Secular changes in Earth's shape and surface mass loading

Abstract

The changing distribution of surface mass (oceans, atmosphere, hydrology and cryosphere) causes detectable changes to the solid Earth’s shape on timescales from hours to millennia. Transient changes in Earth’s shape can be readily identified, but the tectonic plate movements and Glacial Isostatic Adjustment (GIA) will also influence the secular trends of Earth’s shape. To analyse secular trends in surface mass loading, these two confounding factors must be quantified. A suite of GPS-derived surface loading models, including both secular and transient terms is presented. Raw velocities are estimated from over 10 years of high quality combined global GPS position solutions, submitted as part of the first International GNSS Service (IGS) reprocessing campaign. A fiducial-free network approach is used with attention to estimating linear offsets and periodic signals. Consideration is given to realistic formal errors for station coordinates. A robust method is used for estimating horizontal and vertical linear velocities for all stations. Tests of the reprocessed data quality show that there is a dramatic improvement of the RMS of the weekly combined global network in comparison to the operational data used previously. The estimated Helmert transformations, when aligning the reprocessed frame to the IGS05 reference frame also show the stability and homogeneity of the new dataset. This permits a more precise estimate of individual station velocities, ~75% reduction to variability of Helmert parameters. Several a priori GIA models are applied to produce corresponding plate velocity estimates, leaving a range of computed residual surface displacements. Present-day surface mass loading is estimated from these residuals, using gravitationally consistent mass-conserving basis functions. GIA models are assumed to be error-free, so only nominal formal errors, with a white noise assumption, can be calculated, these will be adjusted to produce a realistic uncertainty value. Surface mass loading estimates show significant secular mass loss in Alaska and Greenland. The Greenland values (-140Gt/yr, 1999-2010) fall within published GRACE gravity mission values (-66 to -248Gt/yr, 2002-2009).