PERMANENT DISPLACEMENTS OF THE GROUNDS. 19 
opposite sides of the fault since that date amounts to about 3.2 meters, a little more than 
half enough to account for the slip on the fault-plane; therefore 50 years ago the elastic 
strain, which caused the rupture in 1906, had already accumulated to nearly half its 
final amount. It seems not improbable, therefore, that the strain was accumulating 
for 100 years, altho there is no satisfactory reason to suppose that it accumulated at a 
uniform rate. 
We can picture to ourselves the displacements and the strains which the region has 
experienced as follows: let AOC (fig. 6) be a straight line at some early date when the 
region was unstrained. By 1874-1892, A had been moved to A’ and C' to C’, and AOC 
had been distorted into A’OC’; by the beginning of 1906, A had been further displaced 
to A” and C to C”, the sum of the distances AA” and CC” being about 6 meters; and 
AOC had been distorted into A”0C”. When the rupture came, the opposite sides of 
the fault slipt about 6 meters past each other; A”’O and C”O straightened out to A”O” 
and C”(”; and the straight lines which occupied the positions A”O” and C’Q” just before 
the rupture, were distorted afterward into the lines A”B” and C”D”, these lines being 
exactly like the lines A”O and C”O but turned in opposite directions. The straight lines, 
which occupied the positions A’O’ and C’Q’ in 1874-1892, were distorted into A”O’ and 
C”(’ in the beginning of 1906; at the time of the rupture their extremities on the fault- 
line had the same movements as other points on that line; O’ moved to B’ and @ to D’. 
If we should move the left half of our figure so as to make A’O’ continuous with C’Q’, 
fig. 6 would then be practically similar to fig. 5 and similar letters in the two figures 
would refer to the same points; in fig. 5, however, we have supposed C’ to remain sta- 
tionary and have attributed all the relative movement to A’, whereas in fig. 6 we have 
divided the movement equally between A’ and C’; as we do not know the actual, but 
only the relative, movement this difference has no significance. 
What was actually determined by the two surveys were the distances of points on the 
line C’D’ and A”B’ in fig. 5 measured from the line C’A’; and this is equivalent in fig. 6 
to the distances of the line C”D’ from C’Q”, and A”B’ from Q”A” less the distance O’Q’. 
The divergence of the lines A’B’ and C”D’ from straight lines does not represent 
the strains which existed in the region just before the rupture, but only the strains 
accumulated before 1874-1892; we have seen that the total strains set up by 1906 are 
represented by the divergence from straight lines of the lines A”O and C”O, or their 
counterparts, A”B” and C”D”. 
ILLUSTRATIVE EXPERIMENTS. 
The following very simple experiments were made to illustrate the conclusions we have 
arrived at regarding the elastic strains and the relations between the slip at the fault- 
plane and the displacements of distant points. A sheet of stiff jelly about 2 cm. thick 
and 4 cm. wide was formed between two pieces of wood (fig. 7) to which it clung fairly 
well. <A straight line AC was drawn on the jelly, which was then cut by a sharp knife 
along the line tt’; the left piece of wood was then moved about 1 cm. parallel with ti’, 
as shown in fig. 8; a slight pressure on the jelly prevented slipping along the cut line; 
the jelly was thus subjected to an even shear thruout and the original straight line AC 
was distorted into the line A”C; when the pressure on the jelly was removed, the elastic 
stresses set up by the distortion came into action, the two sides of the jelly slipt past 
each other along the line tt’, A”O straightened out to A”O”, and CO to CQ”, the slip Q”O” 
being equal to the distance AA”; and all the strain in the jelly was relieved. (The 
difference in the straight line A”OC in the jelly and the curved line A”OC” (fig. 6) in the 
rock will be explained later.) 
A second straight line A”O”C” was drawn across the jelly after A had been displaced, 
but before it was allowed to slip on the line ¢t’; when the slip took place, this line broke 
