Spatial spectral inversion of the changing geometry of the Earth from SOPAC GPS data

Abstract

Estimates of station displacements, derived from global GPS networks, contain valuable information about surface deformations. For this reason, we performed a temporal surface spherical harmonics (SH) expansion (up to degree-11) solely for vertical station variations. Our goal was to estimate stable height displacement coefficients, which in turn can be converted into equivalent height coefficients of a column of seawater. New in our approach is a new data set, plus a new method of creating constraints to deal with the truncation and spatial aliasing problem. To achieve our goal, the data of 675 stations were obtained from the Scripps Orbit and Permanent Array Center (SOPAC). To overcome aliasing effects, high-frequency spatial local variations, and high correlations between the estimated harmonic coefficients, we performed an iterative 2D-interpolation approach, based on input from low-degree spherical harmonic coefficients, with a spatial resolution of 10 deg for longitude and latitude. Considering the assumption that the radius of a spherically symmetric Earth model is not supposed to change because of Earth’s constant mass, we tested two options: one with and one without setting degree-0 to zero. One half of the amplitude of degree-0 coefficient will be absorbed by the zonal degree-2 coefficient, if the former is not estimated. This study agrees with recent results on higher load coefficients published in Kusche and Schrama (2005).