Resonant seismic and microseismic ground motion of the Cascadia subduction zone accretionary prism and implications for seismic velocity

Seafloor pressure and seismic observations have been made along a transect of sites off southwestern Canada using connections to the NEPTUNE Canada cabled network beginning in the fall of 2009. A comparison of the vertical ground motion response to oceanographic and seismic loading at a site on the outer Cascadia subduction zone accretionary prism to that at a site on the adjacent Juan de Fuca Plate shows generally stronger ground motion at the prism site across the full bandwidths of infragravity waves and microseisms and a strong sharp peak in the relative response at a period of 9 s. This peak is seen in the response to loading by local storm waves and dispersive swell sequences, as well as in the average response to storm‐ and swell‐generated pressure fluctuations averaged over long periods of time. Tuned response to teleseismic surface waves is also seen at the same frequency. We infer that this behavior results from quarter‐wavelength harmonic resonance of the prism, with the two‐way travel time of compressional waves between the seafloor and underlying igneous crust being one half the resonance period. The consistency of the anomalous spectral peak from year to year at this particular site suggests that the behavior might be used to track small (≈1%) changes in the vertical seismic velocity of the prism if variations related to strain or pore fluid pressure changes through a subduction thrust earthquake cycle were to occur. Key Points Nine seconds resonant seismic ground motion observed at Cascadia subduction zone prism Resonance inferred to reflect quarter‐wavelength compressional wave harmonic Tightly constrained resonant peak useful for monitoring prism seismic velocity