A NEW ANALYSIS OF THE GREAT 1970 COLOMBIA EARTHQUAKE AND ITS ISOTROPIC COMPONENT

Several methods of low-frequency analysis are used to determine the so urce mechanism of the July 31, 1970, Great Colombia earthquake and to reexamine two controversial results previously obtained for this event . In a classic study of the earthquake, Dziewonski and Gilbert [1974] concluded (1) that low-frequency (f < 4 mHz) mode spectrum observation s require an isotropic compression at the source of a magnitude simila r to the deviatoric moment release and (2) that the isotropic componen t of moment release precedes the short-period onset of the earthquake by 80 s. The original data set collected by Dziewonski and Gilbert is used in a reanalysis of the Great Colombia earthquake, taking advantag e of the theoretical and computational advances that have been made du ring the past 20 years in predicting normal mode spectra. In particula r, the splitting and coupling of modes induced by rotation, ellipticit y, and three-dimensional (3-D) mantle structure are considered. When s plitting and coupling are taken into account in the analysis of the ob served spectra, the isotropic component is reduced to an insignificant size, and the data are well explained by a purely deviatoric source w hich does not require the initiation of moment release before the shor t-period onset time of the earthquake. When the effects of splitting a nd coupling are ignored in the calculation, the results are instead si milar to those of Dziewonski and Gilbert [1974]. Experiments with synt hetic spectra generated for the deviatoric part of the moment tensor c onfirm that the distortion of the low-frequency end of the normal mode spectrum caused by splitting and coupling is incorrectly interpreted as an isotropic component when these effects are ignored in the invers e problem. The detailed effects of modeling errors of this kind on the retrieved source parameters will, in general, depend both on the sour ce mechanism and the distribution of observing stations.