DETERMINATION OE EABTHQUAKE ORIGINS. 193 



and sufficiently far separated from each other, the direc- 

 tion of movement of these normal motions, by drawing 

 lines parallel to. these directions through our two stations, 

 the lines would intersect at a point above the required 

 origin. If instead of two points we had three, or, better 

 still, a large number, the results we should obtain ought 

 to be still more certain. Unfortunately, it seems that 

 earthquakes seldom originate from a given point, and, 

 further, normal motions are not always (sufficiently) pro- 

 minent. Sometimes, as has already been shown in the 

 chapters on earthquake motion, they may be non-existent. 

 It is probable, however, that difficulties of this sort are 

 more usually associated with non-destructive earthquakes. 

 Mallet regards the destructive effects of an earthquake as 

 almost solely due to normal motions. If this be true, for 

 destructive earthquakes, the problem is shorn of many 

 of its difficulties. In cases where normal vibrations are 

 not prominent, where we have only transverse vibrations, 

 motions due to the interference of normal or transverse 

 motions, or directions of motions due to the topographical 

 or geological nature through which the disturbance has 

 passed, the determination of the origin of an earthquake 

 by observations on the direction in which the ground has 

 been moving appears to be a problem which is practically 

 without a solution. "We will, therefore, only consider the 

 determination of the origin of those earthquakes which 

 have predominating directions in their movements, which 

 directions we will consider as normal ones. The question 

 which is, then, before us, is the determination of the direc- 

 tion of these normal movements. First of all we may 

 take the evidence of our senses. In exceptional cases 

 these have given results which closely approximate to the 

 truth, but in the majority of cases such results are not to 

 be relied upon, as the inhabitants of a town will, for the 



