Vol. 6] Beid: The Elastic-Rebound Theory of Earthquakes. 421 



the greatest change in direction of any part of the lines between 

 the two surve^ys being about one minute of arc. What kind of 

 forces could have caused this movement? Gravity could not 

 have been the immediate cause of the sudden and nearly hori- 

 zontal displacements, nor could volcanic explosions; the only 

 forces capable of producing such movements are elastic forces. 

 Since the material composing the earth's crust is elastic, and 

 cannot rupture until it is strained beyond its strength, it is 

 evident that an earlier relative displacement of regions on oppo- 

 site sides of the fault had set up an elastic strain in the inter- 

 mediate zone, which exceeded the strength of the rock, causing 

 the rupture along the fault surface ; and that the rock on oppo- 

 site sides of the fault, under the action of its own elastic stresses, 

 then suddenly sprang back to positions of equilibrium, the oppo- 

 site sides moving in opposite directions, and relieving the elastic 

 strain. This is the only satisfactory explanation of the obser- 

 vations and determined displacements. If a curved line, AOC 

 (fig. 2), continuous at 0, and with its two sides exactly like the 

 two lines A" B' and D'C, but bent in opposite directions, had 

 been drawn on the ground just before the earthquake, it would 

 have broken and straightened out into two lines, AB and DC, 

 at the time of the rupture. The line A'O'C, straight at the time 

 of survey II, must have been changed into the line A"0"(" be- 

 fore the rupture, and, as we have seen, into A"B' and D'(" im- 

 mediately afterwards. 



All changes in the shape of a solid body may be reduced to 

 two types, — changes in volume, either compressions or extensions, 

 and slipping of one part past another, as the various cards of 

 a pack may be made to slip over each other. When a beam is 

 bent, the convex side is stretched and the concave side com- 

 pressed, and the elastic forces thus brought into play resist the 

 bending; and if the forces which bend the beam are removed, 

 the elastic forces will cause it to straighten out again. If the 

 cards in our illustration are held together by an elastic cement, 

 we readily see that when they are made to slip slightly over 

 each other there will be a resistance to the movement and on 

 releasing the disturbing pressure they will return to their original 

 position. This kind of a change in shape, when each card be- 



