GLOBAL GEODYNAMIC EFFECt OF THE MACQUARIE RIDGE EARTHQUAKE

Besides generating seismic waves, which eventually dissipate, an earthquake also generates a static displacement field everywhere within the Earth. This global displacement field rearranges the Earth's mass distribution, using the Earth's rotational properties and gravitational field to change. The size of these changes depends, in general, upon the size of the earthquake. The Macquarie Ridge earthquake of May 23, 1989 is considered to be the largest earthquake to have occurred since the 1977 Sumba and Tonga events. As such, the coseismic effect of this earthquake upon •e Earth's length-of-day, polar motion, and low-degree harmonic coefficients of the gravitational field are computed. It is found that this earthquake should have caused the lengthof-day to decrease by 0.06 gsec, the position of the mean rotation pole to shift 0.11 mil!i-arcsec towards 323 øE longitude, and selected degree I = 2-5 gravitational field coefficients to change by about 1 part in 1013. These changes are all smaller than can be detected by current observational techniques. However, changes of this size could perhaps be detected in the future with the implementation of proposed improvements o the techniques of monitoring the Earth's rotation, and (especially for the low-degree gravitational field coefficients) with the placement of GPS receivers onboard orbiting spacecraft.