Earth, Ocean and Atmospheric Sciences

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.