THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 129 
For one point, Chaparral, observations made in 1860 furnish a determination of the 
position before 1868, and hence the displacement of this point in 1868 (see table 2) is 
determined as well as its displacement in 1906. The displacement of 1868 agrees closely, 
within less than 0.13 meter (0.4 feet) in amount and 9° in direction, with the displace- 
ment at that time at Ross Mountain, 5.7 kilometers (3.5 miles) to the eastward. 
A study of the possible accumulated errors in the triangulation shows that five of the 
observed displacements in this group, as referred to Mocho and Mount Diablo, are clearly 
beyond the range of possible errors of observation; namely, those at Fort Ross, Funcke, 
Timber Cove, Stockhoff, and Pinnacle Rock. For the remaining seven displacements, 
there are from one to two chances out of ten that they are due entirely to errors of observa- 
tion, and these displacements are therefore reasonably certain. The relative displacements 
of pairs of points on opposite sides of the fault and near to each other in this group are 
certain, being in every case clearly beyond the range of possible errors of observation. 
The apparent displacements in 1906 of the twelve points in this group conform closely 
to the four deduced laws governing such displacements. The seven points to the west- 
ward of the fault moved in a northerly direction, in azimuth varying from 137° to 158°, 
with a mean of 144° (86° W. of N.). The azimuth of the fault in this region is about 141° 
(39° W. of N.). All five points to the eastward of the fault moved southerly, in azimuth 
varying from 301° to 328° with a mean of 318° (42° E. of 8.). All of the points in this 
group are within 3.2 kilometers (2.0 miles) of the fault and therefore give little opportu- 
nity to ascertain whether the amounts of the displacements show any relation to distances 
from the fault. Such a relation is not clearly discernible among the observed displace- 
ments. The evidence of the apparent displacement at Ross Mountain (see table 1), 
6.2 kilometers (4.2 miles) to the eastward of the fault, indicates a decrease of displacement 
with increase of distance from the fault in that direction. The average displacement of 
the five points to the eastward of the fault is 1.44 meters (4.7 feet) and that of the seven 
points to the westward is 1.5 times as great, namely, 2.11 meters (6.9 feet). 
Group 6. Point Arena. — In this group there are ten points determined by secondary 
triangulation in 1870 to 1892 that were redetermined by secondary triangulation in 1906, 
starting from the stations Fisher and Cold Spring, 11.2 and 13.5 kilometers eastward from 
the fault respectively. (See map 25.) A study of the possible errors in the triangulation 
shows that all of the observed displacements in this group are certain, each being clearly 
greater than the maximum possible errors of observation. There is a possibility that the 
assumption that the two stations, Fisher and Cold Spring, remained unmoved in 1906 is 
in error. The movement, if any, of these stations was probably about the same for both 
stations and in a southerly direction and parallel to the fault. If such a movement of 
these stations occurred, the computed displacements in 1906, shown in table 1 and on map 
25, are all too small for stations to the eastward of the fault, and too great for stations to 
the westward of it. 
The agreement of the observed displacements of the ten points in this group with the 
four deduced laws is close. The six points to the westward of the fault moved in azi- 
muths varying thru a range of 5° only, from 159° to 164°, with a mean of 162° (18° W. of 
N.). The fault in this vicinity is said to change in azimuth, near the point where it crosses 
the coast-line, from about 144° to about 164° (16° W. of N.), curving to the eastward. 
The four points to the eastward of the fault moved in azimuths varying from 324° to 340° 
with a mean of 330° (30° E. of S.). The station Shoemake, comparatively near to the 
fault, 1.5 kilometers (0.9 mile), on the west side, showed a displacement much larger than 
any of the other five points on that side, all of which are from 5.7 to 7.6 kilometers from 
the fault. The two points to the eastward of the fault which are within less than 1 
kilometer of it were displaced nearly twice as much as the other two which are nearly 
4 kilometers from the fault. The average displacement for the four points to the east- 
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