THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906, 
THE FAULT-TRACE. 
The successive movements which in the past have given rise to the peculiar geomorphic 
features of the Rift, either directly or by control of erosion, have with little question 
been attended in every case by an earthquake of greater or less violence. The earth- 
quake of April 18, 1906, was due to a recurrence of movement along this line. The 
movement on that day was of the nature of a horizontal displacement on an approxi- 
mately vertical fault plane or zone, whereby the country on the southwest side was 
moved to the northwest and the country on the northeast side to the southeast. This 
displacement was manifested at the surface by the dislocation and offsetting of fences, 
roads, dams, bridges, railways, tunnels, pipes, and other structures which crost the line 
of the fault. The surface of the ground was torn and heaved in furrow-like ridges. 
Where the surface consisted of grass sward, this was usually found to be traversed by 
a network of rupture lines diagonal in their orientation to the general trend of the fault. 
Small streams flowing transverse to the line of the fault had their trenches dislocated 
so that their waters became impounded. These and similar phenomena of disruption 
constitute the fault-trace. 
The width of the zone of surface rupturing varied usually from a few feet up to 50 feet 
or more. Not uncommonly there were auxiliary cracks either branching from the main 
fault-trace obliquely for a few hundred feet or yards, or lying subparallel to it and not, 
so far as disturbance of the soil indicated, directly connected with it. Where these 
auxiliary cracks were features of the fault-trace, the zone of surface disturbance which 
included them frequently had a width of several hundred feet. The displacement appears 
thus not always to have been confined to a single line of rupture, but to have been dis- 
tributed over a zone of varying width. Generally, however, the greater part of the 
dislocation within this zone was confined to the main line of rupture, usually marked by 
a narrow ridge of heaved and torn sod. 
The amount of the horizontal displacement, as measured on dislocated fences, roads, 
etc., at numerous points along the fault-trace, was commonly from 8 to 15 feet. In some 
places it exceeded this and at one place it was as much as 21 feet. Toward the south 
_end of the fault the amount of displacement was notably less and finally became inap- 
preciable. Nearly all attempts at the measurement of the displacement were concerned 
with horizontal offsets on fences, roads, and other surface structures at the point of their 
intersection by the principal rupture plane, and ignore for the most part any displace- 
ment that may be distributed on either side of this in the zone of movement. The 
figures thus obtained may, therefore, in general be considered as representing a minimum 
for the amount of differential movement. In one or two cases, however, when the dis- 
placement has been measured on soft ground subject to slumping, and the measured 
offset is higher than usual, the results may be in excess of the true crustal displacement. 
Besides this horizontal displacement of the crust, there was also, particularly in the 
region north of the Golden Gate, a distinct uplift of the country to the southwest of the 
Rift, relatively to that on the northeast. This differential vertical movement was made 
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