Anomalous ocean tide loading displacements in western Europe suggest mantle anelasticity

Using GPS data from over 200 continuously-operating sites in western Europe, at a range of latitudes and distances from the coast, we observe ocean tide loading displacements (OTLD) at a selection of semi-diurnal and diurnal tidal periods. Our processing strategy uses the GIPSY-OASIS II software with reprocessed satellite orbits and clocks, and either (1) 24-hour batch solutions with 3-d harmonic motions estimated at fixed tidal periods and later combined over the 2+ year data span, or (2) kinematic solutions from which 3-d harmonic motions are later estimated over the whole data span. Excepting the K1 and K2 tidal periods which are problematic because of their coincidence with the sidereal rotation period and GPS satellite geometry repeat interval, and GPS orbital period, we obtain good (better than 0.5 mm) agreement between these two strategies, with little sensitivity to the quality of a priori OTLD model used. Compared with OTLD computed from modern numerical ocean tide models convolved with Green's functions derived from the PREM global Earth model, we observe differences in the vertical component of OTLD for the M2 tidal constituent approaching 3 mm in SW England and NW France (nearly 10 per cent of the signal amplitude). These differences are too large to be explained by errors in the numerical ocean tide models, which show good agreement with each other and with local tide gauge data, and too large and widespread to be explained by plausible changes to the Green's functions reflecting regional crustal structure. Instead, they show clear correlation with the contribution to OTLD from ocean tidal signals at distances of 100-200 km, suggesting an asthenospheric upper mantle origin for the discrepancy. OTLD offers a window onto the possible anelastic behaviour of the upper mantle at tidal timescales, which cannot be detected by solid Earth tides that are largely sensitive to lower mantle rheology.