Average annual motions at the PGC sites are shown as black arrows. The silent slip motions of August 1999 are shown as red arrows.
Silent slip on the Cascadia subduction zone
During August 1999 the equivalent of a magnitude 6.7 earthquake occurred deep beneath Vancouver Island and northwestern Washington. That’s about the same magnitude as the February 2001 Olympia earthquake, which shook the nerves of millions of people, and caused around $3 billion damage. But no one felt any seismic shaking in August of 1999, because the movement of the rocks took place over a period of more than a month.
The August 1999 movement is known as a “silent slip” event (Dragert et. al., 2001). It was not recorded on any seismometer, and the only reason we know that it happened is that the crustal movements of this area are closely monitored, using GPS technology, by geologists at the Pacific Geoscience Centre (PGC) in Sydney and their counterparts at universities and government institutions in Washington State.
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The GPS
network in Washington and British Columbia. The stations in green are operated
by the Pacific Geoscience Centre (Geol. Survey of Canada).
Average annual motions at the PGC sites are shown as black arrows. The silent slip motions of August 1999 are shown as red arrows. |
The PGC GPS array has now been in operation since 1992, and during that time it has become clear that the southern part of Vancouver Island is slowly being squeezed towards the mainland – at rates of between 0.5 and 1 mm per year. This compression is part of the evidence that the subducting Juan de Fuca plate is locked against the North America Plate, and it is along this locked zone that great earthquakes – such as the 1700 Juan de Fuca earthquake – have occurred in the past. The locked zone - which is approximately 100 km wide – represents the uppermost part of the subduction system. A transition zone extends for another 100 km or so, and beyond that the two plates were assumed to be sliding past each other in a continuous fashion.
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Cross-section of the Cascadia Subduction zone. The locked portion - which is where major earthquakes take place - is shown in yellow. The area of the silent slip of August 1999 is shown in red. |
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The GPS data from August 1999 show that rather than sliding continuously in this region, the two plates are sticking over relatively short periods (perhaps a few years), and then sliding silently - without seismic shaking - over a period of several days or weeks. The 1999 event started in the south – beneath Seattle – on August 18th, and progressed slowly towards the north, reaching Sidney by about the 28th August and the Nanaimo area by about the 24th of September. The upper locked part of the plate boundary is where major earthquakes occur, and it is evident that no displacement took place within this region during the 1999 slip event. In effect, therefore, although some stress may have been released on the lower part of the boundary, Coulomb stress transfer theory suggests that the event actually led to an increase in the stress on the upper part of the boundary – bringing it closer to eventual failure and a large earthquake. The amount of stress build-up during the silent slip event is estimated to be only equivalent to about one-half of a year of steady stress build-up (Thatcher, 2001), but it is speculated that slip events of this type on the lower parts of subduction boundaries can trigger the major earthquakes which take place in the upper parts of the boundaries.
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GPS motion data for 5 locations in Washington and BC. (see map above). The vertical axis is eastward motion in mm . |
References
Dragert, H., Wang, K., and James, T., A silent slip event on the deeper Cascadia subduction interface, Science, V. 292, p. 1525-1528, May, 2001
Thatcher, W., Silent slip on the Cascadia subduction interface, Science, V. 292, p. 1495-1496, May, 2001
The figures above were adapted from figures at the Pacific Geoscience Centre crustal deformation website: http://www.pgc.nrcan.gc.ca/geodyn/geodyn.htm
Further
information on the Geodynamics program at PGC, including the 1999 event, is also
available at this website.
Steven Earle, 2000. Return to Earth Science News
