Episodic Tremor and Slow Slip

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, https://doi.org/10.1126/science.1084783). Using VLFEs detected from a grid-search centroid moment tensor inversion algorithm in the 2011 (Ghosh et al., 2015, https://doi.org/10.1002/ 2015GL063286) and 2014 (Hutchison & Ghosh, 2016, https://doi.org/10.1002/2016GL069750) 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, https://doi.org/10.1002/2017GL076122). 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.

Source geometry and simple propagation of S waves generated by low- frequency earthquakes (LFEs) in northern Cascadia results in strong waveform coherence on horizontal channels for stations at smaller epicentral distances. As recognized by previous workers, this cross-station similarity can be exploited for detection of LFEs in tremor. We develop a cross-station correlation approach based on waveform coherence and travel-time consistency that exploits a full complement of network stations, and we demonstrate its application to separate arrays on Vancouver Island and Washington state. Our approach yields thousands of impulsive LFE detections per slow-slip episode and hundreds of events between episodes. LFE epicentral distributions reveal the presence of well-defined regions of high asperity density with those regions farther downdip that are active during interepisodic tremor and slow-slip periods. The LFE epicenters also display pronounced spatiotemporal clustering that compares favorably with independent tremor and LFE template catalogs and that defines rapid tremor reversals with complex propagation patterns. The LFE source depths can be estimated for some detections in which P waves are identified on the vertical channel through correlation with the horizontal-component S waveforms. LFE hypocenters cluster between two recently developed plate interface models below southern Vancouver Island. Our approach enables the identification of high signal-to-noise-ratio impulsive LFE detections from targeted regions on the plate boundary for generation of LFE templates to be used in studies of structure and seismogenesis in Cascadia.