Vertical crustal motions from the DORIS space-geodesy system

Five years (1993-1997) of DORIS data on the SPOT and Topex-Poseidon satellites have been analyzed to estimate vertical motions at 33 sites of the permanent DORIS network. Rates have been determined assuming constant velocity over the 5-year time span. Except for a few stations, vertical rates are lower than 5 mm/yr. Comparison with solutions provided by GPS, SLR or VLBI at colocated sites shows good agreement for most stations. Some space-geodesy vertical rates however disagree significantly. This may result from interannual to decadal fluctuations affecting the vertical motions, giving rise to different apparent linear trends if estimated over different time spans. Except for six high-latitude stations, post-glacial rebound produces negligible signal and cannot explain the observed motions. Other local or regional phenomena from geophysical or anthropogenic origin may be responsible for the estimated vertical motions.