SEISMIC METHODS 649 



The method employed at seismological stations to determine the 

 focus of an earthquake will be evident from Figures 405 and 406. In 

 Figure 405, the P wave is recorded at 4 minutes and 24 seconds past 

 9 A.M., and the 5" wave at 11 minutes and 12 seconds past 9 A.M. The 

 .S wave, therefore, requires 6 minutes and 48 seconds longer than the P 

 wave to reach the recording station. From Figure 406 it is seen that a 

 difference of 6 minutes and 48 seconds (408 seconds) between the P and 

 6" curves corresponds approximately to 46.9 degrees or 5100 kilometers. 

 Therefore, the earthquake must have occurred at about this distance from 

 the Pasadena station at which the record shown in Figure 405 was 

 obtained. 



One of the most interesting applications of earthquake seismology is the inves- 

 tigation of the structure of the earth at great depths. Time-distance curves and tables 

 derived from comprehensive and reliable data obtained at a great number of seismo- 

 logical stations have shown that over most of the earth the outer crust of the earth 

 consists, essentially, of two layers, an upper granitic layer approximately seven and 

 one-half miles thick and a lower basaltic layer approximately fifteen miles thick.t 

 Below the basaltic layer, there is a layer some two thousand miles thick. Underneath 

 the latter there lies a central core through which the primary waves pass with a 

 velocity only two-thirds as great as the velocity just outside the core. The central 

 core does not transmit the secondary waves, as is shown by their disappearance at 

 large distances from the epicenter. (An epicenter is the point or area on the earth's 

 surface vertically above the focus or point of origin of an earthquake.) This disap- 

 pearance of the secondary waves has been taken to indicate that the central core is 

 probably fluid or plastic. 



A more detailed discussion of earthquake seismology will be found, for example, 

 in B. Gutenberg's "Handbuch der Geophysik," Volume 4; G. W. Walker's "Modern 

 Seismology"; or Galitzin's "Seismometrie." Of most recent interest is the book by 

 Macelwane.* 



Operating Principles of Seismic Prospecting.— General seismic 

 prospecting makes use of artificially produced elastic waves which are 

 the same as the longitudinal waves studied in earthquake seismology, 

 except that they have a higher frequency of vibration. Travel-time curves 

 are computed from prospect field seismograms in much the same way that 

 travel-time curves are computed for earthquake waves. Characteristic 

 features of seismic prospecting methods, however, are: (a) the location 

 of the artificial "earthquake" and its instant of occurrence are known 

 precisely; (b) far more sensitive instruments are used to detect the vibra- 

 tions; and (c) more accurate time measurements must be made than 

 are necessary in earthquake studies. 



In seismology, the instrument used to receive and record the dis- 

 turbances is called a seismograph. (Compare p. 642.) In seismic pros- 

 pecting, however, it is customary to distinguish between the recording 



t Charles Davison, Article on Earthquakes, Encyclopaedia Britannica, 14th Edition, 1929. 

 * J. B. Macelwane, "Introduction to Theoretical Seismology," John Wiley & Sons, New York, 

 1936. 



