Terrestrial spectroscopy following the Rat Island earthquake

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Abstract
Fourier analysis of the four-component strain recordings at Ogdensburg, New Jersey, after the Rat Island earthquake of February 4, 1965, has yielded spectra of the earth oscillations. Three horizontal components were used to calculate synthetic longitudinal, transverse, and shearing strain components. Analysis of the different components or their combinations yielded spectra of the torsional and spheroidal oscillations. The spectral peaks corresponding to l = 3 through l = 24 were resolved and were significantly above the 95 per cent confidence level. The novelty of this analysis is the appearance of 9 first overtones and 8 second overtones of the spheroidal oscillations from cross-correlation of two sections of the vertical component of the strain recording. The observed periods are compared to theoretical periods of four earth models: M1 of Landisman; R1 of Dorman; Jeffreys-Bullen B; and Gutenberg-Bullen A. The M1 and R1 models gave the most satisfactory agreement with the observations of the fundamental modes, whereas the Jeffreys-Bullen B model had a better agreement with the observations of the overtones than other models considered. Assuming the azimuthal order number, m, of the source as a combination of m = 0, m = 1, and m = 2, the variations of the spectral amplitudes at Ogdensburg indicated that the main contributing component of the azimuthal order number of the source was not only zero, but it was 1 for the modes corresponding to l = 2,4,7,9,12, and 2 for the modes corresponding to l = 3, 5,6,8,10, and 13.

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