SECULAR POLAR MOTION, CRUSTAL MOVEMENTS, AND INTERNATIONAL LATITUDE SERVICE OBSERVATIONS

The latitude variations observed by optical astrometry at five Interna tional Latitude Service (ILS) stations since the beginning of the cent ury are analyzed to estimate the secular motion of the pole and the co rrection of the proper motions of the observed stars. The geophysical model of tectonic plate motions NUVEL-1 is used to correct the observe d latitude drifts, thus referring the derived secular polar motion to the ''no net rotation'' terrestrial reference system. It is shown that one of the ILS stations, Ukiah (near the U.S. west coast), very proba bly exhibits an anomalous drift which differs significantly from the m otion of both the Pacific and North American plates.