MODERN SEISMOLOGY SCEASE 195 



which develop within it. The deformation frequently takes the 

 form of a sudden fracture in which the energy stored up by the 

 strain is converted into the energy of internal motion. At the 

 surface of fracture a displacement occurs which sets up vibrations 

 that spread throughout the earth. The majority of severe earth- 

 quakes are produced in this way; such earthquakes are said to 

 be tectonic in origin to distinguish them from shocks caused by 

 stresses arising from volcanic activity. The class of tectonic earth- 

 quakes is by far the larger and more important one; most of the 

 shocks that occur in volcanic regions are small in total energy. 

 Sometimes, however, shocks of gi^eater violence appear to be associ- 

 ated with volcanic eruptions, but it is doubtful whether these earth- 

 quakes are truly volcanic in origin. 



Whatever its origin, an earthquake sets up a disturbance of the 

 ground which is communicated to other parts of the earth in the 

 form of waves. Since the waves spread out in all directions, the 

 motion of the ground at places distant from the origin of the shock 

 is in general very small. With suitable recording apparatus, how- 

 ever, it can be detected even at the opposite end of the earth. In 

 general, any earth movement may be considered as made up of three 

 linear displacements along directions at right angles to each other. 

 A fully equipped observing station therefore requires three seis- 

 mographs, and the components of the displacement which these 

 are set to record are usually the north-south, the east-west, and the 

 vertical. Many stations, however, only record the horizontal com- 

 ponents, since vertical seismographs are generally more trouble- 

 some to operate. 



THE SEISMOGRAPH 



The main principle of the seismograph may be explained briefly 

 as follows : A body attached rigidly to the ground will undergo the 

 same displacement as the ground itself. If, however, the attach- 

 ment is not rigid there will be some relative motion between the 

 body and the ground, and it is this relative motion which enables 

 us to determine how the ground itself is moving. Thus the essential 

 part of a seismograph is like a pendulum, the bob of which tends to 

 remain stationary in space while everything to which it is attached 

 is moving. Actually the bob does not remain stationary, because it 

 is impossible to eliminate entirely the effect of the attachment, but 

 in certain circumstances it is possible to make allowance for this 

 and so obtain the true motion of the ground. The modern devel- 

 opment of seismographs has been in the direction of providing 

 increased magnification of the relative motion and of insuring ade- 

 quate damping of the moving system. The latter is necessary, since 



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