Repeating VLFEs During ETS Events in Cascadia Track Slow Slip and Continue Throughout Inter-ETS Period

Episodic tremor and slip (ETS) events in Cascadia include slow earthquake phenomena such as slow slip events, tremor, low frequency earthquakes (LFEs), and very low frequency earthquakes (VLFEs; Rogers & Dragert, 2003, ). Using VLFEs detected from a grid-search centroid moment tensor inversion algorithm in the 2011 (Ghosh et al., 2015, ) and 2014 (Hutchison & Ghosh, 2016, ) ETS events as templates, we apply a matched filter algorithm to create a VLFE catalog for each ETS event. We also use the 2011 template events to create a VLFE catalog from July 2011 through the end of 2012, which encompasses both the 2012 ETS event and an inter-ETS period. The successful application matched filtering intrinsically suggests that VLFEs are repeating events, of which thousands were detected during each ETS event. The findings contained herein come shortly after the successful application of a similar matched filter methodology in Western Japan where a number of VLFEs were also successfully detected (Baba et al., 2018, ). The high temporal resolution of these VLFE catalogs show a significant increase in VLFE activity during ETS events that drops off immediately before and after the ETS period. A comparison of VLFE activity to GPS data shows that VLFE tracks slow slip, even when tremor is not occurring or is behaving anomalously during ETS periods. Finally, we find continued VLFE activity during the inter-ETS period for all template events, though the extent of reactivation of VLFE asperities and their spatiotemporal coincidence with tremor is varied among the template events.