Loading Deformation On Various Timescales Using Gps And Grace Measurements

Abstract

Tidal, seasonal and long-term surface mass movements cause the earth to deform and the gravity field to change. Current geodetic satellites, GPS and GRACE, accurately measure these geophysical signals. I examine the effect on GPS solutions of using inconsistent reference frames to model ocean tidal loading (OTL). For seasonal loading, I choose two study areas, Nepal Himalaya and southern Alaska, and compare GPS-measured and GRACE-modeled seasonal hydrological ground loading deformation. Globally distributed stations are employed to compare GPS coordinate solutions with OTL corrections computed in different reference frames: center of mass of the solid Earth (CE), and center of mass of the Earth system (CM). A strong spectral peak at a period of ~14 days appears when inconsistent OTL models are applied along with smaller peaks at ~annual and ~semi-annual periods. Users of orbit/clock products must ensure to use OTL coefficients computed in the same frame as the OTL coefficients used by the analysis centers; otherwise, systematic errors will be introduced into position solutions. Continuous GPS measurements of seasonal deformation in Nepal Himalaya are compared with load model predictions derived from GRACE observations. The GPS seasonal height variation and GRACE-modeled seasonal vertical displacement due to the changing hydrologic load exhibit consistent results, for both amplitude and phase. GRACE indicates a long-term mass loss in the Himalaya region, which leads to crustal uplift since the earth behaves as an elastic body. We model this effect and remove it from GPS observed vertical rates. Then most GPS vertical rates can be explained by interseismic strain from the Main Himalayan Thrust. In southern Alaska, vertical seasonal loading deformation observed by GPS stations and modeled displacements due to seasonal hydrological loading inferred from GRACE are highly correlated. The effects of atmosphere and non-tidal ocean loading are important. Adding the AOD1B de-aliasing model to the GRACE solutions improves the correlation between these two geodetic measurements, because the displacements due to these loads are present in the GPS data. Weak correlations are found for some stations located in areas where the magnitude of the load changes over a short distance, due to GRACE's limited spatial resolution.