Shear velocities in Cascadia from seafloor compliance measurements

Compliance is defined as the transfer function between the vertical displacement of the seafloor and the corresponding pressure expressed as a function of frequency. It is sensitive to the elastic parameters of the underlying sediments, particularly the shear modulus. We have measured normalized compliance from 0.001 to 0.049 Hz, using ocean surface gravity waves as a source, at sites in Cascadia near the Ocean Drilling Program (ODP) hole 889B. A differential pressure gauge, datalogger and self‐levelling gravimeter were lowered to the seafloor and each site was occupied for eight hours. The compliance estimates are reproducible and are consistent with other available data and simple models of sediment physical properties. Shear strength is increased from a normal profile in the uppermost few hundred meters, possibly an effect of the presence of a known hydrate layer, and is decreased between 1.2 and 2 km below the seafloor, suggesting the presence of a low velocity zone.