Page 58 - Geotechnical Engineering Soil and Foundation Principles and Practice

Special Problems with Sedimentary Rocks
Special Problems with Sedimentary Rocks 53

material that is designated a fault zone. Active faults are acknowledged sources
of earthquakes.

A fracture is similar to a fault except that there is no relative movement and no
gouge. Fractures are common in granite areas where as previously mentioned they
influence drainage patterns, and criss-crossing of fractures allows weathering into
boulders.

Because fault zones are weak and easily eroded, they strongly influence stream
patterns as sections of streams follow the fault lines. Strike-slip faults where
one side moves laterally with respect to the other cause obvious offsets of linear
features including streams, hogbacks, roads, fences, and the pattern of trees in
orchards. The San Andreas fault is a strike-slip fault that eventually will put
Los Angeles close enough to San Francisco that they may be able to share the
same fire department.

Normal Faults
Faults where one side is elevated relative to the other are revealed by fault scarps
that represent an exposed surface of the fault itself (Fig. 3.8). The high side
of a fault scarp is quickly invaded by gullies, giving the scarp a triangular or
trapezoidal facet that can aid in its identification. A similar topographic feature
occurs where toes of hills are truncated by lateral erosion by a river.

Dating of a fault is important to establish whether a fault is active or not, and
to establish earthquake recurrence intervals. Indirect evidence for recent activity
includes (a) freshness of the fault scarp, (b) ages of trees growing on the exposed
scarp, (c) ages of sediments cut by the scarp, and (d) age of earthquake-related
features such as sand boils, determined from age of the overlying sediments.

Figure 3.8
Diagram showing
stream erosion of a
fault scarp.

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