New Estimates of Earthquake Probability during Episodic Tremor and Slip in Cascadia

In Cascadia, characteristic repeating episodic tremor and slip (ETS) events have been shown to temporally modulate the probability of the next great earthquake, using both Coulomb failure and rate-and-state friction delayed failure models (Beeler et al., BSSA, 2014; Mazzotti and Adams, BSSA, 2004). The significance of this modulation, however, is highly dependent on both the duration and amplitude of shear stress change on the plate interface at the location of future earthquake nucleation due to ETS, as well as the product of effective normal stress and the frictional parameter a. Previous studies have used approximate earthquake cycle parameters, such as stress drop and repeat interval, to estimate the ETS-caused shear stress perturbations. The work presented here updates these prior estimates of ETS-caused temporal modulation of megathrust earthquake probability by using empirically estimated ETS shear stress changes on the plate interface. We calculate shear stress from simultaneous inversion of Earth Scope's Plate Boundary Observatory GPS and borehole strainmeter (BSM) observations with the Network Inversion Filter (NIF) for select ETS events. The joint inversion takes advantage of the BSM's greater sensitivity to shear stress compared to GPS. Time dependent inversions using the additional strain observations require inferred slip and slip rate amplitudes that vary more in time, thus generating more precise estimates of shear stress along regions of the interface during ETS. We estimate that the average ETS-caused shear stress increase up dip from the zone of ETS is closer to 1 MPa/yr, which is much greater than the estimate of 90 kPa/yr used by Beeler et al., 2014. For low effective normal stresses (<2-3MPa), we estimate the probability of fault rupture is increased by an order of magnitude during and immediately following ETS. It is therefore likely that the next large earthquake will occur during or immediately after an ETS event in Cascadia. This approach may form a basis for future real-time monitoring rupture probability of a recent or ongoing ETS event.