New slip and stress distributions hold implications for the next great earthquake in Cascadia
Over the past decade, advances in detection of seismic tremor and slow slip have revealed a new spectrum of fault behavior in subduction zones around the world. These phenomena live between the mechanisms of brittle failure and aseismic creep, and reduce stress on the deep portion of the subduction interface. Cascadia is recognized for having a uniquely strong correlation of tremor and slow slip known as episodic tremor and slip (ETS) that occurs roughly every 14 - 16 months and propagates along strike at kilometers per day. While ETS reduces shear stress in the deeper unlocked regions of the subduction interface, stress increase at the base of the shallower locked zone has the potential to initiate the next major earthquake. Prior assessments of the probability of such an event have used a fault failure model which depends on shear stress rate, but found that the probability increase during ETS was negligible. This study generates new estimates of stress rates on the Cascadia subduction interface based on slow slip distributions determined from simultaneously inverting Earth Scope's Plate Boundary Observatory (PBO) GPS and borehole strainmeter (BSM) observations during ETS events with the Network Inversion Filter (NIF). While GPS directly measures total displacement and cumulative moment from ETS, BSMs directly measure spatial derivatives of displacement with high precision and thus stress. Forward models presented here illustrate this enhanced sensitivity to stress and stressing rate during ETS. Stress rates for the 2011 and 2012 ETS events are higher than the rates used in previous estimations of time-dependent megathrust probabilities (Mazzotti & Adams, 2004; Beeler et al., 2013) by an order of magnitude, suggesting that the most recent estimates of probability increase during ETS are too low, and probability calculations with these new stress rates con rm that ETS events influence megathrust likelihood by an order of magnitude greater than previously estimated.
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