Characterizing Episodic Tremor and Slip in Cascadia

Episodic Tremor and Slip, or ETS, occurs frequently in Cascadia with recurrence intervals of roughly 8-22 months for large ETS. Characterizing these events is critical to our understanding of subduction plate interface mechanics, plate motion budgets, and the potential for damaging earthquakes. Here we combine a novel technique for separating ETS and inter-ETS velocities with the Network Inversion Filter [Segall and Matthews,1997; McGuire and Segall, 2003; Miyazaki et al., 2006] to fully characterize ETS slip using daily GPS time series. The velocity separation technique allows for an inversion of the stacked, time-averaged ETS velocities to obtain a time-averaged ETS slip rate on the plate interface for the last 10-20 year time period. These time-averaged velocities are directly comparable to plate rate to characterize the overall slip budget. We use time dependent NIF inversions with our newly derived inter-ETS velocities to create a catalog of ETS events. Slip and tremor track closely in all ETS events, consistent with prior results [Bartlow et al., 2011; Wech and Bartlow, 2014]. We generate heterogenous elastic Green's functions for both of our inversions using the PyLith finite element code [Aagaard et al., 2013], based on the velocity model of Stephenson [2007] to better estimate slip amplitudes. We find that while 75-100% of the plate rate is accommodated in the northern segment, consistent with prior results [Chapman and Melbourne, 2009], in the central segment and parts of the southern segment ETS accommodates only 0-50% of the plate rate, leaving additional slip to be released as inter-ETS creep, in an earthquake, as posteseismic relaxation, or as ETS slip at other points in the megathrust earthquake cycle. Currently published locking models [Schmalzle et al., 2014; Pollitz and Evans, 2017] indicate that inter-ETS creep is likely to take up most of the remaining slip budget, but some coupling may remain in the ETS zone.