Abstract
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.