DID THE 1977 SUMBA EARTHQUAKE EXCITE THE CHANDLER-WOBBLE

The potential role of large earthquakes on the excitation of the Earth 's free wobble has been the subject of various investigations. It is n owadays recognized that the coseismic moment released by large earthqu akes is not large enough to maintain the wobble, and that they cumulat ively only cause a very modest secular motion of the pole. Nevertheles s, renewed attention was paid to this subject following the observatio n of a sharp jump in the Lageos-derived polar motion excitation functi on at the time of the great 1977 Sumba earthquake. The static dislocat ion 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 co uld reconcile theoretical predictions with observations. Geological ev idence suggests that the 1977 Sumba earthquake was caused by a slab pu ll force acting along the subducting lithosphere, the main shock was f ollowed by a down-dip motion of the slab. We estimate quantitatively t he potential role of this aseismic mass redistribution. We find that t he 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 pol ar motion data sets. However, we also show that the short-term inertia variations due to the slab movements are considerably larger than tho se due to the main rupture, thus possibly making earthquakes produced by slab pull forces an efficient mechanism for exciting the Chandler w obble. (C) 1997 Elsevier Science B.V.