A Study of Low-Frequency Earthquake Magnitudes in Northern Vancouver Island

Abstract:

Tectonic tremor and low frequency earthquakes (LFE) have been extensively studied in recent years in northern Washington and southern Vancouver Island (SVI). However, less attention has been directed to northern Vancouver Island (NVI) where the behavior of tremor and LFEs is less well documented. NVI represents an important warm endmember on the spectrum of global subduction zones, and the characterization of tremor in this region could afford important clues on the physical properties that govern slow slip behaviour. We investigate LFE properties in NVI by assembling templates using data from the POLARIS-NVI experiment. The POLARIS-NVI experiment comprised 27 broadband seismometers arranged along two mutually perpendicular arms with an aperture of ~60 km centered near station WOS (lat. 50.16, lon. -126.57). It recorded two ETS events in June 2006 and May 2007, each of duration less than one week. For these two episodes, we constructed 68 independent, high signal to noise ratio LFE templates representing spatially distinct asperities on the plate boundary in NVI, along with a catalogue of more than 30,000 detections. The precisely located LFE templates include simple direct P-waves and S-waves at many stations thereby enabling magnitude estimation of individual LFE detections. After correcting for radiation pattern, 1-D geometrical spreading, attenuation and free-surface magnification, we solve a large, sparse linear system for 3-D path corrections and LFE magnitudes for all detections corresponding to an individual LFE template. NVI LFE magnitudes range up to 2.54, and like southern VI are characterized by high b-values (b~6.2). In addition, we will quantify NVI LFE moment-duration scaling and compare with results for SVI (where LFE moments appear to be controlled by slip, largely independent of fault area), Tokai, and the San Andreas fault.