Along-strike changes in ETS behavior near the slab edge of Southern Cascadia

Abstract

Episodic tremor and slip (ETS) is well-documented along the entire length of the Cascadia subduction zone. We explore how the occurrence of ETS varies at the southernmost edge of the subduction zone, where geometric complexity and a slab window likely alter conditions along the plate interface. This work uses tremor and GNSS time series data to identify nineteen of the largest ETS events in southern Cascadia between 2016.5-2022 and document source properties for events approaching the slab edge. Distributed slip models for these events show that cumulative fault slip along the megathrust reaches a maximum near 40.5° N latitude and that large ETS events accommodate up to 85% of plate convergence at this location. However, ETS fault slip and tremor terminate near 40° N latitude, some 50 km before the southern lateral edge of the subducting plate. After considering a range of explanations, we propose that the complex geometry and progressive heating of the subducting plate modifies ETS behavior and does not allow seismic slip to occur along the plate interface in southernmost Cascadia below 35 km depth.