of about 50 km by 300 km (about 30 mi by 190 mi) at depths of about 25 km to 45 km (about 15 mi to 28 mi). If this slip had occurred suddenly, it would have been a magnitude 6.7 earthquake, similar in magnitude to the 2001 Nisqually earthquake near Seattle. But because it took several weeks to complete, no shock waves were generated, and it was only detected with the sensitive GPS measurements.

The Nisqually earthquake is an example of an "intraslab" earthquake, one of the other kinds of earthquakes that can occur in the area. Intraslab earthquakes occur in the subducting oceanic slab and shallow crustal earthquakes occur in the North America plate above the subduction zone. These earthquakes are smaller than megathrust earthquakes, but are also hazardous because they can occur near populated areas, in contrast to the offshore megathrust earthquakes. GPS observations from Washington State have documented the crustal displacement from the Nisqually earthquake and illustrate the numerous uses of the technique.

Understanding the relationship between the different kinds of earthquakes and the silent slip is a new area of study. More GPS stations are needed to better understand how frequently, and over what area, these slip events occur. Future study of this new aspect of fault behavior will lead to a better understanding of earthquake hazard and may guide the first steps towards monitoring pressure build-up on the subduction fault as it is occurring.
 

BE INFORMED, BE WARNED, BE PREPARED: BEFORE, DURING and AFTER an EARTHQUAKE

2.13.2007

49-49 125-55 is, as far as I can make out, between Gold River and Campbell River BC, in a fairly remote area of Vancouver Island.

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Alex
Calgary Canada

reply

Thanks Alex.

Brian

2.26.2007

Brian

3.16.2007

The latest episode of subterranean tremors beneath Vancouver Island has ended earlier than expected, seismologists at the Pacific Geoscience Centre in Sidney, B.C., said Saturday.

"Things have quietened down very quickly. We don't know why," Natural Resources Canada seismologist John Cassidy told the Victoria Times Colonist newspaper.

The minor earth tremors under the floor of the Pacific Ocean indicate that stress is being placed on the shallow, locked part of the Cascadia fault line.

The tremors are caused by the slipping of the bottom plate in the fault, which runs off the west coast of the island and beneath it. If the plate gives way, a major earthquake would result.

Scientists say they began picking up the low-level shaking in 2003 with sensitive equipment. The episodes seem to occur every 14 months, with the latest detected on Jan. 31. The next expected episode of tremor and slip is expected to occur in April 2008.

The tremor activity has, in the past, lasted for a week or two, moving along the fault line from Puget Sound in Washington state to Vancouver Island.

Cassidy told the newspaper the seismic activity usually travels across Juan de Fuca Strait up the island to Lake Cowichan and on to the Alberni Inlet, but this time, it appeared to stop north of Duncan.

reply

Hi, thanks very much, and will also post this news story.

Brian

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