SEISMIC METHODS 6\7 



this wave-like line, at P, represents the beginning of a larger distur- 

 bance or vibration. The amplitude of the disturbance is small and the 

 period of oscillation is fairly short. Also, the motion is very irregular 

 indicating that additional impulses arrive at irregular intervals. The 

 record shows that a few minutes after this first motion, at the point 

 marked S, there begins another type of motion. This motion has a 

 somewhat larger amplitude and about the same period as, or a greater 

 period than, the first disturbance. Following the initial large amplitude 

 the motion becomes quite irregular indicating, as before, the arrival of 

 additional impulses. 



,,09 2 4 e a '0 li I* I* '» *o '« •^■» '^ ■=* ■"' " ^^ 



Fig. 405. — Schematic representation of an earthquake seismogram. 



A short time later, at the point marked L, a different type of motion 

 begins. The amplitude of this motion is relatively large and the period 

 relatively long. This portion of the disturbance is followed by a complex 

 vibration M of even greater amplitude and somewhat shorter period. 

 These two parts of the motion are referred to as the principal portion. 

 In the final portion, the amplitude is greatly decreased and the period is 

 somewhat lengthened. 



The maximum displacement of the ground usually amounts to a frac- 

 tion of a millimeter during light shocks. In destructive earthquakes, 

 however, the maximum acceleration often exceeds two hundred millimeters 

 per second per second and has been known to exceed 10,000 millimeters 

 per second per second.* 



Figure 406 shows the travel-times for the P, S, and L waves for 

 various distances.** The 6" wave travels faster than the L wave and the 

 apparent speed increases with distance, due to penetration of the wave 

 into zones of higher velocity. Likewise, the P wave travels faster than 

 the 6" wave and its speed also increases with depth down to the core 

 of the earth. 



* It is of interest to contrast these displacements with the earth motion corre- 

 sponding to a readily detectable reflection in the reflection shooting type of seismic 

 prospecting. The amplitude of the motion at the seismometer due to the reflected .wave 

 may be a few millionths of a millimeter, while the acceleration may be a few thou- 

 sandths of a millimeter per second per second. (Compare D. H. Gardner, "Measure- 

 ment of Relative Ground Motion in Reflection Recording," Geophysics, vol. 3, No. 1, 

 1938, p. 40.) 



** Essentially similar time-distance curves are obtained everywhere on the earth's 

 surface. This fact attests to the uniformity of the earth's structure. 



