TIDAL FRICTION AND ITS CONSEQUENCES IN PALAEOGEODESY, IN THE GRAVITY-FIELD VARIATIONS AND IN TECTONICS

Fossils and tidal deposits as well as the possibility to compute value s of the lunar tidal torque for different geological epochs allow us t o model the variations in time of the Earth's figure, assuming that th e latter remains, on a global scale, close to a hydrostatic equilibriu m figure. On this basis we were able to infer the variations of the Ea rth's most important kinetic parameters over much of the geological pa st. Thus, the geometrical oblateness of the outer surface has decrease d from 0.005 to 0.003 over the last two and a half billion years. This slow but continuous change of the Earth's curvature brought about by tidal friction must have led to continuous stress accumulation in the uppermost part of the lithosphere, where the temperature is below 400 degrees C and the rheological behaviour is likely to remain brittle ov er geological time scales. We investigate the inevitable tectonic cons equences of this stress buildup, and try to find some evidence in pres ent-day worldwide seismicity, with a negative result. An interesting r esult of our study, which may open a new field of gravimetric research , is embodied in the fact that tidal friction causes a secular increas e of the Earth's normal gravity component at the equator at a rate of about 2 ngals yr(-1), and a concomitant decrease at the poles of about 0.5 ngals yr(-1) This tiny secular signal may just lie within observa tional reach of superconducting gravimeters. (C) 1997 Elsevier Science Ltd.