A Silent Slip Event on the Deeper Cascadia Subduction Interface

TLDR

Continuous GPS sites in southwestern British Columbia and northwestern Washington show landward motion caused by the locked Cascadia subduction fault offshore. In summer 1999, seven GPS sites briefly reversed direction without seismicity, indicating ~2 cm aseismic slip over a 50 × 300 km area downdip from the seismogenic zone, equivalent to a Mw 6.7 earthquake, and demonstrating that slip in the hotter, plastic part of the subduction interface can occur in discrete pulses.

Abstract

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 approximately 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.