﻿>46 DE. J. W. EVANS ON [Aug. r 9IO> 



13. An Earthquake Model. By John William Evans, 

 D.Sc, Ll.B., F.G.S. (Bead June 15th, 1910.) 



Some eighteen months ago, Mr. B. D. Oldham showed to this Society 

 a model designed to explain the mechanism of the earth-movements 

 at the time of the Californian earthquake of 1906 in the neighbour- 

 hood of the San Andreas Eault. 1 Though of great value in assisting 

 us to realize the nature of the changes which then took place, it 

 does not give prominence to the time-element so as to indicate the 

 succession of events that led up to the earthquake shock ; and it is to 

 illustrate this aspect of the question that I have had the model 

 constructed which forms the subject of the present paper. 



It will be convenient for me, in the first place, to describe suc- 

 cinctly, from my own standpoint, the stages into which the conditions 

 that precede an earthquake may be divided. 



I shall on the present occasion put on one side earthquakes which 

 are in the nature of landslips, as well as those which are the im- 

 mediate result of the explosive action of the gases in igneous 

 magmas, although there are authors who attribute to both of these 

 modes of origin a greater importance and a more frequent occur- 

 rence than is usually accorded to them. 



The typical earthquake, then, including great earthquakes like 

 that of 1906, is, in my belief, the ultimate result of the 

 slow relative movement of great masses of the earth's 

 crust extending in most cases far beyond the area 

 immediately affected by the shock. This differential creep 

 may be in a horizontal, or in a vertical, or in any intermediate 

 direction. As it proceeds, the intervening tract is subjected to 

 stresses resulting in distortion or strain, so that a transverse line 

 which was originally straight becomes curved. When the stress 

 exceeds anywhere the limit of strength of the rocks, fracture will 

 occur. This will relieve the stress, and if the adjoining portions of 

 the rock have not lost their original elasticity through long sub- 

 jection to the stress, they will swing back and resume their former 

 relation to the mass with which they still remain connected. 2 



This motion of release appears to be identical with the molar 

 displacement or ' mochleusis ' of Mr. Oldham. It will obviously be 

 greatest near the fault (except so far as it is affected by friction 

 between the moving portions of the rock on opposite sides), and 

 gradually diminish, as was the case in the earthquake above 

 mentioned, as the distance from the fault increases. 



Under the accelerating force of the elasticity of the rock the 



1 Quart. Journ. Geol. Soc. vol. lxv (1909) p. 1. 



2 I have assumed in my descriptions that there is only one fracture, and 

 that the movements are everywhere parallel to it. In practice there will in 

 most cases be smaller secondary fractures, especially near the surface, each with 

 its local movements of release. The great' length of the San Andreas Fault 

 shows, however, that it is not of this secondary character. 



