SHEARING MOVEMENTS IN THE FAULT-ZONKE, 
CHANGES IN THE LENGTH OF LINES. 
In the general descriptions of the fault-trace it is shown that when the rupture occurred 
there was a zone of varying width between the shifting sides which did not partake of 
their simple movements, but was more or less distorted by the shearing forces to which 
it was subjected. The existence of this zone in alluvium or disintegrated rock may be 
explained even tho the fault were a sharply defined crack in the underlying solid rock. 
Let us suppose that the straight line AOC in the rock (fig. 17) has been broken at the 
fault and displaced into the two parts A’O’ and D/C’. If the alluvium were brittle and 
with little plasticity, it might be broken and displaced in the same way, but if it were 
plastic, as it would be if it were to some extent composed of clay, a part of the displace- 
ment would be accomplished by shearing distortion, and the offset at the fault-plane 
would be less than that of the underlying rock. Close to the rock the displacement of 
the alluvium would be very nearly the same as that of the rock (lines 1 in the figure) ; 
at greater distances, however, the distortion in the vertical plane would make itself felt ; 
the offset would be less, and the displacement would be distributed more like the lines 2. 
The alluvium might be so thick or plastic that it would suffer no break at the surface 
along the fault-line, the whole displacement being distributed like line 3; this seems to 
be the condition which produces the echelon phase 
of the fault-trace in very wet alluvium, as described 
by Mr. Gilbert (vol. 1, p. 66). 
Special phenomena were exhibited in this zone 
of shearing distortion which might easily be mis- 
understood, but which can be explained fully on 
mechanical principles. 
The zone was in some places only 2 to 6 feet 
wide, in others several hundred yards. Where it 
was broad the shift was divided in some cases 
among a number of cracks; in others it was dis- 
tributed more or less evenly over the zone; in all 
cases, we have a zone of greater or less width 
subjected to shear; let us see what compressions 
and extensions take place in it. Let W and 
(fig. 18) be the eastern and western boundaries of 
the sheared zone, whose width is 7 and let W move 
a distance s, short in comparison with /; and let 
all other lines parallel with the boundaries also 
move a distance proportional to their distance 
from E. WN will be the direction of this motion; 
the line Ec, which makes an angle a with WN, a being positive to the right of WN, is 
shortened by an amount cd; and the simple geometry of the figure shows that the total 
shortening equals s cos a; and this is independent of the length Ec, provided only that 
the line He does not materially change its direction during the motion; this is, in general, 
D 33 


Fig. 18. Fie. 19. 
