136 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
amounted from 2 to 6 meters (7 to 20 feet). These relative displacements were evident 
at every road, fence, or line of trees crossing the fault, but such evidence does not enable 
one to ascertain how far back from the fault in each direction the displacement extended. 
The repetition of the triangulation after the earthquake showed that many points at 
various distances from the fault had all been displaced parallel to the fault, that the dis- 
tribution of the displacements is regular, and that for points nearest the fault, the relative 
displacements corresponded in amount to those observed at roads, fences, tree lines, etc., 
at the fault and which were known to have taken place suddenly. Hence it is certain 
that the widely distributed displacements shown by the triangulation are a part of the 
same phenomenon and took place at the same time as the displacements at the fault, 
that is, suddenly on April 18, 1906. 
For the displacements credited to the year 1868 in this report, the case is different. It 
had been known from previous examination of the evidence given by triangulation that 
Mount Tamalpais had moved between 1859 and 1876. In the course of the detailed 
studies of the triangulation in connection with the present investigation, it was found that 
other triangulation stations had moved at or about 1868. It was discovered that wherever 
triangulation in this part of California before 1868 had been connected with triangulation 
done after 1868, it was necessary, in order to obtain consistent results, to apply abnor- 
mally large corrections to the observed angles. By trial it was found that wherever the 
observations of angles were separated into two groups and separate computations made 
connecting identical points marked upon the ground, one group comprizing observations 
before 1868 and the other observations after that year, that the corrections necessary to 
obtain consistent results from each set of angles were much smaller than before, and 
about the normal size to be expected from the instruments and methods of observation 
used. The evidence proves that permanent displacements took place at or about 1868 
of a magnitude which the triangulation could detect with certainty. The particular year 
in which the displacements took place is not fixt, however, by the triangulation, but 
sniply the fact that they occurred within the interval of several years which elapsed in 
each part of the triangulation between the last observation before 1868 and the first ob- 
servation after that year. For this reason considerable care has been taken in stating 
the dates of the triangulation for each locality. In 1906, it was known that sudden per- 
manent displacements took place on a certain day, hour, and minute along a great fault- 
line and these displacements were similar to those detected later by triangulation. So 
far as the writers know, no evidence has been found that such large sudden relative dis- 
placements took place in 1868 or about that year, but it is known that a very severe earth- 
quake in this region occurred in 1868. Hence the observed displacements, referred in this 
report to 1868 for the sake of brevity, may have occurred in some other year near 1868 
and may have occurred by a gradually creeping motion extending over several years. 
No other abnormal discrepancies in the triangulation within this region are known to 
exist. If there are such discrepancies, produced by displacements of the triangulation 
stations by earthquakes, they are so small as to be effectually masked by the unavoidable 
errors of observation. In other words, any other permanent horizontal displacements by 
earthquakes within this region between 1850 and 1907 must have been much smaller 
than the displacements of 1906 and 1868. 
It has been assumed that there was na permanent displacement of stations Mocho and 
Mount Diablo during the earthquake of 1906. What is the evidence that this assumption 
is true? : 
The true direction or azimuth from Mocho to Mount Diablo was determined by observa- 
tions upon the stars in 1887 and found to be 144° 57’ 35.71”. In 1907 it was redetermined 
by observations upon the stars and found to be 144° 57’ 35.66”, differing by only 0.05” 
