THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 133 
three stations named were displaced to the southeastward in 1868, being about 3 meters 
(10 feet) more than Santa Cruz Azimuth Station and Loma Prieta, and consequently the 
width of Monterey Bay was increased then by about one part in 13,000. 
The combined effects of the earthquakes of 1868 and 1906 have increased the distance 
between Mount Tamalpais and Black Mountain, see map 24 and table 3, by 3 meters (10 
feet). The distance is 79 kilometers (49 miles) and the increase is therefore one part in 
26,000. The Golden Gate lies between these two stations. It is interesting to note 
that the length of part of the Pacific Coast including the Golden Gate has been increased 
just as the distance across Monterey Bay has been increased. 
During the earthquake of April 18, 1906, displaced points on opposite sides of the great 
fault accompanying the earthquake moved in opposite directions, those to the eastward of 
the fault in a southerly and those to the westward in a northerly direction. Among all the 
points there are but two apparent exceptions to this rule, namely, Rocky Mound and 
Red Hill. For both these stations the apparent exceptional movement is so small as to 
be probably due simply to errors of observation and therefore it is not significant. 
During the earthquake of 1906, the permanent displacements of all disturbed points 
were approximately parallel to the fault. When the difficulties encountered in determin- 
ing the direction of these displacements are considered, it is remarkable that the observed 
displacements follow this law so accurately as they do. The nearest fixt points to which 
each displaced point is referred are from 30 to 140 kilometers distant (20 to 90 miles). 
The total displacements are from 0.5 to 4.6 meters (2 to 15 feet). Among all the points 
examined, there are but five for which the apparent changes in distance from the fault 
are not so small as to be probably due to errors of observation. The Farallon Light-house 
apparently moved at an angle of about 27° with the fault and its increase in distance from 
the fault of 0.8 meter is reasonably certain. As Mount Tamalpais, nearly opposite to Fa- 
rallon Light-house across the fault, moved practically parallel to the fault, there was 
either an opening of the fault beneath the sea in this region or an increase in length of the 
earth’s crust, in a direction at right angles to the fault, of one part in 50,000 (0.8 meter on 
44 kilometers, or 3 feet on 27 miles). Point Reyes Light-house also apparently receded 
from the fault, moving in about the same direction (within 5°) as the Farallon Light-house, 
but the determination of the displacement of the Point Reyes Light-house is so weak that 
this apparent displacement has little significance. It is reasonably certain that Bodega 
Head approached the fault from the western side, while Bodega, on the eastern side 
of the fault, about opposite, moved parallel to the fault. The apparent closing up of 
the fault or shortening of the crust at right angles to the fault is 1.6 meters (5.2 feet) 
between these two points only 5.4 kilometers (3.4 miles) apart. This is one part in 3,400. 
It is possible that as much as one-half of this apparent closing up is due to errors of 
observation, but it is reasonably certain that not all of it is due to that cause. Similarly 
it is reasonably certain that Peaked Hill in the Fort Ross group receded from the fault on 
the east side and Pinnacle Rock approached it on the west side, the apparent amounts 
being 0.4 meter (1.3 feet) and 0.7 meter (2.3 feet) respectively. It is reasonably certain 
that San Pedro Rock in the Colma group approached the fault from the west side, the 
apparent amount being 1.1 meters (3.6 feet). 
During the earthquake of 1906, the displacements on each side of the fault were less 
the greater the distance of the displaced points from the fault. On the eastern side of the 
fault, ten points at an average distance of 1.5 kilometers (0.9 mile) from the fault have an 
average displacement of 1.54 meters (5.1 feet) ; three points at an average distance of 4.2 
kilometers (2.6 miles) have an average displacement of 0.86 meter (2.8 feet), and one 
point, Mount Tamalpais, at 6.4 kilometers (4.0 miles) from the fault, has a displacement 
of 0.58 meter (1.9 feet). These fourteen points are the only ones on the eastern side of the 
fault for which the observed displacements were determined with reasonable certainty. 
For no point to the eastward of the fault at a greater distance than 6.4 kilometers (4.0 
