On P'P' and Related Waves

Abstract

The new vertical seismographs (1) designed by Mr. BENIOFF for routine use at this Laboratory can be set up to give records with high magnification for periods of about one second, with considerably less magnification for periods over 4 seconds. Instruments of this type T₀ = 0.5, T₁ = 0.2 or 1.5) are in service at all seven stations of the southern California network. Such seismograms give excellent detail in the preliminary phases of teleseisms, while the surface waves are either not recorded at all, or with such small trace amplitudes that they do not interfere with the record of the preliminaries. The seismograms of deep-focus earthquakes are especially satisfactory. When a strong teleseism is followed by aftershocks, records of this type make it easy to pick out the beginning of each separate shock. Numerous cases of the kind are to be found in our files; but it has recently appeared that some supposed instances are spurious, being due to an entirely different phenomenon. This forms the principal subject of the present paper.