The following is the abstract from an interesting article on reversal of plate
motions in the Pacific northwest during late August-early September 1999.
The author makes a case that this was equivalent to a slow earthquake of
Mw 6.7. I would suggest that motion added to the North American plate
by Hurricane Dennis may have temporarily reversed the motion of the plate
with respect to the Pacific and given the observed results. Observations we
made on the Parkfield creep at the time suggest creep stopped and may have
reversed in that area at the same time as this event in the northwest.

A Silent Slip Event on the Deeper Cascadia Subduction Interface

Herb Dragert 1*, Kelin Wang 1, Thomas S. James 1

1 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@pgc.nrcan.gc.ca.

Continuous GPS sites in southwestern British Columbia and northwestern Washington 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.

• A Silent Slip Event on the Deeper Cascadia Subduction Interface