Seafloor tilt induced by ocean tidal loading inferred from broadband seismometer data from the Cascadia subduction zone and Juan de Fuca Ridge

Mass-balancing voltages from four buried broadband seismometers connected to the NEPTUNE Canada seafloor cable are being recorded at 24-bit resolution. Sites are located on the Vancouver Island continental shelf, the nearby Cascadia accretionary prism, the eastern flank of the Juan de Fuca Ridge, and the western flank close to the Juan de Fuca Ridge axis. Tidal variations are present throughout the records. Variations in vertical acceleration at three of the sites match predicted gravitational attraction variations very well; those at the fourth site show a small residual that is probably caused by sensitivity to tilt resulting from sensor inclination. Horizontal accelerations, which at tidal periods are sensitive primarily to tilt, are anomalously large relative to standard-earth model results. After removal of predicted tidal body and ocean attraction and loading terms, the residuals are seen to follow ocean pressure variations. Responses range from 0.4 μrad dbar−1 (0.04 μrad kPa−1) at 10° true (down under positive load) at the continental shelf site, to 0.6 μrad dbar−1 at 243° at the Cascadia prism, 0.4 μrad dbar−1 at 90° at the eastern Juan de Fuca Ridge flank, and 0.2 μrad dbar−1 at 116° true on the western ridge flank. Except at the continental shelf site, tilts are roughly perpendicular to structural strike. The tilt observations can be explained by loading-induced deformation in the presence of local lithologic gradients or by the influence of faults or structurally controlled anisotropic elastic properties. The observations highlight the utility of using mass position data from force-feedback broad-band seismometers for geodynamic studies.