SEA-FLOOR COMPLIANCE OBSERVED BY LONG-PERIOD PRESSURE AND DISPLACEMENT MEASUREMENTS

Ocean surface waves with periods longer than 30 s create periodic, horizontally propagating pressure fields at the deep seafloor. Seafloor displacements resulting from these pressure fields depend on the density and elastic parameters of the oceanic crust. The displacement to pressure transfer function, the seafloor compliance, provides information about ocean crustal density and elasticity, and we outline a linearized inversion method to determine ocean crustal shear velocity from the compliance. By computing compliance partial differences with respect to changes in ocean crust shear velocity, we provide estimates of inversion stability and of the compliance sensitivity to crustal properties. Seafloor compliance, measured from pressure and acceleration spectra, is presented for two different sites: Axial Seamount on the Juan de Fuca Ridge and the West Cortez Basin in the California continental borderlands. The compliances and inverted structure for these two sites show significant differences; in particular, a zone of low shear strength is observed at depth within Axial Seamount, suggesting the presence of at least 3% partial melt within the upper 2500 meters of the ediface. These results suggest that the method provides a useful new geophysical prospecting tool.