POSTGLACIAL REBOUND FROM VLBI GEODESY - ON ESTABLISHING VERTICAL REFERENCE

Difficulty in establishing a reference frame fixed to the earth's inte rior complicates the measurement of the vertical (radial) motions of t he surface. I propose that a useful reference frame for vertical motio ns is that found by minimizing differences between vertical motions ob served with VLBI [Ma and Ryan, 1995] and predictions from postglacial rebound predictions [Peltier, 1995]. The optimal translation of the ge ocenter is 1.7 mm/yr toward 36 degrees N, 111 degrees E when determine d from the motions of 10 VLBI sites. This translation gives a better f it of observations to predictions than does the VLBI reference frame u sed by Ma and Ryan [1995], but the improvement is statistically insign ificant. The root mean square of differences decreases 20% to 0.73 mm/ yr and the correlation coefficient increases from 0.76 to 0.87. Postgl acial rebound is evident in the uplift of points in Sweden and Ontario that were beneath the ancient ice sheets of Fennoscandia and Canada, and in the subsidence of points in the northeastern U.S., Germany, and Alaska that were around the periphery of the ancient ice sheets.