THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 137 
from its former value. The maximum possible difference between the two determinations 
of azimuth which could occur simply as errors of observation is about 1”. Hence these 
observations show positively that the true direction from Mocho to Mount Diablo had not 
changed between these dates by as much as 1” and probably had not changed by as much 
as 0.3”. 
The true direction or azimuth of the line Mount Tamalpais to Mount Diablo was 
determined by observations upon the stars in 1882 and again in the same manner in 
1907. In 1882 it was found to be 274° 15’ 15.04” and in 1907, 274° 15’ 14.49”, 0.55” 
less than before. The azimuth of the line Mount Tamalpais to Mount Diablo was com- 
puted separately from the triangulation between 1868 and 1906, and from the triangu- 
lation after the earthquake of 1906 and the two values found to be 274° 15’ 19.46” and 
274° 15’ 17.89” respectively, the second being 1.57” less than the first. This apparent 
decrease of azimuth as determined by the triangulation agrees within 1.02” with the 
decrease of 0.55” determined independently by astronomic observations.” This agree- 
ment is within the range of possible errors of observation. In the two computations of 
the triangulation, the line Mocho-Mount Diablo was assumed to have the same azimuth 
before and after April 18, 1906; hence the close agreement noted indicates that the 
azimuth Mocho-Mount Diablo remained unchanged. 
In the investigation which has been made, it was found that the absolute displacement 
decreased with increased distance from the fault and that no displacement sufficiently large 
to be detected with certainty was found farther to the eastward of the fault than Mount 
Tamalpais, 6.4 kilometers (4.0 miles) from it. _Mocho and Mount Diablo are 53 kilometers 
(33 miles) from the fault ; hence it seems certain that the displacements, if any, at Mocho 
and Mount Diablo must have been extremely small. It may be objected that this is reason- 
ing in a circle, inasmuch as the computed displacements depend upon the assumption 
that Mocho and Mount Diablo stood still. Cleared of this objection, the argument reduces 
to the following. The triangulation shows no relative displacements in 1906, large enough 
to be determined with certainty, of Mocho, Mount Diablo, Rocky Mound, Red Hill, and Lick 
Observatory, a group of points far to the eastward of the fault, whereas many points 
nearer to the fault showed large relative displacements as referred to each other, with a 
marked tendency to be greater the nearer to the fault are the groups of points compared. 
Hence the reasoning is valid that Mocho and Mount Diablo remained unmoved, these being 
two points in a group showing no displacements relative to each other, the whole group 
being far from the fault and these two particular stations being the two points most distant 
from, the fault. 
If either Mocho or Mount Diablo had moved in April, 1906, in such a direction as to 
decrease (or increase) the azimuth of the line joining them, the effect of the erroneous 
assumption, used in the computation of the triangulation done after the earthquake 
that the azimuth had remained unchanged, would have been to produce a set of computed 
apparent displacements which would be represented by red arrows on map 24, all indi- 
cating a rotation in a clockwise (or counterclockwise) direction around Mount Diablo, 
the lengths of the arrows being proportional to their distances from Mocho and Mount 
Diablo. The fact that the computed apparent displacements of 1906, as shown by the red 
arrows on maps 24 and 25, do not show any such systematic relation to each other, jndi- 
cates that the line Mocho-Mount Diablo remained unchanged in azimuth on April 18, 1906. 

1 The probable error of observed azimuth in 1887 was + 0.21” and in 1907 £0.20”. The expression 
“»robable error’”’ is here used in the technical sense in which it is used in connection with the least square 
method of computation. 
2 The discrepancy of about 4’” on each date between the azimuth determined by astronomic observa- 
tions and the azimuth determined by triangulation is what is known as “station error” in azimuth and is 
due to the deflection of the vertical at the observation station. It does not enter into the present dis- 
cussion, which is based on differences of azimuths of the same kind, either astronomic or geodetic, on 
different dates at the same station, : 
