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Coseismic Excitation of the Earth’s Polar Motion

Published online by Cambridge University Press:  12 April 2016

B.F. Chao  and
R.S. Gross
B.F. Chao
Affiliation:
Space Geodesy Branch, NASA Goddard Space Flight Center Greenbelt, Maryland 20771, USA
R.S. Gross
Affiliation:
Jet Propulsion Laboratory, California Institute of Technology Pasadena, California 91109, USA

Abstract

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Apart from the “shaking” near the epicenter that is the earthquake, a seismic event creates a permanent field of dislocation in the entire Earth. This redistribution of mass changes (slightly) the Earth’s inertia tensor; and the Earth’s rotation will change in accordance with the conservation of angular momentum. Similar to this seismic excitation of Earth rotation variations, the same mass redistribution causes (slight) changes in the Earth’s gravitational field expressible in terms of changes in the Stokes coefficients of its harmonic expansion. In this paper, we give a historical background of the subject and discuss the related physics. We then compute the geodynamic effects caused by earthquakes using Chao and Gross’ (1987) formulas based on Gilbert’s (1970) normal-mode summation scheme. The effects are computed using the centroid moment tensor (CMT) solutions for 15,814 major earthquakes from Jan., 1977, through Feb., 1999, as provided in the Harvard CMT catalog. The computational results update those of Chao and Gross (1987) and Chao et al. (1996), further strengthening their findings and conclusions: (i) the strong tendency for earthquakes to make the Earth rounder and more compact (however slightly) continues; (ii) so does the trend in the seismic “nudging” of the rotation pole toward the general direction of ~ 140°E, roughly opposite to that of the observed polar drift, but two orders of magnitude smaller in drift speed.

Type
Part 4. Long-term Polar Motion
Information
International Astronomical Union Colloquium , Volume 178: Polar Motion: Historical and Scientific Problems , 2000 , pp. 355 - 367
Copyright
Copyright © Astronomical Society of the Pacific 2000

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