The change in the Earth's density distribution caused by an earthquake dislocation will change thc Earth's rotation and gravitational field. In this papcr we first develop the analytical formulae based on the normal-mode theory. Equipped with these formulae and using a spherically symmetric earth model (1 066B) 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 harinonics of the gravitational field for the pcriod 1977 4 5 (altogether 2 146 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 nonrandomness eithcr in their polarity or in their directions. ( 3 ) The parameters that show the strongest non-randomness are the dynamic oblateness J 2 , 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 t o make the Earth roundcr, and t o pull in mass toward the centre of the Earth. (4) The earthquakes continue t o make the rotational pole drift towards a preferred direction of -150"E. This direction is roughly the opposite t o that inferred from polar motion observations. (5) The earthquakeinduced changes in the polar principal moment of inertia are considerably larger than those in the two equatorial counterparts combined. ( 5 ) The earthquakeinduced changes in higher zonal harmonics that we have computed ( J 3 , J 4 , J , ) are much smaller than that in J,: and they show little sign of nonrandomness. (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 raisc, can presumably only be explained in terms of and in conjunction with the dynamics of plate tectonics.Suppose we have an arbitrary body lying inside a spherical region of radius a (Fig. I). Let the mass density distribution of the body be p(r). Then the gravitational potential U at any Y J X Figure 1. Geometry of a gravitating body in relation to the field point r o .
Because of the action of various geophysical excitation mechanisms, the Earth does not rotate about its figure axis, so it wobbles as it rotates. Here, the effectiveness of atmospheric and oceanic processes in exciting the Earth's wobbles during 1980–2000 is evaluated using estimates of atmospheric angular momentum from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis project and estimates of oceanic angular momentum from the Estimating the Circulation and Climate of the Ocean (ECCO) consortium's simulation of the general circulation of the oceans. On intraseasonal timescales, atmospheric surface pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining about twice as much of the observed variance as do either atmospheric wind or ocean bottom pressure changes and nearly 4 times as much of the observed variance as do oceanic currents. However, on interannual timescales, ocean bottom pressure changes are found to be the single most effective process exciting the Earth's wobbles, explaining more than 5 times as much of the observed variance as do atmospheric wind and pressure changes combined, and more than twice as much of the observed variance as do oceanic currents. Within the Chandler band it is found that during 1980–2000 atmospheric and oceanic processes have enough power to excite the Chandler wobble and are significantly coherent with it. The single most important mechanism exciting the Chandler wobble is found to be ocean bottom pressure variations. Atmospheric and oceanic processes do not appear to have enough power to excite the Earth's wobbles to their observed levels on pentadal and longer timescales, although series longer than the 21‐yearlong series used here need to be studied in order to obtain greater statistical significance of this result.
Abstract.The Chandler wobble is an excited resonance of the Earth's rotation having a period of about 14 months. Although it has been under investigation for more than a century, its excitation mechanism has remained elusive. Here, the angular momentum of the atmosphere computed from the products of a numerical weather prediction analysis system and the angular momentum of the oceans computed from a global oceanic general circulation model driven by observed surface winds and fluxes are used to show that during 1985.0-1996.0 the Chandler wobble was excited by a combination of atmospheric and oceanic processes, with the dominant excitation mechanism being oceanbottom pressure fluctuations.
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