Stacking global GPS verticals and horizontals to solve for the fortnightly and monthly body tides: Implications for mantle anelasticity

Abstract

AbstractThe availability of long‐term position measurements from permanent GPS stations distributed around the Earth makes it feasible to extract small, globally coherent, long‐period geophysical signals from the data. Using 11 years of daily vertical and horizontal positions from over 600 permanent GPS stations worldwide, we solve for the amplitude and phase of the fortnightly and monthly body tides by stacking the surface displacements against spherical harmonics and isolating the fortnightly and monthly signals in the stacks. We use our solutions to help constrain the Earth's anelastic properties, which are not well understood within the tidal frequency band. Our error estimates include the effects of the following: (1) random noise across the fortnightly and monthly tidal bands; (2) fortnightly and monthly ocean tide model errors; (3) errors in the diurnal and semidiurnal ocean tide models that alias into the fortnightly and monthly frequency bands; (4) errors in the solid Earth Green's functions used to compute ocean tidal loading corrections; and (5) leakage from GPS draconitic signals at periods close to the fortnightly and monthly frequencies. The mantle anelasticity coefficients we infer from our solutions are consistent with a ωα frequency dependence of mantle Q with α in the range 0.1–0.3.