Geomatics for Informed Decisions - GEOIDE

Watching the earth move, one millimetre at a time

Saving lives and reducing damage due to earthquakes are the ultimate goals of the Natural Hazards and Disaster Monitoring Project, part of the Geomatics for Informed Decisions (GEOIDE) Network of Centres of Excellence.

West coast scientists with the project have made progress in the application of continuous GPS (Global Positioning System) technology. This progress has paid off with a new discovery about how the subduction zone - the area where one geological plate moves under another one - along Canada's west coast behaves.

The researchers recently observed the earth "rippling" westward along a line from Puget Sound to central Vancouver Island at the breakneck speed of 6 kilometres per day. That's a slow walking pace for us, as the average person could easily walk a single kilometre in about 15 minutes, but for a geological event, it's pretty significant. Called a "slow earthquake," its detection was a direct result of innovations in GPS data analysis that now allows scientists and researchers to continuously monitor surface motions as small as 1 millimetre.

Detecting the recent "slow earthquake" is a good example. Researchers observed surface displacements ranging from 1 to 4 millimetres at seven GPS locations from Seattle to central Vancouver Island over a two-week period, systematically delayed at the more northerly locations. These surface displacements were caused by slip - about 2 centimetres over a total area of about 50 by 300 kilometres - across the deeper portion of the fault between the two plates. While the amount of slip is equivalent in energy to a magnitude 6.7 earthquake, this slow earthquake did not generate any shock waves and there was no damage.

"The previously much sparser GPS coverage and the noisier GPS time series prevented us from observing such events before," says Dr. George Spence of the School of Earth and Ocean Science at the University of Victoria and the project's coordinator. "That's a very exciting result that has come from this careful analysis."

And what did this event tell us?

"It changed our perception of how and where stress is built up on this interface," says Dr. Spence. "It now appears that the deeper, hotter portion of the interface is subject to stress accumulation that is released by episodic slip." He adds that each deep slip event actually increases the stress across the shallower locked megathrust zone, in discrete pulses. In the future, one such pulse could trigger "the big one."

The project has also been instrumental in helping to map the location and extent of the locked zone - the region where stress is accumulating for the next megathrust earthquake. It has been confirmed that the "stuck" region extends no further landward than the west coast of Vancouver Island and probably no further northward than about central Vancouver Island. What is somewhat reassuring about this discovery is that the locked zone does not lie directly beneath the large population centres of Victoria and Vancouver.

The benefits of this GEOIDE project are tremendous. "It's absolutely essential to have improved probability estimates for earthquake hazard, in terms of both size and location," says Dr. Spence. "That new knowledge and understanding can be incorporated into seismic capabilities of buildings, dams, bridges and highways." It will also help target funds and preparation efforts to areas where they would be most effective, saving lives and reducing damage.

GEOIDE is one of 20 federally funded Networks of Centres of Excellence, the objectives of which are to enhance the Canadian economy and our quality of life. The program is funded by the Natural Sciences and Engineering Research Council (NSERC), the Canadian Institutes of Health Research (CIHR) and the Social Sciences and Humanities Research Council (SSHRC), in partnership with Industry Canada.

To learn more about the Natural Hazards and Disaster Monitoring project and GEOIDE, visit www.geoide.ulaval.ca.