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Originally published in Science Express on 19 April 2001
Science 25 May 2001:
Vol. 292. no. 5521, pp. 1525 - 1528
DOI: 10.1126/science.1060152
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Reports

A Silent Slip Event on the Deeper Cascadia Subduction Interface

Herb Dragert,* Kelin Wang, Thomas S. James

Continuous Global Positioning System sites in southwestern British Columbia, Canada, and northwestern Washington state, USA, have been moving landward as a result of the locked state of the Cascadia subduction fault offshore. In the summer of 1999, a cluster of seven sites briefly reversed their direction of motion. No seismicity was associated with this event. The sudden displacements are best explained by ~2 centimeters of aseismic slip over a 50-kilometer-by-300-kilometer area on the subduction interface downdip from the seismogenic zone, a rupture equivalent to an earthquake of moment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress loading of the megathrust earthquake zone, can occur in discrete pulses.

Geological Survey of Canada, Pacific Geoscience Centre, 9860 West Saanich Road, Sidney, British Columbia, Canada V8L 4B2.
*   To whom correspondence should be addressed. E-mail: dragert{at}pgc.nrcan.gc.ca

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