Limitations in one-dimensional (an)elastic earth models for explaining GPS-observed M2 ocean tide loading displacements in New Zealand

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 compare these to modeled values 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 across the North Island where they reach 0.4 mm (2%). The best fit in the North Island is obtained when combining the FES2014b tide model with spatially variable ocean density and water compressibility, and the STW105 Earth model. The residuals exhibit a change of ∼0.3 mm in magnitude between the Taupo Volcanic Zone and the east coast (∼100 km), suggesting that this region's laterally varying, shallow rheological structure may need to be considered to explain these observations.

Plain Language Summary

The solid Earth changes shape due to the changing weight of the ocean as the ocean tides rise and fall. Measuring this change and comparing it to predictions can yield insights into the interior properties of the Earth, tens to hundreds of kilometers below the surface. We used GPS to measure the changing shape of New Zealand and compared it with predictions based on a range of Earth and tide models. The difference between the observed and modeled displacements revealed a complicated pattern over New Zealand, especially in the North Island and specifically near the Taupo Volcanic Zone. Due to the high accuracy of our GPS analysis and the ocean tide models, the observed residuals provide information about the elastic properties of the Earth and the complex geological structure of the region. The observed significant misfits show limitations of the 1D Earth model that varies only with depth which is standard in geodetic analysis.