Identification of Free Oscillation Spectral Peaks for 1970 July 31, Colombian Deep Shock using the Excitation Criterion

Abstract

The excitation criterion was applied to identify spectral peaks of free oscillations excited by the Colombian deep shock of 1970 July 31. About 160 new overtones were identified. They range from 1T to 3T torsional modes and from 1S to 6S spheroidal modes. Some of the modes identified have periods below 90 s. The standard deviation of the measured eigen‐ periods is in general smaller than 0.1%. Comparison of 0S and 0T eigenperiods measured for the Alaskan (1964) and Colombian shocks shows a systematic difference. The difference is larger than the error of the measurement, and this indicates that the average eigenfrequencies measured for each event are biased by the epicentre location. The effect of lateral heterogeneities of the Earth, aftershocks, and the choice of source and Earth model when using the excitation criterion are discussed in detail. Copyright © 1973, Wiley Blackwell. All rights reserved