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.

Schematic of Subduction Zone showing fully Locked and partially locked Transition zones (source of next great earthquake) and the deeper zone where the slip events occur. larger image [GIF, 21.7 kb, 600 X 213, notice]

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 Network: In Canada the GPS sites are operated by the Geological Survey of Canada and in the U.S. by numerous agencies, including the Pacific Northwest Geodetic Array (PANGA) - a university consortium, NOAA (US Coast Guard and NGS), the USGS Cascades Volcano Observatory, NASA Jet Propulsion Laboratory and others. larger image [GIF, 36.1 kb, 497 X 396, notice]

	The red stars in the picture represent the earthquakes greater than magnitude 7 on North American Plate
	The blue star represents the earthquake greater than magnitude 7 on Juan de Fuca Plate
	The red square is the reference GPS station used in our data processing.
	The green squares are the GPS sites in Canada
	The yellow square yellow squares are the GPS sites in the USA
The greenred dotted lines show the nominal downdip limits of the locked and transition zones, respectively, from the model of Flueck et al. (1997).

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.

GPS derived long term (black arrows) and slip event (red arrows) motion larger image [GIF, 57.1 kb, 600 X 479, notice]

Daily position plotted as a function of time (click to see all sites) - vertical bar indicates beginning of slip event larger image [GIF, 68.9 kb, 648 X 446, notice]

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.