FOUNDATIONS OF THE GREAT DEEP 15 



initial Main wave is followed by a train of waves, and these 

 may keep coming to the station for hours, until the planet 

 ceases to tremble. 



Figure 6 represents an actual seismogram traced at the 

 Harvard University station when the waves arrived from the 



SCALE ON SEISMOGRAM 



I 2 3 4 5 

 INCHES 

 25 



P S 



\ 10 II 12 14 15 16 17 18 19 



ERZINCAN. TURKEY 

 DECEMBER 26.1939 



FIGURE 6. SEISMOGRAM OBTAINED BY L. D. LEET FOR THE DESTRUCTIVE 

 EARTHQUAKE IN TURKEY IN DECEMBER, 1 939. 



center of the terribly destructive shock of 1939 in Turkey. The 

 onset of the Push wave is represented by the point P; that of 

 the Shake wave by the point S; and that of the Long or Main 

 waves, with large maximum of swinging, by the point L. 



The Push and Shake waves, starting at the center of shock, 

 spread out in all directions through the depths of the earth; 

 they are therefore often referred to as Body waves. In contrast, 

 the Main waves are propagated along the rocky surface of the 

 globe and bear another appropriate name, Surface waves. 

 While they are running, the oscillation of rock particles is 

 unimportant at and below a level only a few scores of miles 

 down. 



