78 



C. Davison — The Cowrie Earthquake. 



My object in drawing special attention to this shock is not on 

 account of any peculiar interest which it in itself possesses, so much 

 as from the evidence it supplies as to the direction in which the 

 great fault hades. "There is," as Sir Archibald Geikie has kindly 

 informed me, "no sufficient section of the Highland Border fault 

 to show its hade. Normally, of course, it should he to S.E., and 

 perhaps it is so on the whole. But there are traces of great and 

 irregular disturbance along the line, so that the hade may here 

 and there be to N.W." The seismic evidence in the present case 

 of course applies only to the Conine district, but, if 1 am right in 

 associating the recent earthquake with a slip of the fault, there can 

 be little doubt I think that the fault near there hades to the north- 

 west, and that it is therefore in this part a reversed fault. 



As this conclusion is of some importance from a geological point 

 of view, it may not be out of place to refer briefly to the evidence 

 in its favour. 1 





Fig. 2 



If the seismic focus, F (Fig. 2), were a point, the intensity of the 

 shock on the earth's surface AD would be greatest at the point E 

 vertically above it ; that is to say, the epicentre would lie on that 

 side of the fault-line towards which the fault AB hades. 



But, in the case of earthquakes produced by fault-slipping, the 

 seismic focus is a surface one, and, it may be, of considerable extent 

 both along the strike and the dip of the fault-surface. Supposing 

 for a moment that the intensity of the shock undergoes no dimi- 

 nution as the distance from the focus increases, it is evident that the 

 intensity at any point on the earth's surface will be greater the 

 shorter the duration of the shock. Now, the duration will be least 

 at the point D, where the line perpendicular to the fault-surface 

 through the centre F of the focus meets the earth's surface. 2 But 



1 In a future paper I hope to enter more fully into this question, as well as to 

 summarize the evidence in favour of the fault-slip theory of earthquakes. 



2 The argument may also be stated as follows: The point D is that which is 

 affected simultaneously by the disturbances proceeding from a larger part of the focus 

 than any other point on the earth's surface. 



