High-resolution images of tremor migrations beneath the Olympic Peninsula from stacked array of arrays seismic data

Episodic tremor and slip (ETS) in subduction zones is generally interpreted as the manifestation of shear slip near the base of earthquake-generating portion of the plate interface. Here we devise a new method of cross-correlating stacked Array of Arrays seismic data that provides greatly improved tremor locations, a proxy for the underlying slow slip, beneath the Olympic Peninsula. This increased resolution allows us to image many features of tremor that were not visible previously. We resolve the spatial transition between the rupture zones of the inter-ETS and major ETS episodes in 2010, suggesting stress redistribution by the former. Most tremor migrations propagated along the slowly advancing main tremor front during both the inter-ETS and the major ETS episodes, even though the main front of the former deviated strongly from its usual (along-dip) orientation. We find a distinct contrast between along-dip rupture extent of large-scale rapid tremor reversals (RTRs) to the south and that to the north in our study region that anticorrelates with the locations of inter-ETS events. These RTRs originate from the main front, similar to smaller-scale RTRs previously observed at high-resolution, and many start by propagating along the main front. This could be consistent with RTRs being triggered by a cascading failure of brittle asperities. After initiation, the RTRs repeatedly occupy the same source region, and the early repetitions appear not to be tidally driven. Their stress drop may come from continuing fault weakening processes within the tremor zone, or loading by aseismic slip in surrounding regions.