Earthquake-induced tendency of the Earth Oblateness change

It has been known (since Chao and Gross, 1987) that globally earthquakes have a strong tendency in making the Earth less oblate, or rounder; that is, the Earth's oblateness parameter J2 has been decreasing slightly but secularly because of the cumulative effect of earthquakes. As the dynamic explanation is still outstanding, we calculate the earthquake-induced, cumulative-to-date change in J2 as a function of the radius in the Earth interior, ΔJ2(r), of all earthquakes according to the Global CMT catalog (close to 30,000 events from 1976 to October 2007). The calculation is via the normal-mode summation scheme using the CMT moment tensor for the seismic source mechanism on the SNREI Earth model PREM. The radial profile function ΔJ2(r) thus has the physical meaning of the contribution, per unit (normalized) radial distance, to the whole-Earth oblateness change cumulated of earthquakes since 1976. The result for ΔJ2(r) indeed shows a decrease through the mantle and core (including the core-mantle boundary), except near the crustal depth where most earthquakes happen. We then attribute ΔJ2(r) to two major earthquake types: normal faulting and thrust faulting. The result shows that the secular tendency of ΔJ2(r) are distinctive according to source mechanisms: decreasing ΔJ2(r) by thrust faulting and increasing ΔJ2(r) by normal faulting. The thrust faulting dominates the ΔJ2(r) tendency over that of normal faulting by about six times. We further examine the implications of this result in terms of global earthquake dynamics.