On the interpretation of gravity tide residuals in the Iberian Peninsula

The interpretation of tidal gravity final residuals is an issue of high debate in geodesy. Whereas some authors suggest that final residuals are related to possible instrumental and measurement errors or to errors in the evaluation of the oceanic load computations, others propose certain empirical relationships between the gravity tidal residuals and the heat flow, or the tectonothermal age of the lithosphere which, in turn, reveals structural variations with respect to a homogeneous elastic model. We examine such relationships in the Iberian Peninsula by studying correlations between the cosine component of the final residual vector for the M₂ and O₁ waves of the gravity tide potential and different structural parameters for a total of 21 gravity tide stations. The selected parameters, which are related to the capability of the lithosphere to deform, include surface heat flow as the main parameter and, to a lesser extent, lithospheric strength, Moho temperature and tectonothermal age of the lithosphere. Our study agrees with previous results by other authors and does not demonstrate the existence of empirical relationships between the gravity residuals and structural parameters. Actually, when we consider the most precise observations and recent ocean tide models the final residuals are mostly uncorrelated noise. Our results also agree with theoretical studies proposing that the distortions of the tidal gravity field from spherical symmetry are very small and hardly to observe.