P and S amplitudes of two earthquakes of the single force couple type

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Abstract
Fault plane solutions of the Alaska earthquake of April 7, 1958 and of the Taiwan earthquake of April 26, 1959 are presented; the solutions are based on the initial motion directions of P and S waves at many stations all over the world. Qualitatively, the data concerning both shocks relate to a focal point-source of a single force couple with moment. The field data of Davis (1960) have been used to determine the position of the fault plane in the Alaska earthquake. It is shown that the initial motions are not implicitly needed to determine the polarization angle of the S waves, and that if the initial motion is obscured, the maximum amplitudes of S in two horizontal components will suffice. Incident amplitudes of P and S waves were determined and compared with theoretical amplitudes in the point-source model. For the individual shocks there is a consistent difference between the observed amplitude ratio ub/ua and the theoretical ratio. This is confirmed by a frequency analysis of some of the P and S wavelets. The difference is not the same for the two shocks investigated. It seems likely that the difference is related to the size of the seismic source.

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