Using seafloor pressure gauges and satellite altimetry data to constrain deformation models in a partially locked subduction zone, Central Vanuatu

Subduction zone deformation is difficult to fully constrain, as the plate boundary generally lies in deep waters, preventing the use of common GNSS based methods. While the subduction zone plate boundary in Central Vanuatu is indeed submerged, two shallow seamounts located on either side of the plate limit allow us measure vertical motions on both sides, using diver-installed seafloor pressure gauges. In 1999, we installed one gauge on the subducting plate, at Sabine Bank, and the other one on the overriding plate, at Wusi Bank. The two gauges have been recording nearly continuously since 1999. The signal we are looking for is small and on the same order of magnitude than sea level changes; thus we use satellite altimetry as an external reference to monitor sea surface changes. The Sabine Bank site is close to an ERS/Envisat crossing point, while the Wusi Bank site is close to a crossover point for Topex-Poseidon, Jason and Jason2. Seafloor pressure, converted to depth, and satellite altimetric measurements of the sea surface are combined to derive the height above ellipsoid of both seafloor points. In addition, kinematic GPS data on a floating platform above the pressure gauges, contribute to the calibration of the measurement system. We show the vertical deformation and uncertainties obtained at both underwater points and use these results to complement the deformation profile perpendicular to the plate boundary, obtained with on-land GPS measurements. The whole profile, comprising at sea and on land measurements is then modeled in 3D to investigate the role played by topographic features lying on the subducting plate on the locking of the subduction and on the vertical uplift of the overriding plate, which is responsible for the existence of both Santo and Malekula, the two largest islands of the Vanuatu archipelago.