Abstract.—

In the case of the seismological tables developed by seismologists during the first decade of the present century, it was always assumed that the focus lay in the surface. Under such circumstances, the so-called PR1-wave, the wave once reflected at the surface at a point midway between the focus and the point of observation, would be expected to have a travel-time double that for the compressional or P-wave to the mid-point. Observation failed to support the earlier PR1-tables, which were constructed from the P-tables on this assumption. It was later found that earthquake foci do not always, nor even generally, lie in the surface. Furthermore, the P-curves for earthquakes known to have shallow foci are now found to differ considerably from the earlier curves based on group data. It became desirable to study the travel-times for the reflected waves from the data of a single well-defined earthquake. The seismometric study of the Tango earthquake, Japan, March 7, 1927, recently completed by the writer, furnished the opportunity for such a study, the results of which are presented in this paper.

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