Modern observations of the effect of earthquakes on the Chandler wobble

• The complicated signatures of great earthquakes on the pole path are explored.
• Earthquakes can excite or de-excite the Chandler wobble.
• A very long baseline interferometric (VLBI) polar motion sequence is used.
• The maximum entropy method (MEM) is used to extend the pole path in both directions.
• Chandler and annual components are separated in the discrete Fourier transform (DFT).

Earthquakes have long been postulated as the source of excitation of the Chandler wobble (Mansinha and Smylie, 1967). More recently, the classical astronometric observations of the polar motion have been replaced by very long baseline interferometric (VLBI) observations with an improvement in accuracy by a factor of several thousand. We analyze the record of nearly 29 years of VLBI polar motion observations from the Goddard Space Flight Center.In addition to the Chandler wobble, the polar motion has annual components making the analysis more difficult. The present study extends the polar motion sequence in both directions by the maximum entropy method (MEM). This allows the annual components, both the prograde motion and a weaker retrograde motion, to be identified and removed, leaving a pure Chandler wobble and secular polar shift. In the absence of excitation, the free Chandler wobble is closely a prograde circular motion. Circular arcs are fitted to the pole path, free of the annual components, to determine breaks corresponding to sudden excitations. The event times of earthquakes of magnitude greater than or equal to 7.5 are shown on the plotted pole paths. Often, the effects on the pole path precede the earthquake by many days, confirming the establishment of the far-field displacements in advance of the earthquake. The precursory rise in P-wave attenuation before the 2004 Parkfield earthquake, as discovered by Chun et al. (2010), may indicate a similar effect from local deformations.