22 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
40 km. (25 miles) long was felt slightly in Boston, a distance of 1,350 km. (850 miles). If 
we assume that the vibrations from the two disturbances had about the same periods and 
that a certain acceleration is necessary for a shock to be felt, we find that the ampli- 
tude of the vibration must have been about the same at Boston and at Winnemucca, for 
the two shocks, respectively ; as the amplitude would diminish inversely as the distance 
for the Charleston earthquake, but much more slowly for the California earthquake on 
account of the length of the fault-line, the amplitude of the former disturbance must 
have been many times as great as that of the latter at the same distance from the origin ; 
and the intensity must have been very many times greater per unit area of the fault-plane 
for the Charleston earthquake than for the California earthquake. 
The above calculation of stresses applies especially to the region north of San Francisco ; 
to the south the slip at the fault-line was, in places and perhaps for all this part of the 
fault, somewhat smaller. At Wright the slip on the fault-plane in the tunnel is given 
by the engineers as 5 feet, and the west side was shifted toward the north (vol. 1, fig. 42, 
and pp. 111-113). This is a case of elastic rebound as at other parts of the fault. The 
character of the material in the tunnel and the numerous cracks in the surrounding moun- 
tain, one of which shows a relative shift opposite to that generally observed (p. 35), 
lead us to expect more or less irregularity in the distortion of the tunnel, which is con- 
firmed by the figure. The greatest angle of shear must be something more than half 
the slip at the fault-plane divided by the distance over which the distortion is distributed; 
this gives 2.5/5,150 or 1/2,000, approximately. The angle of distortion is apparently 
slightly less here than further north. The smaller slip in the neighborhood of Colma, a 
little south of San Francisco, may be due to the partial relief of strain by the earthquake 
of 1868; for it shows that this region was under less strain at the time of the II survey 
than the region further north. 
THE WORK DONE BY THE ELASTIC STRESSES. 
We can also determine the work done at the time of the rupture; it is given by the 
product of the force per unit area of the fault-plane multiplied by the area of the plane 
and by half the slip. If we take the depth of the fault at 20 km. (12.5 miles), the length 
at 435 km. (270 miles), the average shift at 4 meters (13 feet), and the force at 1 x 10° 
dynes per square centimeter (1,450 pounds per square inch), we find for the work 1.75 x 
10* ergs (1.38 < 10*7 foot-pounds), or 130,000,000,000,000,000 foot-pounds.* This energy 
was stored up in the rock as potential energy of elastic strain immediately before the 
rupture; when the rupture occurred, it was transformed into the kinetic energy of the 
moving mass, into heat and into energy of vibrations; the first was soon changed into 
the other two. When we consider the enormous amount of potential energy suddenly 
set free, we are not surprised, that, in spite of the large quantity of heat which must have 
been developt on the fault-plane, an amount was transformed into elastic vibrations 
large enough to accomplish the great damage resulting from the earthquake and to shake 
the whole world so that seismographs, almost at the antipodes, recorded the shock. 
THE DISTRIBUTION OF THE DEFORMING FORCES. 
In examining what forces could have caused the slow displacements which brought 
about the strains existing in the region before the rupture, we note that gravity does 
not seem to have been directly active, as the displacements were practically horizontal. 
Any force except gravity could only have been applied to a boundary of the region 


+ It is probable that the maximum strain was not produced at all parts of the fault-plane, and espe- 
cially not near its ends; but when the rocks broke at one place, the stress was thrown upon adjacent 
parts and the fracture thus carried along; in this way the fault was probably made much longer than it 
would otherwise have been. This consideration leads us to put the maximum stress at three-quarters 
the value determined from the distortion of the rock. 
