A silent slip event on the deeper Cascadia subduction interface

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 dir ection of motion. No seismicity was associated with this event. The sudden displacements are best explained by similar to2 centimeters of aseismic sli p over a 50-kilometer-by-300-kilometer area on the subduction interface dow ndip from the seismogenic zone, a rupture equivalent to an earthquake of mo ment magnitude 6.7. This provides evidence that slip of the hotter, plastic part of the subduction interface, and hence stress Loading of the megathru st earthquake zone, can occur in discrete pulses.