DID THE 1977 SUMBA EARTHQUAKE EXCITE THE CHANDLER WOBBLE?

Abstract

The potential role of large earthquakes on the excitation of the Earth's free wobble has been the subject of various investigations. It is nowadays recognized that the coseismic moment released by large earthquakes is not large enough to maintain the wobble, and that they cumulatively only cause a very modest secular motion of the pole. Nevertheless, renewed attention was paid to this subject following the observation of a sharp jump in the Lageos-derived polar motion excitation function at the time of the great 1977 Sumba earthquake. The static dislocation theory proved unable to explain these abrupt variations simply in terms of coseismic moment release, and it was suggested that aseismic movement of blocks of ruptured lithosphere following the main shock could reconcile theoretical predictions with observations. Geological evidence suggests that the 1977 Sumba earthquake was caused by a slab pull force acting along the subducting lithosphere, the main shock was followed by a down-dip motion of the slab. We estimate quantitatively the potential role of this aseismic mass redistribution. We find that the polar motion excitation due to the motions of ruptured lithosphere is not compatible with the Lageos-observed excitation function, being in better agreement with the excitation functions derived by other polar motion data sets. However, we also show that the short-term inertia variations due to the slab movements are considerably larger than those due to the main rupture, thus possibly making earthquakes produced by slab pull forces an efficient mechanism for exciting the Chandler wobble.