Amplification and Tuning of Ground Motion at the Outer Cascadia Accretionary Prism

Ground‐motion amplification in the outer Cascadia subduction zone accretionary prism has been documented previously by comparing earthquake and microseismic vertical ground‐motion and pressure records from an ONC/NEPTUNE Canada cabled observatory site on the outer prism to those from a site on the flank of the Juan de Fuca Ridge. Since then, four additional instruments became operational, and data, now spanning 10 years, include more than 100 large (Mw > 7) distant earthquakes. Well‐tuned response at the outer prism sites is observed in both vertical and horizontal components, with peaks in the spectral ratio of vertical velocity relative to nearby ocean crustal and continental bedrock sites (V/Vref) at 9 s, and in the intra‐site horizontal to vertical spectral ratio horizontal‐to‐vertical spectral ratio at 14 s, both with band widths of ±10% to 15% at half amplitude. The vertical response is consistent with ¼ wavelength compressional wave reinforcement, while the horizontal motion tuning most likely reflects the effects of the low velocity prism sediments on surface‐wave propagation. At the periods of maximum relative motion, outer prism surface‐wave vertical and horizontal accelerations are enhanced by factors of up to 15 and 25, respectively. Similar behaviour is seen in microseismic records from these sites, and, to a lesser extent, in earthquake records from temporarily deployed Cascadia Initiative seismometers along the outer Cascadia prism to the south. Such tuning and amplification must be accounted for when assessing the dynamic behavior of the prism and its basal fault at the time of large local earthquakes. Plain Language Summary Sedimentary basins are known to amplify ground motions, a behavior that must be considered in the assessment of seismic hazards. The same is true for subduction zone accretionary prisms, where enhanced seismic shaking can lead to enhanced dynamic stressing at the plate interface of the subduction zone, weakening of the accretionary prism sediments, and triggering of submarine slope failures. Here we examine data from seafloor seismometers deployed on, seaward of, and landward of the outer Cascadia accretionary prism, spanning up to 10 years and including signals from more than 100 large earthquakes. Response of the outer prism to seismic waves shows amplification and tuning, with peaks in anomalous vertical‐ and horizontal‐motion centered at 9 and 14 s periods. At these wave periods, accelerations at the outer prism can