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Bulletin of the Seismological Society of America; June 1980; v. 70; no. 3; p. 717-734
© 1980 Seismological Society of America
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A technique for the recovery of the seismic moment tensor applied to the Oaxaca, Mexico earthquake of November 1978

STEVEN N. WARD

GEOLOGICAL RESEARCH DIVISON SCRIPPS INSTITUTION OF OCEANOGRAPHY, UNIVERSITY OF CALIFORNIA, SAN DIEGO, LA JOLLA, CALIFORNIA 92093

Abstract

The potential for using seismograms in a linear inversion process to recover the seismic moment tensor has been theorized for sometime. Unfortunately, practical application of the theory is difficult. Lateral variations of the Earth and imprecise knowledge of the source often severely contaminate real seismic data relative to synthetic models which must be supplied. This paper outlines an experiment which minimizes the contamination sufficiently to permit a successful inversion yet retains much of the data's sensitivity to source parameters. Routineness of technique was maintained in data selection and preparation by an unambiguous on-line windowing of P phases from the SRO network. Duplicate preparation of the theoretical P phases assured their synchronization with the observations. A time domain weighted least-squares inversion was implemented including double couple and nondouble couple constraints. Estimates of the total variance in addition to variances of the moment tensor eigenvalues and eigenvectors were returned, forming the basis for statistical tests. Seismic moments of 1.4 ± 0.4, 2.8 ± 0.3, and 2.7 ± 0.2 x 1027 dyne-cm were found for the Oaxaca, Mexico earthquake of November 1978, using 10 P phases in the distance ranges 20 to 120°, 25 to 120°, and 40 to 120°, respectively. Corresponding azimuth and dips of the probable slip vector are 21.5°, 11.8°; 30.3°, 9.3°; and 32.1°, 9.0°. No evidence was found to support the hypothesis that unconstrained source models fit the data significantly better than a double couple. We believe an automated procedure such as this is a viable means for the routine recovery and cataloging of earthquake sources.




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