Characteristics of very low frequency earthquakes (VLFEs) in Cascadia

Abstract:

Very low frequency earthquakes (VLFEs) are rich in 20-50 sec energy and depleted in higher frequencies compared to their regular counterparts. At the downdip edge of the subduction zone, they are often associated with tectonic tremor and slow slip, and thought to be a seismic manifestation of slow earthquakes [Ghosh et al., 2015; Ito et al., 2009]. They are, however, notoriously elusive and found only in a handful of subduction zones. We used a grid-search moment tensor inversion method to detect and locate VLFEs during multiple episodic tremor and slip events in Cascadia. Their magnitudes range between Mw 3.3 and 3.9. They are located in areas with elevated tremor activity and migrate with tremor with the progression of the episodic tremor and slip event. The correlation of spatiotemporal distribution between tremor and VLFEs suggest a causal relationship. To increase the detection level of VLFEs, a match-filter technique is applied. We are using the detected VLFEs as template events to find similar repeating events using a cross-correlation method over a subset of the seismic network. Preliminary results indicate multiple repeats over an episodic tremor and slip events. Such repeats demonstrate response of fault asperities generating VLFEs to the dynamic stress loading during the passage slow slip. Investigations of VLFEs and its source properties are giving valuable insights into the dynamics of fault slip.