Changes in the Earth's rotation and low-degree gravitational field induced by earthquakes

The change in the Earth's density distribution caused by an earthquake dislocation will change the Earth's rotation and gravitational field. In this paper we first develop the analytical formulae based on the normal-mode theory. Equipped with these formulae and using a spherically symmetric earth model (1066B) and the centroid-moment tensor solutions for earthquake sources, we then compute the earthquake-induced changes in the Earth's rotation (polar motion and length of day) and low-degree harmonics of the gravitational field for the period 1977–85 (altogether 2146 earthquakes). We then conduct simple spectral and statistical analyses on these changes. Major findings are that: (1) the earthquake-induced changes computed are, in general, two orders of magnitude smaller than the observed that are available. (2) Most of these changes show strong evidence of non-randomness either in their polarity or in their directions. (3) The parameters that show the strongest non-randomness are the dynamic oblateness J2, the total moment of inertia, the length of day, and the sum of as well as the difference between the two equatorial principal moments of inertia. They are all inclined towards negative changes, indicating the tendency of earthquakes to make the Earth rounder, and to pull in mass toward the centre of the Earth. (4) The earthquakes continue to make the rotational pole drift towards a preferred direction of ˜150°E. This direction is roughly the opposite to that inferred from polar motion observations. (5) The earthquake-induced changes in the polar principal moment of inertia are considerably larger than those in the two equatorial counterparts combined. (6) The earthquake-induced changes in higher zonal harmonics that we have computed (J3, J4, J5) are much smaller than that in J2; and they show little sign of non-randomness. (7) The above behaviours appear to be independent of time and the size of the earthquake causing the changes. These findings, as well as the geophysical questions they raise, can presumably only be explained in terms of and in conjunction with the dynamics of plate tectonics.

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