Abstract
The Hindu Kush earthquake of July 6, 1962 produced anSa phase (vertical component or Rayleight type) which was clearly recorded at stations in North America in the distance range 90° to 110°. The propagation of this phase is studied in detail, both by the usual method of measuring velocity of first arrival and also by determining group velocities from the derivative of the phase of the Fourier transform (in the period range 20 to 80 seconds). The energy in theSa phase propagates in a manner intermediate between that of a single ray and that of a single isolated mode. The expected interference effects which vary as a function of distance are observed and complicate a straight-forward measurement of group velocities. The results are most reliable if only the group velocities corresponding to parts of the spectra with relatively high amplitudes are used.
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