M2 Ocean Tide Loading displacements in New Zealand with GPS

Abstract:

GPS observations of ocean tide loading displacements can help infer the regional anelastic properties of the asthenosphere. We estimate M₂ ocean tide loading displacements at 170 GPS sites in New Zealand and model these using a range of numerical tide and radially symmetric (1D) elastic and anelastic Earth models. Regardless of the model combination we are unable to reduce the strong spatial coherence of the M₂ residuals in the North Island where they reach 0.4 mm. The best fit is provided with FES2014b tide model, spatially-variable ocean density with water compressibility and S362ANI earth model. The residuals exhibit ~0.3 mm magnitude change between the Taupo Volcanic Zone and the east coast (~100 km), and this suggests that this region’s laterally varying, shallow rheological structure may need to be considered to explain these observations. As such, until such three-dimensional models are available, residual ocean tide loading displacements will propagate into long-period GPS time series noise in this region.