Tectonic tremor can be used to constrain seismic wave attenuation for use in ground motion prediction equations in regions where moderately sized earthquakes occur infrequently. Here we quantify seismic wave attenuation by inverting tremor ground motion amplitudes in different frequency bands of interest, to determine frequency dependence of and spatial variations in seismic wave attenuation in Cascadia. Due to the density of tremor data, we are able to resolve along-strike variations in the attenuation parameter. We find that tectonic tremor exhibits the frequency dependence expected for attenuation, as determined from ground motion prediction equations developed from moderate-to-large magnitude earthquakes. This implies that attenuation along these paths is independent of the source mechanism. This study demonstrates that tectonic tremor can be used to provide insight into the physical factors responsible for attenuation and to refine estimates of attenuation for ground motion prediction, thus having important implications for hazard assessment and engineering seismology.Plain Language Summary Earthquake ground motion models use estimates of seismic wave attenuation, that is, the decrease in amplitude of a seismic wave along its path from the earthquake source. Seismic wave attenuation is typically determined by analyzing ground motion from moderate-to-large earthquakes. Yet Cascadia also hosts tremor, a group of many small seismic signals accompanying slow sliding of the subducting plate. Because tremor occurs frequently when compared to regular earthquakes in Cascadia, it presents an opportunity to better refine attenuation parameters for use in ground motion models. We quantify seismic wave attenuation using tremor ground motion amplitudes to determine the extent of regional variations and frequency dependence of seismic wave attenuation in Cascadia. Incorporating spatial modifications and allowing for varying frequencies would increase the accuracy of the ground motion model. We are able to resolve spatial variations in the attenuation parameter along strike in Cascadia and observe the frequency dependence expected for attenuation, as seen in ground motion models developed from moderate-to-large magnitude earthquakes. Hence, we show that tectonic tremor can be used to provide insight into the physical factors responsible for attenuation and refine estimates of attenuation for ground motion models, thus having important implications for seismic hazard assessment. This is especially helpful in regions where moderate-to-large earthquakes are sparse, such as Cascadia.

The Queen Charlotte triple junction offshore British Columbia has existed as an unstable configuration for about 40 Ma (Riddihough, 1982). Historically, the triple junction itself is considered the broad, seismically active region between the Explorer ridge and southern end of the Queen Charlotte fault (i.e., it does not occupy a single point) (e.g.,

High-resolution earthquake locations and structural inversions using body waves rely on precise delay-time measurements. Subsample accuracy can be realized for P waves using multichannel cross correlation (MCCC), as developed by VanDecar and Crosson (1990), which exploits redundancy in pairwise cross correlations to determine delays between similar waveforms in studies of mantle structure using teleseismic sources (common source and multiple stations) and regional studies of structure and seismicity (multiple sources and common station). For regional S waves, alignment is complicated by the additional degree of freedom in waveform polarity that is expressed for sources with different moment tensors. Here, we recast MCCC within a principal component framework and demonstrate the equivalence between maximizing waveform correlation and minimization of various singular value–based objective functions for P waves. The singular-value framework is more general and leads naturally to an MCCC linear system for S waves that possesses an order of magnitude greater redundancy than that for P waves. Robust L1 solution of the system provides an effective means of mitigating outliers at the expense of subsample precision. Residual time shifts associated with higher-order singular vectors are employed in an iterative adaptive alignment that achieves subsample resolution. We demonstrate application of the approach on a seismicity cluster within the northern Cascadia crustal fore-arc.

In the Coast Mountains of western British Columbia, an anomalous seismicity concentration exists near the intersection of the Coast Shear Zone, a major NW-SE trending Eocene-age shear zone that accommodated deformation between the Pacific and North America plates, with the Anahim Volcanic Belt, an ENE-WSW trending zone of volcanic features that decrease in age to the east. To better characterize seismicity in the Coast Mountains, we augment the existing Natural Resources Canada seismicity catalog by applying an automatic detection and location algorithm to both permanent Canadian National Seismic Network stations and temporary stations from the 2005-2006 BATHOLITHS deployment, resulting in 837 relocated events with at least 3 paired P and S phase picks. Double-difference relocation reveals several small-scale linear strands subparallel to the Coast Shear Zone and within the Anahim Volcanic Belt, and three clusters of events striking at a high angle to the Coast Shear Zone that occurred as swarms in 2015 and 2017. First-motion focal mechanisms exhibit extensional and strike-slip faulting. Our observations indicate that most of these events are not associated with surficial processes such as landslides, but rather, we hypothesize that interaction of the Anahim Volcanic Belt and Coast Shear Zone has weakened the lithosphere in this region leading to current-day strain localization and high heat flow that manifest seismicity including swarm-like activity.