Just out in the Journal of Geophysical Research – Solid Earth (accepted articles online), are two articles on ocean tide loading (OTL) research that Nigel Penna, Peter Clarke and collaborators Machiel Bos (University of Beira Interior) and Trevor Baker (formerly at the National Oceanography Centre, Liverpool) have been working on for the last few years (originally funded by NERC).
Paper 1 uses known, synthetic OTL signals inserted in real GPS data to validate the use of kinematic GPS for the recovery of ocean tide loading displacements to within ~0.2 mm amplitude error, using at least 3-4 years’ data, and establishes the importance of correct, simultaneous, estimation of tropospheric delay parameters. The measurements are shown to be robust with respect to typical levels and patterns of data loss, and levels of reference frame noise, and a heuristic relationship between site background noise and OTL measurement error is proposed.
Paper 2 presents OTL measurements at the M2 tidal period across western Europe, and shows how discrepancies of up to 2-3 mm with respect to modelled values cannot be explained by credible errors in the numerical ocean tide models or elastic solid Earth response models. The discrepancies can be explained only by changes in the solid Earth response that correspond to anelastic behaviour in the asthenosphere at tidal periods.This anelastic behaviour corresponds to 8-10% reduction in the shear modulus in the asthenosphere, consistent with an absorption band of constant Q~70 stretching from seismic frequencies (1 Hz) through to the semi-diurnal tidal band (2 cycles per day).
