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Amplitude spectra of the spheroidal modes 0St (1 = 2-40) excited by the 1960 Chilean earthquake and observed at Pasadena, Los Angeles, and Isabella show distinct holes at oSxo (T = 580 s) and 0S:x (T = 336 s). These holes can be explained as an interference pattern caused by a composite source consisting of a finite propagating source (main shock) and a slow precursory source, 15 rain before the main shock. A total seismic moment of 4-5 X 10 dyn cm is required to explain the observed amplitude.
Kanamori et al. (Mon,) studied this question.