Toward a complete catalog of Very Low Frequency Earthquakes (VLFEs) in Cascadia using a Match Filter Technique

Abstract:

During episodic tremor and slip (ETS) events in the Cascadia subduction zone, tremor is accompanied by very low frequency earthquakes (VLFEs) that are responsible for the majority of the total moment release during an ETS event [Ghosh et al., GRL, 2015]. VLFEs characteristically emit energy in the 20-50s frequency range, but release minimal energy in higher frequency bands [e.g., Ito and Obara, GRL, 2006]. They can occur up- and downdip of the seismogenic zone [e.g., Walter et al., GRL, 2013; Asano et al., EPS, 2008] and are thought to be a result of the shear slip process on the subduction fault [Ghosh et al., GRL, 2015]. This study attempts to improve the efficiency and ability to detect VLFEs in Cascadia using a match filter technique [e.g., Shelley et al., Nature, 2007] that can detect events in data despite low signal to noise ratios. VLFE templates are selected from a 2011 ETS event. These template events are identified using a grid-search centroid moment tensor (CMT) inversion method [e.g., Ito and Obara, GRL, 2006], and typically consist of M_w 3.3 - 3.7 events with focal mechanisms consistent with the Cascadia subduction fault [Ghosh et al., GRL, 2015]. The templates are cross-correlated to continuous waveforms from the August 2011 ETS event. Candidate events are identified using six times the median absolute deviation. After eliminating the time windows with earthquakes listed in the Advanced National Seismic System composite catalog, the preliminary findings include a significant number of additional events. To further validate the match filter technique results, a grid-search CMT inversion algorithm is applied, providing focal mechanisms and source locations. Most of the events occur within or south of the Straight of Juan de Fuca, near the template event locations. Candidates with inconsistent focal mechanisms and low variance reduction values are discarded. Because these events have been confirmed with a match filter technique, visual inspection, and finally a CMT method, they can be used in future studies as additional templates to detect other VLFEs for a more robust VLFE catalog. Such a catalog can serve as a resource for a spatial-temporal analysis of their distribution, to inquiry further into the cause and mechanisms involved in slow earthquakes, and to investigate the relationship between tremor and VLFEs.