LAUSR

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.