Earthquake spectra and near‐source attenuation in the Cascadia subduction zone

Models of seismic source displacement spectra are flat from zero to some corner frequency, f c , regardless of source type. At higher frequencies spectral models decay as f −1 for slow events and as f −2 for fast earthquakes. We show that at least in Cascadia, wave propagation effects likely control spectral decay rates above ∼2 Hz. We use seismograms from multiple small‐aperture arrays to estimate the spectral decay rates of near‐source spectra of 37 small ‘events’ and find strong correlation between source location and decay rate. The decay rates (1) vary overall by an amount in excess of that inferred to distinguish slow sources from fast earthquakes, (2) are indistinguishable for sources separated by a few tens of km or less, and (3) separate into two populations that correlate with propagation through and outside a low‐velocity zone imaged tomographically. We find that some events repeat, as is characteristic of low‐frequency earthquakes (LFEs), but have spectra similar to those of non‐repeating earthquakes. We also find no correlation between spectral decay rates and rates of ambient tremor activity. These results suggest that earthquakes near the plate boundary, at least in Cascadia, do not distinctly separate into ‘slow’ and ‘fast’ classes, and correctly accounting for propagation effects is necessary to characterize sources.