Cascadia Tremor Spectra from Beamforming Fall Off as Frequency Squared

The spectral decay of non-volcanic tremor (NVT) contains important information about the physical processes involved in Episodic Tremor and Slip (ETS). Using a small-aperture seismic array deployed on Big Skidder Hill, WA, we employ frequency-domain beamforming to obtain spatiotemporal spectral estimates of NVT activity in Cascadia on May 6-19, 2008. By shooting rays up from the plate boundary, potential tremor source locations are found where they best match the beamformer output. While energy from 2.5 Hz up to 25 Hz can radiate simultaneously from different patches, they often tend to come from the same region and migrate from the south to the north. An advantage of using beamforming for spectral analysis is that we can suppress noise and focus on the tremor phase by deriving the cross-power spectrum. This enables extracting tremor spectra for higher frequencies. We then compute the tremor source spectrum by using nearby small earthquakes to estimate empirical path and attenuation corrections. Our results show that displacement spectral amplitudes of the Cascadia tremor fall off roughly as the inverse of frequency squared over 5-20 Hz, agreeing with standard frequency-squared spectral models for earthquakes, but disagreeing with prior tremor analyses that have indicated a falloff proportional to frequency rather than frequency squared.