200 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 3 4 



The waves which penetrate to a depth of 2,900 kilometers travel to 

 a distance of about 10,000 kilometers. When this distance is ex- 

 ceeded the ordinary P and S waves become very feeble and difficult 

 to identify, but at about 16,000 kilometers a new compressional wave 

 appears and remains recognizable to the antipodes of the epicenter. 

 The delay of this wave, compared with the ordinary P, shows that 

 the velocity, instead of continuing to increase with depth, must suf- 

 fer a decrease at some considerable depth. This led Oldham to 

 suggest that at this depth, which is now known to be about 2,900 kil- 

 ometers, there is a discontinuity which forms the boundary of a 

 central core of the earth. The core acts like a lens and waves which 

 are transmitted through it undergo refraction at the boundary. The 

 effect is to produce a shadow zone in which neither the waves which 

 just miss the core nor those which are refracted by the core appear; 

 thus on seismograms recorded at distances between 10,000 and 16,000 

 kilometers neither the ordinary direct compressional wave nor the 

 compressional wave which has traveled through the core can be 

 recognized. Another point of very great importance is that the core 

 is apparently unable to transmit distortional waves; the S waves 

 which strike the core are transformed on refraction into compres- 

 sional waves which can change back to the S type when the boundary 

 is crossed the second time.* Since for the transmission of distor- 

 tional waves a material possessing some rigidity is necessary, it is 

 inferred that the central core of the earth is nonrigid and is there- 

 fore composed of material in the liquid state. On the seismogram 

 of a distant earthquake a number of phases can usually be recog- 

 nized in addition to the P and S waves or the waves transmitted 

 directly through the core. Some of these phases are due to reflection 

 of waves at or near the earth's surface. Reflection can also take 

 place at the core boundary, and the fact that such reflected waves 

 produce sharp pulses on the records shows that the transition 

 between the solid shell and the liquid core must be a rapid one. 



DEEP EARTHQUAKES 



Although there is still some difficulty in deciding what figure 

 we may take as the average depth of focus of normal earthquakes 

 it is almost certain that the majority of shocks originate at depths 

 of less than 50 kilometers below the surface. That some earthquakes 

 occur at much greater depths than this, however, was suggested by 

 the late Professor Turner, who noticed that for some shocks the 



* Some investigators have claimed that pulses representing distortional waves through 

 the core can be observed in the seismograms of distant earthquakes. A correct interpre- 

 tation of such records is often very difficult, however, ovping to their complexity. It 

 seems clear that generally no prominent waves of this type occur. 



