18 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
is still more convincing, we shall shortly see that not only was the displacement of 1.8 
meters of the Farallons between the survey of 1874-1892 and 1906-1907 insufficient to 
account for the slip on the fault, but the additional displacement of 1.4 meters which 
they experienced between the surveys of 1851-1865 and 1874-1892 leaves this quantity 
still too small. 
We must therefore conclude that the strains were set up by a slow relative displace- 
ment of the land on opposite sides of the fault and practically parallel with it; and that 
these displacements extended to a considerable distance from the fault. Let us consider 
this process; suppose we start with an unstrained region, fig. 6, in which the line AOC 
is straight; suppose forces parallel to B’D” to act on the regions on opposite sides of the 
line B’D” so as to displace A and C to A” and C”; the straight line AOC will be distorted 







































Fia. 6. 
into the line A”OC”; if the distortion is beyond the strength of the rock, a rupture will 
occur along B’D”; the line A”OC” will be broken and the two parts will become straight 
again and will take the positions A”O” and C’Q”; and O”Q” will represent the relative 
slip at the line of rupture, which will be equal to A”A”, the sum of the opposite dis- 
placements which A and C gradually experienced when they were brought to A” and C”. 
All points on the western face of the fault will move a distance OO” to the north, and all 
points on the eastern face a distance OQ” to the south. The straight line which occu- 
pied the positions A”O” and C”Q” just before the rupture will be distorted to A”B” and 
C”D”, these lines being exactly like A”O and C”O, but turned in opposite directions. 
The sum of O”B” and Q”D” will exactly equal 0”Q”, the total slip. 
When we examine the actual displacements about the fault-line, we find that the slip 
B'D’, fig. 5, about 6 meters, is fully 4 meters greater than the relative displacement of 
A’ and C’ since the survey of 1874-1892; this means that the region was not unstrained 
at that time, but that A’ and C’ had already suffered a relative displacement of about 
4 meters from their unstrained positions; that is, two-thirds of the stress which caused 
the rupture had already accumulated 25 years ago. Going still further back to the 
surveys of 1851-1865, we find that the total relative displacement of distant points on 
