ABSTRACT

Phases of the 600 km. deep earthquake of April 16, 1957, which have not been reflected at the earth's surface show very sharp beginnings arriving within the limits of error of a few seconds at the calculated times. However, phases which have been reflected at the surface, for example PP, pP', pPP, have more or less emergent beginnings which arrive between a few seconds and as much as 30 seconds too early. These emergent beginnings are followed by an impulse which arrives at nearly the calculated time. In agreement with earlier findings, it is concluded that the emergent beginnings of such waves are caused by waves which have been reflected at the Mohorovičić discontinuity or other discontinuities below the surface. Observations of such early emergent arrivals also include waves of the types P'P', PKSP', P'P'P' which have passed twice or more through the mantle and the core.

P'P'P'P' has been observed for the first time. The travel times of P'P'P'P' and the coefficient of absorption calculated from P'P'P'P' and P' agree within the limits of error with the expected values; however, these limits of error are so large that the results cannot be used to improve earlier findings. The periods of P'P'P'P' which has traveled about 50,000 km. through mantle and core do not show a marked change from those observed in P waves which have traveled only a few thousand km. through the mantle.

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