On the observability of isotropic seismic sources: The July 31, 1970 Colombian earthquake |
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| Authors: | Emile A Okal Robert J Geller |
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| Institution: | Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, U.S.A. |
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| Abstract: | 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. |
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