SHEARING MOVEMENTS IN THE FAULT-ZONE. 35 
them would trend about north and south, as the fault runs about northwest. The sub- 
sequent movement seems in many places not to have obliterated this arrangement of the 
turf in strips, which is so characteristic that it indicates the position of a fault-trace 
without possibility of error, and shows the direction, tho not the amount, of the relative 
movement of the sides. Its appearance is shown in plates 168, 39B, 43B, and it is 
sketched diagrammatically in figs. 18, 19, 20, vol. 1.1 An interesting example is shown 
in plate 65a and fig. 57, representing an auxiliary fault at the Morrell ranch; the direction 
of the diagonal cracks across the road shows that the northeastern side moved relatively 
to the northwest, a direction contrary to the movement observed elsewhere. This 
unexpected result is confirmed by the offsets of the fences bordering the road; a picture 
of the right-hand fence is shown in plate 648, and a measure of the offset shows a relative 
movement of 3.75 feet. This anomaly is local and apparently very superficial, as it 
does not appear in the tunnel which is nearly under the point observed; the tunnel is 
offset normally a short distance to the east of the auxiliary fault.” 
In places the subsequent motion has so broken up and so confused the earth clods 
that the regular diagonal cracks have been obscured; in places a slight compression or 
extension at right angles to the fault has entirely closed the cracks or made others more 
nearly parallel with the fault; but it is surprising how generally traces of the diagonal 
cracks can be seen when they are lookt for. They are frequently described by the word 
splintering. 
If the sheared zone runs along a slope, gravity acts as a tension on the higher part of 
the zone, increasing the tendency to form cracks and making them more nearly parallel 
with the fault-trace; in the lower part it produces a compression which tends to prevent 
the formation of cracks. This is the condition near San Andreas Lake (vol. 1, p. 93). 
Other cracks were made which apparently were not due to the shearing movements 
in alluvium which we have been considering; some, such as those in the Point Reyes 
region and those on Black Mountain (vol. 1, pp. 75, 107) seem due to a general shattering 
of the mass, and may be caused by vibratory motion (vol. 1, p. 40); others (vol. 1, 
pp. 106-109) which are nearly parallel with the fault may in some cases be due to the 
topography, and in others to a small relative upthrust of one side of the crack. 
In all parts of this report special efforts have been made to distinguish between 
cracks and dislocations due to the actual rupture along the fault, and those due to 
landslides, the settling of unconsolidated material, the slumping of river banks, the 
effects of vibrations, etc. This distinction is very important in order to interpret 
correctly the true movements of the underlying rock. 
OFFSETS OF FENCES AND PIPES. 
The distribution of shear over a broad zone is well illustrated by the distortion of 
fences; a number have been described in the preceding pages and illustrated by photo- 
graphs and figures; we may refer especially to figs. 15, 31, 32, vol. 1. In some cases 
anomalies occur, which are probably not real, but which may be due to a misinterpreta- 
tion of the observations; in fig. 29, for instance, the fence on both sides of the fault-line 
is dragged in the same direction, with shifts of 13 feet and 5 feet 9 inches on the two sides, 
respectively; at a distance from the fault-line there is only a very small, relative dis- 
placement of the opposite sides; this is so opposed to the general character of the dis- 
placements that it probably does not represent the true movements. In fig. 38 a fence 
is represented as having been distorted to the south on the eastern side of the fault, for 



1 Professor Omori describes and explains this effect of the shear in his account of the earthquake in 
Bull. Imperial Earthquake Investigation Commission, vol. 1, No. 1, pp. 12-15. 
2 See Mr. Johnson’s description, vol. 1, top of page 277. Fig. 57 is badly drawn and shows the offset 
in the wrong direction. 
