Ocean tide loading displacements in western Europe: 2. GPS‐observed anelastic dispersion in the asthenosphere

Abstract

GPS‐observed vertical ocean tide loading displacements show in Cornwall, southwest England, and in Brittany, northwest France, discrepancies of 2–3 mm with predicted values based on the isotropic Preliminary Reference Earth Model for the main tidal harmonic M2, yet in central Europe the agreement is better than 0.5 mm. By comparison of ocean tide models and validation with tide gauge observations, we demonstrate that the uncertainties in the former are too small to cause this disagreement. Furthermore, we find that different local models of the crust and different global elastic reference models derived from seismological observations can only reduce the observed discrepancies to 1–2 mm, which still exceeds the GPS observational uncertainty of 0.2–0.4 mm. It is customary to use the elastic properties of the Earth as given by seismic models. Previously, there has been insufficient evidence to determine how to modify these properties during the transformation from seismic to tidal frequencies to account for possible anelastic dispersion in the asthenosphere, and so this effect has been ignored. If we include this effect, then our discrepancies reduce further to 0.2–0.4 mm. This value is of the same order as the sum of the remaining errors due to uncertainties in the ocean tide models and in the GPS observations themselves. This research provides evidence in western Europe of a reduction of around 8–10% of the seismic shear modulus in the asthenosphere at tidal frequencies. In addition, we find that the asthenosphere absorption band frequencies can be represented by a constant quality factor Q.

Ocean tide loading displacements in western Europe: 1. Validation of kinematic GPS estimates

“…For example, using the simultaneous tropospheric approach, at CAMB a residual 12.42 h period M 2 height signal of 2.9 mm amplitude is obtained, which is explained geophysically by Bos et al . [] to arise from deficiencies in the Gutenberg‐Bullen Green's function used here, whereas it has been erroneously reduced to only 0.5 mm amplitude in the a priori fixed tropospheric solution. It can therefore be concluded that while coordinate time series noise reductions may be obtained using the a priori fixed tropospheric approach of Reuveni et al .…”

“…Crucially, however, M 2 displacement amplitudes with the optimum tuned settings remain at East 0.55 mm, North 0.55 mm, and height 2.88 mm, regardless of the amplitude of the input synthetic displacement (the maximum error, i.e., vector difference, obtained is 0.16 mm), strongly suggesting that these tuned coordinate and ZWD process noise values enable accurate 0–6 mm amplitude semidiurnal and diurnal periodic tidal ground displacements to be detected to better than 0.2 mm accuracy, and that the ~3 mm amplitude M 2 height signals in southwest England, considered by Bos et al . [] are genuine, are not caused by GPS processing error, and may be interpreted geophysically. The tests illustrated for CAMB were repeated for LERW, HERT, and ZIMM, with the same trends and same optimum process noise values found.…”

LoadDef: A Python‐Based Toolkit to Model Elastic Deformation Caused by Surface Mass Loading on Spherically Symmetric Bodies

“…The forcing functions that generate the tides are well known from astronomical ephemerides (e.g., Meeus, 1998;Melchior, 1983). Body and load tides therefore provide valuable opportunities to investigate the interior structures of astronomical objects (e.g., Bos et al, 2015;Ito & Simons, 2011;Khan & Connolly, 2008;Lau et al, 2017;Métivier & Conrad, 2008). Body and load tides therefore provide valuable opportunities to investigate the interior structures of astronomical objects (e.g., Bos et al, 2015;Ito & Simons, 2011;Khan & Connolly, 2008;Lau et al, 2017;Métivier & Conrad, 2008).…”

Constraints on Transient Viscoelastic Rheology of the Asthenosphere From Seasonal Deformation

“…Then, the elastic structure of the Earth at time scales up to a few years has also been questioned independently by tidal studies using GNSS observations. Bos et al (2015) proposed a reduction of the shear modulus of ∼10% in the asthenosphere for western Europe at the M2 tidal frequency, also suggested by Goes et al (2000) at seismic frequencies. These results are in agreement with ours, regardless of the global Earth average asthenospheric viscosity derived from our study.…”