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

Abstract

Fossils and tidal deposits as well as the possibility to compute values of the lunar tidal torque for different geological epochs allow us to model the variations in time of the Earth's figure, assuming that the latter remains, on a global scale, close to a hydrostatic equilibrium figure. On this basis we were able to infer the variations of the Earth's most important kinetic parameters over much of the geological past. Thus, the geometrical oblateness of the outer surface has decreased 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 °C and the rheological behaviour is likely to remain brittle over geological time scales. We investigate the inevitable tectonic consequences of this stress buildup, and try to find some evidence in present-day worldwide seismicity, with a negative result. An interesting result of our study, which may open a new field of gravimetric research, is embodied in the fact that tidal friction causes a secular increase 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 observational reach of superconducting gravimeters.