﻿Tol. 66,~] AN EARTHQUAKE MODEL. 349 



support. That on trie left (f\) is capable of being moved upwards l 

 by the action of a screw. The other on the right (/ 2 ) can move a 

 short distance to the left. To each of these frames a number of 

 steel strips are attached at one end. These are covered with a 

 thin flexible silk membrane, which shares their movements. The 

 free ends of the steel strips attached to each frame are pivoted in a 

 movable vertical rod. The two rods r v r 2 , one for each frame, are at 

 first closely connected by an interlacing loop of string, so that a 

 continuous surface is obtained which comprises both membranes, 

 as shown in fig. 1 (p. 348). It represents the whole region before 

 the commencement of the creep. It is seen to be traversed by 

 continuous horizontal straight lines as well as by an interrupted 

 curved line, the meaning of which will appear immediately. 



The screw is now turned so that the frame f x moves upwards 

 into the position shown in fig. 2, when the lines which were formerly 

 straight and horizontal become curved in a sigmoid manner, the 

 curvature being reversed where they traverse the vertical rods. 

 This position represents the conditions after the slow creep has 

 progressed until the limit of the strength of the rocks has been just 

 reached, so that fracture is on the point of taking place. The 

 interrupted line now stretches straight across, horizontally, from 

 side to side. It may represent straight roads or fences transverse 

 to the fault made in the period immediately preceding the earth- 

 quake. 



The ' fracture ' is effected by cutting the string. This releases 

 the two rods, which at once fly apart, ?\ upwards and r 2 downwards. 

 Shortly after they have passed the position of equilibrium they are 

 brought to a stop by the collision of two metallic masses attached 

 to them, which are thrown into rapid vibration and sound the 

 corresponding notes. As they are not quite in unison, distinct 

 beats are heard. At the same time, the rods swing back and vibrate 

 with a comparatively slow period about the position of equilibrium. 

 The model finally comes to rest in the position shown in fig. 3, 

 where the interrupted line which was straight in fig. 2 has become 

 curved in fig. 3, resembling very closely the lines drawn as a result 

 of the geodetic observations taken since the Californian earthquake. 2 

 At the same time, the ends of the lines are seen to be turned back 

 by the friction between the two sides of the fissure. 



If the model is to be considered as a representation of the course 

 of events in connexion with the Californian earthquake, the slow 

 motion of the left-hand frame expresses a gradual creep of the bed 

 of the Pacific to the north-west relatively to the North American 

 continent, giving rise to a region of strain in the neighbourhood 

 of the coast-line, which increases until fracture occurs in the San 

 Andreas Fault. 



1 Namely, upwards in the photograph. The model works best, however, 

 when the frames are horizontal. 



2 R. D. Oldham, Quart. Journ. Geol. Soc. vol. lxv (1909) fig. 2, p. 4. 



