ABSTRACT

Ocean surface gravity waves produce temporally correlated pressure and vertical displacements on the ocean floor. Compliance is the ratio between the pressure and the resulting displacement. Pressures are measured by a differential pressure gauge, whereas a broadband seismometer measures velocities that are directly related to the displacements in the frequency domain. These instruments are installed on the Cascadia accretionary prism as part of NEPTUNE Canada’s ocean-floor network. Gas hydrates have the property of stiffening marine sediment, i.e., reducing the compliance. A compliance measurement is dependent on the elastic parameters of the sediment, particularly, the shear modulus. We have computed compliance from raw data, daily, for 203 days and determined the temporal variation in shear modulus of a region approximately 390 m northeast of the undersea area known as Bullseye Vent. Our aim was to investigate the evolution of the elastic properties of the subsurface in this region. The compliance was determined over a bandwidth of 0.01–0.03 Hz, and had peak sensitivity to the sediment elastic structure at approximately 275 m below seafloor, for a water depth of 1250 m — determined by the gravity wave dispersion relation. Bullseye Vent is not a steady-state system! We observed a linear decrease in compliance of approximately 1% between October 2010 and May 2011, corresponding with an average change of 1% in the shear modulus over the depth of investigation. This corresponds to up to a 13% change solely in gas hydrate content in the first 100 m; a change that resulted from a simplified 1D four-layer model, which does not reflect the complexity and three-dimensionality of the hydrate system. A 13% change in hydrate content was likely an overestimate because the compliance function at 1250 m had limited sensitivity to the uppermost sediments.

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