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Published April 1978 | Published
Journal Article Open

Time-domain observation and synthesis of split spheroidal and torsional free oscillations of the 1960 chilean earthquake: Preliminary results


The rotationally and elliptically split normal modes of the earth are observed for the 1960 Chilean earthquake by analysis in the time domain. One hundred and fifty hours of the Isabella, California, strain record are narrow band filtered about the central frequency of each split multiplet to isolate the complex wave form resulting from the interference of the different singlets. We compute synthetic seismograms using our previous theoretical results, which show the dependence of the amplitude and phase of the singlets on source location, depth, mechanism, and the position of the receiver. By comparing these synthetics to the filtered record, we conclusively demonstrate the splitting of modes whose splitting had not been definitely resolved: torsional modes (_0T_3, _0T_4) and spheroidal modes (_0S_4, _0S_5). The splitting of _0S_2 and _0S_3 is reconfirmed. We obtain good agreement between the synthetics and the filtered data for a source mechanism (previously determined from long-period surface waves) of thrust motion on a shallow dipping fault.

Additional Information

Copyright © 1978, by the Seismological Society of America. Manuscript received May 24, 1977. We thank Stewart Smith and Hiroo Kanamori for providing us with data from and information about the Isabella strain record. Yoshio Fukao, Hiroo Kanamori, and Emile Okal critically read the manuscript. We have also benefited from the comments of an anonymous reviewer. Seth Stein was supported by a fellowship from the Fannie and John Hertz Foundation. This research was supported by the Division of Earth Sciences, National Science Foundation, NSF Grants EAR76-14262, EAR74-22489, and EAR77-14675.

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