Cooperative Institute for Research in Environmental Sciences

Estimated Shallow Crustal Shear Velocity Structure Off the South Island, New Zealand from Seafloor Compliance Measurements

Justin S. Ball (1), Anne F. Sheehan (2)

(1&2) CIRES and Geological Sciences Department, University of Colorado, Boulder

Ocean surface gravity wave energy at sufficiently long periods can propagate to the seafloor producing a time-varying pressure load. The transfer function between applied pressure and resulting displacement fields measured at the seafloor is known as the seafloor compliance and depends strongly on the shear structure beneath the measurement site. The Marine Observations of Anisotropy Near Aotearoa (MOANA) experiment in 2009-2010 deployed 30 broadband ocean bottom seismometers (OBS) with collocated differential pressure gauges (DPGs) off the South Island of New Zealand with the goal of using regional anisotropy measurements to quantify the strain field at depth and elucidate the mantle rheology. Teleseismic methods of resolving anisotropy such as receiver function analysis and shear-wave splitting depend on high signal-to-noise ratios that are difficult to achieve at OBS sites due largely to the effects of low-velocity sediments. Methods of removing sediment effects from teleseismic data typically require accurate estimates of sediment column velocities, to which seafloor compliance is sensitive. Preliminary compliance curves were calculated from pressure and acceleration power spectra at periods between 33-500s and normalized by the coherence between pressure and vertical acceleration signals to suppress non-infragravity noise sources. The resulting compliance values range from 10^-10 - 10^-8 Pa^-1 and are sensitive to changes in basement shear modulus at depths that increase with forcing period. Data uncertainties increase with depth to approximately 5% at 7km. 1D forward velocity models with a depth range of 100-7000m are used to calculate synthetic compliance curves and the best-fitting model for each station is presented. The use of these models for the removal of sediment-converted S-phases from teleseismic receiver functions is demonstrated. Additionally, time-varying compliance observations at one station corresponding with anomalous long-period pressure signals are interpreted as possible gas migration events.