The eastward-moving Juan de Fuca tectonic plate meets the westward moving North American plate at the Cascadia Subduction Zone off the west coast of Canada and the U.S.A. Periodic giant megathrust earthquakes exemplify a catastrophic sliding of the Juan de Fuca plate beneath the North American plate (approximately once every 500 years). In the period between the mega-earthquakes, the Juan de Fuca plate continues trying (unsuccessfully) to slide beneath the North American plate with the consequence that the rocks all along the edges of the plates are compressed or squeezed and uplifted. Knowing where and how fast the rocks are being deformed enables us to estimate the approximate width and length of the fault that may slip in the next megathrust earthquake. Another reason for determining how crustal deformation varies from place to place in the subduction zone is the need to know how and where the crustal stress is changing in the overlying North American plate. Changing stress can result in large crustal earthquakes in southwestern British Columbia and northwest Washington State.
Locked Subduction Zone
Subduction-thrust earthquakes or mega-earthquakes are known to be one stage of a subduction-thrust Earthquake Cycle. In the inter-seismic period between mega-earthquakes the rocks are being continuously deformed. The squeezing, stretching and uplifting of the rocks is determined in two ways: 1) by measuring the slow movement of survey points on the surface relative to survey points in the continental interior, and 2) by measuring the change in gravity with time at those same points. The first method involves continuous daily measurements of position using a permanent network of Global Positioning System (GPS) receivers called the Western Canada Deformation Array (WCDA). The second method involves repeated measurements using a technique called Absolute Gravimetry. We measure the change in gravity at selected survey points to provide additional confirmation of our interpretations. Also, it is conceivable that at some locations a change in gravity may be the only indication of on-going deformation.
The techniques mentioned above are high-precision geodetic techniques which have been developed to the required accuracy only recently. Another method called Very Long Baseline Interferometry (VLBI) uses the most fundamental reference system available, extragalactic quasars. A version of this system is being developed in Canada in a co-operative venture involving a number of Canadian scientific agencies: Geodetic Survey of Canada, Institute for Space and Terrestrial Science (ISTS), National Research Council (NRC) and Geological Survey of Canada.
A number of other ways of measuring crustal deformation are 1) repeated resurveying of old geodetic survey networks, 2) repeated resurveying of old levelling lines, 3) repeated resurveying of gravity networks, and 4) measurement of changes in mean sea level (MSL). All of the high-precision methods used in the study of crustal deformation are being carried out in co-operation with the Geodetic Survey of Canada, while MSL studies are enabled through the cooperation of the Canadian Hydrographic Survey.
Using the Global Positioning System (GPS) satellites, and a network of permanent GPS receivers, the relative motion of points on the earth's surface can be monitored at the level of a few millimetres per year. The arrows in this diagram show the measured annual rates and directions of motion of specific sites of the Western Canada Deformation Array (WCDA) relative to a reference site located at the Dominion Radio Astrophysical Observatory (DRAO) south of Penticton. Points on the outer coast of the North American Margin, which overly the locked portion of the Cascadia Subduction Zone, move at rates of over 10 mm/yr in a north-easterly direction. The fact that inland sites move at half that rate, or less, indicates that the outer margin is slowly being compressed like a giant spring. At the time of the next great earthquake it is expected that the accumulated compression will be totally released and that the outer coast of southern Vancouver Island will move up to 5 metres to the south-west.
Cascadia earthquake activity
Evidence that mega-earthquakes (m>8) have occurred regularly in the the Cascadia region is steadily accumulating; (see giant mega-earthquakes). In addition to these mega-events, large (m>6) damaging crustal earthquakes are known to occur in this region as demonstrated by the 1918 (m=7.0) and 1946 (m=7.3) earthquakes on central Vancouver Island and the 1872 event in northern Washington State (m=7.3).