The transmission of seismic waves

In studying the instrumental results of the Santiago, Cuba, earthquake of February 3, 1932, the author was particularly impressed by the almost linear character of portions of the travel‐time curve for the P‐wave, and for other wave‐groups in the interval between P and S. Two important conclusions have been suggested by this study, both of which pertain to fundamental conceptions of Earth‐structure and to the transmission of seismic waves. If we modify the usual smooth type of travel‐time curve by flattening out certain sections, we introduce sharp bending points in the curve and such points must be taken to indicate the presence of additional shells. The rather limited data for the Santiago shock indicate that one of these sharp changes in slope corresponds to a discontinuity about 350 km deep. A discontinuity near this level has already been suggested by Mohorovičić and others, but seismologists in general have apparently been reluctant to accept it. The data also indicate that another discontinuity may be present near the 800‐km level, but the data on which this is based are indefinite. The first generally accepted major discontinuity near the 1,200‐km level is evident in the Santiago curve by a bend near 5,500 km.