GREAT CASCADIA EARTHQUAKES IN THE CONTEXT OF EPISODIC TREMOR AND SLIP (ETS)

The Cascadia subduction fault has for several decades been considered a sleeping giant, but the discovery of Episodic Tremor and Slip (ETS) coupled with new instrumentation have shown the entire fault to be continuously active over week-to-week timescales. This activity, identified and mapped in near-real time, provides new constraints on the characteristics of the eventual great earthquake that Cascadia will produce.

We argue that the present-day distribution of ETS along the Cascadia convergent margin delimits the landward limit of future seismogenic rupture of the megathrust. The evidence is multi-faceted. First, a suite of over 40 Episodic Tremor and Slip (ETS) events imaged between 1997 and 2010 along the northern Cascadia region appears to bleed off nearly all the convergence-related slip deficit accumulated during the observation period. Second, an interseismic fault coupling model derived from these ETS moment release observations accurately predicts GPS-measured interseismic deformation of the overlying North American plate, as measured by ~50 continuous GPS stations across western Washington State. Third, when this model is extrapolated over the 550-year average recurrence interval of Cascadia megathrust events, the coupling model replicates the pattern and amplitude of coseismic coastal subsidence inferred from previous megathrust earthquakes here. If correct, this conclusion calls into question our current understanding of Cascadia’s seismic hazards- it suggests future coseismic rupture will extend to 25 km depth, or ~60 km inland of the Pacific coast and closer to the metropolitan regions, rather than stopping offshore at 15 km depth. For only the Washington State segment of the Cascadia margin, a 550-yr recurrence interval along this seismogenic zone translates into an M_w=8.9 earthquake, and for the entire margin an M_w=9.2.