PERMANENT DISPLACEMENTS OF THE GROUNDS, 21 
It will be observed that three points are determined on the eastern line near enough 
to the fault to enable us to draw the line fairly well and to extend it to the fault at D’ 
(fig. 5). We have but two points determined on the western line near the fault, which 
are not enough to determine the character of the line; but a third point is determined 
from the fact that B’ must be about 6 meters from D’, and we can therefore draw the 
western line fairly well also. Its general form is like that of the eastern line, but its 
curvature is somewhat less. This is probably in part due to the fact that the rocks on 
the western side of the fault are more rigid than those on the eastern side; for former 
movements on this fault have raised the western side relatively to the eastern and 
brought the more rigid crystalline rocks nearer the surface. 
In fig. 6 B’B” = O’O” = 0.9 meter, that is, half of 1.8 meters, the total relative dis- 
placement of A’ and C” between the two surveys; and since O”B” is a little less than 
half the total slip, on account of the greater rigidity of the western rocks, we may esti- 
mate it at 2.8 meters. Therefore O”B’equals 1.9 meters, and O”B” is 1.47 times O”B’; 
and since the curves A”B’ and A’B” are both curves of elastic distortion of the same sub- 
stance the angle at B” must be 1.47 times that at B’.1| We can measure the angles at B’ 
in fig. 5 and we find it 1/2,500; therefore the angle at B” is 1/1,700; similarly we find 
the angle at D” to be 1/1,000. 
We can determine the force necessary to hold the two sides together before the rupture, 
which must exactly have equaled the stress which caused the break. The force per square 
centimeter is given by the expression ns where n is the coefficient of shear and s is the 
shear, measured by the angle at O or B” for the western side of the fault, or the angle at 
O or D” for the eastern side. We shall see further on that in the crystalline rocks below 
the surface the strain was somewhat greater than at the surface, so that we may assume 
that the angle corresponding to B” lower down may be as high as 1/1,500. 
The experiments of Messrs. Adams and Coker ? give the value of n for granite as 2 x 104 
dynes per square centimeter (2,900,000 pounds per square inch); therefore the force 
necessary to produce the estimated distortion at the fault-plane at a short distance below 
the surface is 1/1,500 of this, or 1.33 x 108 dynes per square centimeter (1,930 pounds 
per square inch). ‘There are no very satisfactory determinations of the strength of gran- 
ite under pure shear; tests made at the Watertown Arsenal * gave values ranging between 
about 1.2 x 10" and 1.9 x 10" dynes per square centimeter (between 1,700 and 2,900 
pounds per square inch), but these values are apparently too small, for the specimens 
were subjected to tensions and compressions as well as to shear. The rock at a distance 
below the surface would probably have a greater resistance to shear on account of pressure 
upon it, and moreover it has not been subjected to the changes of temperature, etc., 
which the surface rocks experience, so that it probably has a strength greater than the 
higher figure given. We must therefore conclude that former ruptures of the fault- 
plane were by no means entirely healed, but that this plane was somewhat less strong 
than the surrounding rock and yielded to a smaller force than would have been necessary 
to break the solid rock. This idea is strongly supported by a comparison of the distance 
to which this shock and the earthquake of 1886, at Charleston, South Carolina, made 
themselves felt. With a fault-length of 435 km. (270 miles), the California earthquake 
was noticed at Winnemucca, Nevada, a distance of 550 km. (350 miles) at right angles 
to the fault; whereas the Charleston earthquake, with a fault-line certainly less than - 


1 This reasoning is not perfectly rigid; the similarity of the lines A”B’ depends upon the similarity of 
strains set up during the intervals between the I and II, and the II and III surveys. These were prob- 
ably fairly similar, as the difference between them represents the strain added between the II and III 
surveys which was only a fraction of the total strain at the time of the break; and the results obtained 
upon this assumption can not be very far wrong. 
2 An Investigation into the Elastic Constants of Rocks. Frank D. Adams and Ernest G. Coker, 
Carnegie Institution of Washington, Publication No. 46, 1906. 
8 Report of Tests of Metals, etc., made at the Watertown Arsenal, 1890, 1894, 1895. Washington, D.C. 
