370 ANNUAX. REPORT SMITHSONIAN" INSTITUTION, 19 31 



the one at 2,900 kilometers that bounds the central core of the earth 

 which geologists believe to be of iron and nickel. The ability of this 

 core to change path and energy of the earthquake waves is beyond 

 argument. It has also been held that the interior, though at very 

 high pressure and temperature, behaves like a liquid in its inability to 

 transmit any but longitudinal waves. Transverse waves are practi- 

 cally lacking, though Doctor Macelwane ^ has recently found evidence 

 that they may exist. 



Of more importance to our discussion are layers near the surface. 

 These are not so definite and there is more difference of opinion. In 

 records of near-by earthquakes there are often two P phases and 

 sometimes three, and the same may be true of S. An explanation is 

 called for. The first was that of Mohorovicic who postulated a layer 

 of discontinuity 60 kilometers beneath the continents, and this seemed 

 to give fairly good agreement with observations (fig. 2). Changes 



Figure 3. — niustratiug the refraction of earthquake waves when reaching the 

 supposed GO-lcilometer layer l)elow the surface. Reflected waves are not 

 shown 



in velocity throughout the layer, taking place in a manner similar to 

 that worked out for the deep interior of the earth, were necessary. 

 Observations of submarine earthquakes gave evidence of a much 

 thinner layer beneath the oceans and this in turn corresponds to the 

 faster surface waves beneath the oceans which have been mentioned. 



Seismologists in general had accepted this conclusion almost uni- 

 versally when Jeffreys suggested a different explanation. As shown 

 in Figure 4, his theory calls for two layers of discontinuity about 10 

 and 30 kilometers, respectively, below the surface. Most of the 

 energy passes along these surfaces and the velocity of the waves 

 remains nearly constant throughout their paths. Seismic prospectors 

 use a somewhat similar theory for layers very near the surface. 

 Investigations in California fit Jeffreys' theory very well. 



• Macelwane, J. B., South Pacific earthquake of June 26, 1924, Gerland's Beitr. 

 Geophys., vol. 28, 1930. 



