Reevaluation of the Chandler wobble seismic excitation from recent data

Abstract

Since 1977, seismic moment tensors of major earthquakes have been systematically determined while high accuracy polar motion data have been made available from space techniques. The seismic excitation of the Chandler wobble during the period 1977–1983 has been obtained using the centroid moment tensor solutions of 1287 strong and moderate earthquakes occurring during that period. Changes in the Earth inertia tensor induced by earthquakes, which are derived from the elastic theory of dislocation, are assumed to be step functions of time. From a comparison between the derived synthetic Chandler wobble and the observed one, it turns out that seismic excitation is far too small (by about two orders of magnitude) to explain the Chandler wobble amplitude variations during 1977–1983, in particular the increase in the wobble amplitude between 1980 and 1983. During the last two decades, seismic moments of major earthquakes remained on the level of 10 27 to 10 28 dyne cm, whereas in the past, events of high magnitudes corresponding to seismic moments on the order of 10 30 to 10 31 dyne cm have been frequently reported. As the later values are those needed to explain the amplitude variations of the Chandler wobble, the problem of the validity of magnitude estimations in the past and frequency of occurrence of great earthquakes is raised, as well as the question of validity of previous positive conclusions on the influence of earthquakes on the Chandler wobble excitation.