EARTHQUAKE MOTION DISCUSSED THEORETICALLY. 55 



Both of these methods are referred to by Mallet, but the 

 first is considered as probably the more correct. 



Earthquake Waves. — Hitherto we have chiefly con- 

 sidered earthquake vibrations ; now we will say a few 

 words about earthquake waves. If we strike a long iron 

 rod at one end, we can imagine that, as in the long 

 spring, a pulse-like motion is transmitted. If the rod be 

 struck quickly, the pulses will rapidly succeed each other, 

 and if struck slowly the pulses will be at longer intervals. 

 Each individual pulse, however, will travel along the rod 

 at the same rate, and hence the distance between any two 

 will remain constant ; but that distance will depend on 

 the interval between the blows producing these pulses 

 being equal to the distance travelled by one pulse before 

 the next blow is struck. 



From this we see that an irregular disturbance will 

 produce an irregular succession of motions ; some will be 

 like long undulations in a wide deep ocean, whilst others 

 will be like ripples in a shallow bay. Again, consider 

 the bar to be struck one blow only, and then left to itself. 

 The bar will propagate a series of pulses along its length, 

 due to the out and in vibration of its end. These will 

 succeed each other at regular intervals, and will be mixed 

 up with the pulses we have previously considered. 



From this we see that in an earthquake, if it be pro- 

 duced by one blow, the motion will be isochronous in its 

 character ; but if it be due to a succession of blows at 

 regular intervals, the motion will be the resultant of a 

 series of isochronous motions, and will be periodical. If 

 the impulses are irregular, you have a motion which is 

 the resultant of a number of isochronous motions due to 

 each impulse, but these compounded together in a differ- 

 ent manner at each instant during the earthquake, and. 

 giving as a result a motion which is in no sense isochron- 

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