THE EARTH MOVEMENT ON THE FAULT OF APRIL 18, 1906. 113 
Engineers’ measurements of displacement. —'The reconstruction of the tunnel at Wright 
Station necessitated an instrumental survey of the displacement in so far as it immediately 
affected the structure. The results of this survey have been placed at the disposal of 
the Commission by Mr. J. D. Matthews, assistant resident engineer in charge of the work. 
The plot of the survey is given in fig. 42. The plot shows that, while the tunnel is trav- 
ersed by only one fault fracture, at a distance of about 400 feet from the northeast portal 
the deformation has been distributed over a distance of nearly a mile. This deformation 
of the tunnel, or its departure from a straight line, is measured from a line drawn from the 
northeast portal to a point on the same side of the tunnel 675 feet in from the southwest 
portal. It indicates a bending of the ground to the northwest in the direction of the 
relative displacement on the southwest side of the fault. That is to say, the bending 
is in the opposite direction to that which would be characteristic of the drag of a fault. 
A possible explanation of this phenomenon is that the ground pierced by the tunnel was 
in a state of excessive elastic stress, even at the time the tunnel was constructed; and 
that the relief effected by the rupture rendered resilience operative and so caused the 
ground to be flexed in the sense opposite to that of adrag. The nature of the deformation 
of the ground on the northeast side of the fault is not yet known. It may be here men- 
tioned, in regard to the effect of the fault upon the steel bridge at Chittenden, that, in 
addition to the cracking and displacement of the supporting piers, as noted by Mr. 
Waring, the distance between the abutments was lengthened about 3.5 feet, according 
to measurements supplied to the Commission by Mr. J. H. Wallace, Assistant Chief 
Engineer, Southern Pacific Company, and illustrated in the accompanying diagram, 
fig. 43. 
