Abstract:We present a joint inversion analysis of the GPSderived radial (vertical) deformation and GRACEderived gravity rates in North America to examine whether the icesheet models (ICE5G and ICE3G) can fit the satellite based observations. The results show that the existing icesheet models can not correctly predict the concentration of gravity change observed by GRACE in two areas of west and southeast of Hudson Bay. Tests of modifying ICE5G show that a reduction of ice thickness by about 20% in the west area of Hudson Bay and an increase by about 40% in the southeast are required to fit both observed vertical deformation and gravity changes by GPS and GRACE.The results of joint inversion demonstrate that the estimated effective lithosphere thickness (150 km, under a statistical confidence level of 80%) is apparently larger than that valuation (chouq 90 km) inferred from relative sealevel data, while the upperand lowermantle viscosity estimates (3.7 × 1020 Pa s; 1.9 × 1021 Pas; 90% confidence level) are about 20% smaller than those derived from sealevel indicators.