THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 139 
even this extremely small displacement of the pole of maximum moment of inertia 
computed above, 0.002 meter, does not immediately affect the latitude of points in Cali- 
fornia, but only tends to change them by that average amount in the course of a year or 
more. The effect of the earthquake on the latitudes of points outside the region of actual 
displacement of the surface is therefore entirely negligible. The earthquake changed the 
latitude of marked points on the earth’s surface within the disturbed region by the amount 
of the northward or southward components of the displacement of the points. 
Similarly, the possible effect of the displacements on the deflections of the vertical, that 
is, upon the direction of gravity at any point, is too small to be considered. 
The displacements near Point Arena were computed upon the assumption that the 
triangulation stations Fisher and Cold Spring remained unmoved during the earthquake 
of 1906. Is this assumption true? The station farthest to the eastward from the 
fault at which a displacement in 1906 has been detected with certainty is Mount Tamal- 
pais, distant 6.4 kilometers and displaced 0.53 meter. Also the rate of decrease of dis- 
placements at this distance has been found to be 0.13 meter per kilometer of increase of 
distance from the fault. At this rate, the displacement would become zero at about 11 
kilometers from the fault. Fisher is 11.2 and Cold Spring 13.5 kilometers from the 
fault; hence it is reasonably certain that if the displacement was not zero, at these two 
stations, it was so nearly zero that it could not have been detected with certainty. 
A high degree of accuracy has been claimed for the triangulation. There is abundant 
evidence available from which to determine the actual accuracy, as has been indicated in 
an earlier part of this report. A large amount of time has been spent in studying this 
evidence in order to insure that the estimates of the accuracy of the determination of the 
various apparent displacements might be reliable. The methods necessarily followed 
in estimating the accuracy are too technical and too complicated to be included in this 
report. Two illustrations of the degree of accuracy attained in the observations may 
prove interesting, however. 
The position of the Lick Observatory small dome was determined after the earthquake 
of 1906 by intersections upon it from four stations, Loma Prieta, Sierra Morena, Red Hill, 
and Mocho. There were discrepancies among these observations which were adjusted 
by the method of least squares and a resulting most probable position adopted and used 
in computing the apparent displacement given in table 1. The mean observation from 
Loma Prieta hit 0.388 meter (1.2 feet) to the left of the position adopted for the dome. 
The mean observation from Sierra Morena hit 0.22 meter (0.7 foot) to the right, that 
from Red Hill 0.01 meter (0.03 foot) to the-left, and that from Mocho 0.11 meter (0.4 
foot) to the left of the adopted position. The words ‘‘right”’ and ‘‘left”’ refer in each 
case to the Lick Observatory dome as seen from the station named. The distance of the 
four observation points from the Lick Observatory were, Loma Prieta 31 kilometers (19 
miles), Sierra Morena 59 kilometers (87 miles), Red Hill 46 kilometers (29 miles), and 
Mocho 17 kilometers (11 miles). 
Similarly the determination of the position of the Lick Observatory before the earth- 
quake depended upon observations taken from seven stations, Santa Ana, Mount Toro, 
Loma Prieta, Sierra Morena, Mount Tamalpais, Mount Diablo, and Mocho. ‘The line 
from Mount Tamalpais, 106 kilometers (66 miles) long, mist the adopted position by 0.36 
meter (1.2 feet). The other six all came nearer than this to the adopted position. 
The Farallon Light-house was determined between 1868 and 1906 by intersections 
upon it from three stations, Mount Helena, Mount Tamalpais, and Sierra Morena. The 
mean observation from Mount Helena, distant 112 kilometers (70 miles), mist the adopted 
position by 0.30 meter (1.0 foot) and the other two lines came closer. In 1906-1907 the 
Farallon Light-house was determined by intersections upon it from the six stations Ross 
Mountain, Tomales Bay, Point Reyes Hill, Sonoma, Mount Tamalpais, and Sierra Mo- 
