134 . | REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
miles) was‘any displacement detected with certainty. 'To the westward, twelve points at 
an average distance of 2.0 kilometers (1.2 miles) from the fault have an average displace- 
ment of 2.95 meters (9.7 feet). Seven at an average distance of 5.8 kilometers (3.6 miles) 
have an average displacement of 2.38 meters (7.8 feet). The only other point to the 
westward of the fault of which the displacement was determined with certainty was Faral- 
lon Light-house, distant 37 kilometers (23 miles) and displaced 1.78 meters (5.8 feet). 
In receding from the fault, either to the eastward or to the westward, the displacement 
decreases more rapidly near the fault than it does farther from the fault. According to 
the averages given in the preceding paragraph, the decrease in displacement on the 
eastern side near the fault is at the rate of 0.25 meter per kilometer (that is, 0.68 meter 
on 2.7 kilometers) and farther away the rate is 0.13 meter per kilometer (that is, 0.28 
meter on 2.2 kilometers). Imagine a straight line before the earthquake of April 18, 1906, 
starting at the fault and extending eastward at right angles to it. According to this 
investigation, after the earthquake this line became a curved line concave to the south- 
ward, the point at the fault being displaced southward and distant points on the line re- 
maining fixt. Also according to the above figures, the part of the line which is from 1.5 
to 4.2 kilometers from the fault was deflected from its former direction about 52 seconds 
and that part from 4.2 to 6.4 kilometers from the fault was deflected about 26 seconds, and 
the deflection probably decreased gradually to zero at distant points. To the westward 
of the fault the rate of decrease of displacement, according to the averages in the preceding 
paragraph, near the fault is 0.15 meter per kilometer (that is, 0.57 meter on 3.8 kilometers), 
and farther away only 0.02 meter per kilometef (that is, 0.60 meter on 31 kilometers). 
Accordingly the imaginary straight line at right angles to the fault and extending west- 
ward from it has become concave to the northward, the point at the fault being dis- 
placed to the northward and very distant points remaining fixt. The deflection from its 
original direction is about 31 seconds for the part from 2 to 6 kilometers from the fault 
and about 4 seconds on an average for the part from 6 to 37 kilometers from the fault. 
For points on opposite sides of the fault of 1906, and at the same distance from it, 
those on the westward side are displaced on an average twice as much as those on the 
eastern side. This statement applies especially to points within 10 kilometers (6 miles) 
of the fault. For points farther away, the ratio becomes more than two to one. It is 
important to notg that this statement applies to displacements, not distortions. The 
distortion, exprest in angular measure, discust in the preceding paragraph, is nearly the 
same on the two sides of the fault, being somewhat less close to the fault on the western 
side than on the eastern side. : 
The amount of relativ@displacement of the two sides of the fault by sliding along the 
fault, as detected by the triafgulation, shows no variations for different parts of the fault 
along its whole length from Point Arena to San Juan, with one exception, which are suffi- 
ciently large to be clearly not due to errors of observation. This exception is the region 
near Colma where, as already noted, relative displacements seem to be unusually small. 
The permanent displacements and distortions which took place at the time of the 
earthquake of April 18, 1906, may be pictured by imagining a series of perfect squares 
drawn on the surface of the ground before the earthquake, with their sides parallel 
and perpendicular to the fault. At the time of the earthquake every square to the east- 
ward of the fault moved bodily in a.southerly direction parallel to the fault, the squares 
more distant from the fault moving less than those near to it. All sides of squares parallel 
to the fault remained straight lines, unchanged in length and direction. For the squares 
to the eastward, the sides perpendicular to the fault became curved lines concave to the 
southward and changed direction as a whole by rotation in a counterclockwise direction, 
the change being 52 seconds or more for squares near the fault, and less for more remote 
squares. The angles of the squares all took new values differing from 90° by quantities 
