DETERMINATION OF EARTHQUAKE ORIGINS. 199 



directions of motion. In certain cases, therefore, our 

 records, if we assume the most permanent motions to be 

 normal ones, give definite and valuable results. In other 

 cases it is necessary to carefully analyse the records, com- 

 paring those taken at one station with those taken at 

 another. 



One remarkable fact which has been pointed out in 

 reference to artificial earthquakes produced by exploding 

 charges of gunpowder or dynamite, and also with regard 

 to certain earthquakes, is that the greatest motion of the 

 ground is inwards^ towards the point from which the 

 disturbance originated. Should this prove the rule, it 

 gives a means of determining, not only the direction of 

 an earthquake, but the side from which it came. 



Determination of earthquake origins by time obser- 

 vations. — The times at which an earthquake was felt at 

 a number of stations are among the most important 

 observations which can be made for the determination 

 of an earthquake origin. The methods of making time 

 observations, and the difficulties which have to be over- 

 come, have already been described. When determining 

 the direction from which a shock has originated, or 

 determining the origin of the shock by means of time 

 observations, it has been usual to assume that the velocity 

 of propagation of the shock has been uniform from the 

 origin. The errors involved in this assumption appear to 

 be as follows : — 



1. We know from observations on artificial earth- 

 quakes that the velocity of propagation is greater between 

 stations near to the origin of the shock than it is between 

 more remote stations ; and also the velocity of propaga- 

 tion varies with the initial force which produced the 

 disturbance. If our points of observation are sufficiently 

 close together as compared with their distance from the 

 10 



