SEISMIC METHODS 641 



reflected earthquake waves fit certain observed events on seismograms. 

 Reflected paths as computed from these data fit the conditions of the 

 layered structure of the interior of the eartli as determined from the 

 refracted travel-time curves for earthquake waves. 



The main events shown on an earthquake seismogram for a shock 

 originating at a long distance are the onsets of the longitudinal refracted 

 wave, the refracted transverse wave, and the surface waves. When the 

 data from a sufficient number of stationary seismological stations are avail- 

 able, a travel-time curve may be constructed, using the known distances 

 between the stations and the measured times of arrival. Analysis of such 

 a curve makes possible inferences as to the path of the seismic wave within 

 the earth's interior, depth of penetration and velocities of the waves along 

 the paths. 



Earthquakes may be classified according to depth of foci, i.e., deep 

 and shallow focus earthquakes. They are also often classified as to distance, 

 i.e., local and distant. Seismologists have learned much about the shallow 

 focus type, but beyond the proof that deep focus earthquakes occur, little 

 is known about the nature of their origin. 



In seismic exploration, the artificial earthquakes are generated by various 

 mechanical or explosive means. Some of the methods used to initiate a 

 shock wave are : a heavy mechanical blow at the surface of the earth or at 

 the bottom of a shallow hole ; the explosion of a charge of dynamite buried 

 at sufficient depth to obtain the desired propagation ; or several charges 

 exploded simultaneously in air above the ground. 



For studies of the response of the ground or structures to continuous 

 vibration at various frequencies, a mechanical method, utilizing a motor- 

 driven shaking device whose operation depends on the rotation of a heavy 

 eccentric mass around an axle, is sometimes employed. 



Just one type of wave is utiHzed in applied seismology, as compared to 

 the several wave motions employed in earthquake studies. Only the longi- 

 tudinal wave is observed because of two factors. First, the distances 

 involved are extremely short ; therefore, the longitudinal and the transverse 

 waves arrive so close together that they are recorded on the seismogram 

 almost superimposed. The transverse wave onset is lost in the train of 

 motion following the arrival of the longitudinal wave. Also, since only the 

 vertical component of vibration is usually observed, the transverse compo- 

 nents of the waves are greatly minimized. 



Longitudinal waves traveling three difi^erent paths are observed from 

 artificial shocksf : these paths are (1) surface longitudinal ; (2) refracted 

 longitudinal, and (3) reflected longitudinal. The only direct wave observed 

 is probably recorded by the up-hole seismometer, which is placed at the 

 surface very near the shot hole to measure the time necessary for the shock 

 wave to travel directly from the point of detonation to the ground surface. 

 The first onsets on the seismograms are for the most part refracted waves. 



t C. Y. Fu, "Studies in Seismic Waves," Geophysics, Vol. XI, No. 1, Jan., 1946. 



