On the observability of isotropic seismic sources: The July 31, 1970 Colombian earthquake

Theoretical calculations are made to study the observability of isotropic components of seismic sources. In particular we consider the 1970 deep Colombian earthquake, for which a precursory isotropic component was previously reported by Gilbert and Dziewonski. We compare an ultra-long period vertical record at Pasadena of the 1970 event to synthetic seismograms calculated both for Gilbert and Dziewonski's source model and for the pure double-couple source of Furumoto and Fukao, and obtain better overall agreement for the latter. The amplitude of the long-period synthetic for the isotropic source is about 5-15 times smaller than the synthetic for the deviatoric source, suggesting that the data may be relatively insensitive to the presence of a small isotropic source. When this possibility was tested, the overall agreement was found to be almost completely insensitive to the presence of even a reasonably large isotropic component. However, the isotropic source was derived from multi-station moment tensor inversion, rather than from single-station studies. A numerical experiment on the effect of lateral heterogeneity of eigenfrequencies and of Q on the inversion for the moment tensor shows that even relatively small amounts of heterogeneity can produce spurious isotropic sources from moment tensor inversion.