THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 127 
kilometers (40 miles)long. They are subject, therefore, to an error of position common to 
them all, which may be as great as 7 meters (23 feet). With the exception of Montara 
Mountain Peak and Bonita Point Light-house these points are all within 13 kilometers 
(8 miles) of San Bruno Mountain and therefore their relative positions were determined 
with considerable accuracy. 
In the triangulation of 1906-1907, the position of San Bruno Mountain, which is in the 
midst of this group, was determined by secondary triangulation in connection with group 
2 as indicated on maps 24 and 25, a direct and strong determination. The new azimuth 
was also carried into the triangulation of group 3 with a high degree of accuracy in this 
same manner. No new determination was made of the starting length in group 3. It 
was assumed that the length San Bruno Mountain to Black Ridge 2 had remained un- 
changed during the earthquake of 1906 and the old value of that length was used in the 
computation of the triangulation of 1906-1907. As a check upon the assumption that 
this length remained unchanged, it is to be noted that the azimuths of this line before 
and after the earthquake of 1906 were found to differ only by 9.3”, which is within the 
possible range of errors of observation in the earlier triangulation. 
For the reasons stated above, the apparent absolute displacements shown in table 1 
for group 3, as referred to Mocho and Mount Diablo as fixt points, are probably due 
to errors of observation. 
On account, however, of the fact that seven of the nine points in this group are within 
a rather small area, their relative displacements are determined with considerable ac- 
curacy, the errors of length and azimuth having less effect in producing errors in relative 
positions, the smaller the area covered by a triangulation. Montara Mountain Peak and 
Bonita Point Light-house are each determined with a low grade of accuracy. They are 
each far from the stations occupied in the triangulation and the lines which determine 
them intersect at a small angle; hence even their relative displacements are uncertain. 
The relative displacements observed for the remaining seven points after omitting these 
two are certain, being beyond the possible range of errors of observation. 
The apparent absolute displacements for this group of points (see table 1 and map 25) 
indicate that all points on the eastern side of the fault moved in a southerly direction, 
and those on the western side in a northerly direction; that the displacements tend to be 
parallel to the fault, the more doubtful displacements showing the greater angles with the 
fault; and that the amounts of the displacement are in the inverse order of the distances 
of the stations from the fault, with two exceptions. ‘These exceptions are San Pedro 
Rock, of which the relative displacement is determined with sufficient accuracy to establish 
this as a real exception; and Bonita Point Light-house, for which the apparent displace- 
ment as observed is so uncertain that this apparent exception has but little significance. 
Of the four points, all on the western side of the fault, of which the relative displacements 
are believed to be certain, as indicated in table 1, the azimuths of the displacements vary 
from 151° to 169°, with a mean of 157° (23° W. of N.). The azimuth of the fault in this 
vicinity is 144° (36° W. of N.). 
The relative displacements on opposite sides of the fault and near to it are less in this 
group (2 to 3 meters) than for points at a similar distance from the fault in group 1; 
namely, Point Reyes Hill, Tomales Bay, and Bodega (5 to 6 meters). 
Group 4. Tomales Bay. — There are seven points in this group (see tables 1 to 3 and 
maps 24 and 25). These were fixt in 1856-1860 by tertiary triangulation extending 
southeastward along Tomales Bay from stations Tomales Bay and Bodega of group 1. 
They were fixt again in practically the same manner in 1906 after the earthquake. 
With these seven points may advantageously be considered the three points, Point 
Reyes Hill, Tomales Bay, and Bodega, which were fixt in group 1. 
