Proactive disclosure Print version ![Print version Print version](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/esst_images/_printversion2.gif) ![ÿ](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/esst_images/_spacer.gif) | ![ÿ](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/esst_images/_spacer.gif) | ![Strong and safe communities Strong and safe communities](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/esst_images/2002iscom_e.jpeg) Natural Resources Canada > Earth Sciences Sector > Priorities > Strong and safe communities > Geodynamics
Geodynamics Cascadia Subduction Zone
![Cascadia Subduction Zone: Cross section Cascadia Subduction Zone: Cross section](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/geodyn/images/mega01a_.jpg) Cascadia Subduction Zone: Cross section
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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.
Measurement of Subduction Zone Deformation |
![Locked Subduction Zone Locked Subduction Zone](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/geodyn/images/intrseis_.gif) 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.
![Cascadia Subduction Zone: Map Cascadia Subduction Zone: Map](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/geodyn/images/mega11_.gif) Cascadia Subduction Zone: Map
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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.
![Horizontal velocities Horizontal velocities](/web/20061103040358im_/http://www.gsc.nrcan.gc.ca/geodyn/images/agu99vel_.gif) Horizontal velocities
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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).
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