Vertical crustal motions from the DORIS space-geodesy system

Five years (1993-1997) of DORIS data on the SPOT and Topex-Poseidon satelli tes have been analyzed to estimate vertical motions at 33 sites of the perm anent DORIS network. Rates have been determined assuming constant velocity over the 5-year time span. Except for a few stations, vertical rates are lo wer 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 intera nnual to decadal fluctuations affecting the vertical motions, giving rise t o different apparent linear trends if estimated over different time spans. Except for six high-latitude stations, post-glacial rebound produces neglig ible signal and cannot explain the observed motions. Other local or regiona l phenomena from geophysical or anthropogenic origin may be responsible for the estimated vertical motions.