THE EARTHQUAKE PEOBLEM HECK 367 



With the instruments that have been described or others with 

 similar characteristics, it is possible to have seismograms to meet the 

 needs of particular investigations. 



It would appear at first thought that there would be such a 

 complexity of waves from an earthquake that they could not be sorted 

 out. However, through absorption of energy and the results of re- 

 flections and refractions within the earth or at its surface only certain 

 waves have sufficient energy to arrive. The definite arrivals of these 



,..,WaveJsWs^^,^^^ 



Figure 1. — The difference In times of arrival of the longitudinal or high-speed 

 wave and the transverse or slower-speed wave is a measure of the distance 

 from the earthquake to the seismological station 



so-called phases are indicated by sudden or gradual increases of 

 amplitude or by sudden change of period, or both, and these form an 

 orderly succession, some of which can be distinguished in all intel- 

 ligible records. Since many of those present have probably not gone 

 far into the subject, I will recapitulate the principal facts. 



We have the preliminary tremors, designated P for primus, which 

 are longitudinal ; that is, with vibration in the direction of progress. 

 These follow a direct path from earthquake to recording station, but 

 a curved one, dipping somewhat below the straight line connecting 

 these two points. Their velocity near the surface is 7 to 8 kilometers 



