640 EXPLORATION GEOPHYSICS 



as elastic rebound; instrumentation to record the passing of earthquake 

 waves ; computation of the space paths of earthquake waves ; depth of foci ; 

 locations of epicenters; nature of displacements of the various wave 

 motions; study of earthquake wave amplitudes and frequencies of vibra- 

 tion; velocities of the various waves through certain portions of our planet; 

 and study of earthquake damage. Even the possibility of predicting earth- 

 quakes probably belongs to this branch of science. If this subject falls 

 under the scope of seismology, then precautions against earthquakes may 

 also be closely related to seismology. However, it is now recognized by 

 most seismologists that the element of time in prediction does not in itself 

 afford protection against earthquakes. Since destructive earthquakes may 

 occur at any time, the best protection lies in the building of structures 

 sufficiently strong to withstand, with a considerable safety factor, the 

 heaviest known shocks for any given area. 



Briefly, the major cause of earthquakes is thought to be the disruptive 

 adjustments produced by a sudden yielding to stresses built up within the 

 earth's crust. These stresses may cause new fractures, faults, or sudden 

 displacements along existing fault planes. Previous to the occurrence of 

 an earthquake, slow movements of earth crustal blocks on opposite sides 

 of a fault plane may cause elastic deformation within the rocks until such 

 a point is reached that the strength of the rocks cannot withstand the strain 

 any longer. At this point relief seems to occur either by rupture of the rocks 

 or by sudden slippage adjustment along fault planes. In either case great 

 energy is released in the form of an elastic disturbance which propagates 

 outward in all directions from the focus of the earthquake. 



Seismological stations are usually located in a network or pattern suit- 

 able for the study of the occurrence of earthquakes in a certain region 

 under investigation. Several dififerent types of earthquake waves are 

 recorded on the seismograms. These waves are recognizable because each 

 is characterized by a unique velocity, a different wave path, or a unique 

 direction of vibration with respect to the direction of wave propagation. 

 In the general analysis of the seismograms only the longitudinal and the 

 transverse waves are considered. However, there are two types of waves 

 which are often observed in the case of the more distant earthquakes. 

 These have been named, after the investigators who first analyzed and 

 described them. Love and Rayleigh waves. The Love wave is a unique 

 type of transverse wave which vibrates in the boundary of a formation, 

 while the Rayleigh wave is a combination of a longitudinal and transverse 

 wave which vibrates in a plane perpendicular to the surface and parallel to 

 the direction of propagation. 



Theoretically several kinds of transverse waves may be demonstrated to 

 exist in addition to those mentioned. These waves originate at geological 

 or physical boundaries but are of minor interest to the seismologist because 

 they are seldom recognized on the seismogram. Indications have been 

 published showing that computed travel-time curves for once- and twice- 



