Geology Reference
In-Depth Information
4
Stress, faults, and folds
When rocks are subjected to stresses that
exceed their strength, they rupture, fold, or
flow. Different varieties of faults (strike-slip,
normal, thrust) characterize contrasting tectonic
settings and stress regimes in the upper crust.
When faults break the Earth's surface either in
a single earthquake or during many seismic
events, they commonly create geomorphic
features that can be associated with a particular
type of fault. Sometimes earthquakes occur
on faults that do not reach the Earth's surface,
and, therefore, no ground ruptures are directly
associated with the fault trace. Nonetheless,
the Earth's surface will deform by folding in
response to earthquakes along buried faults.
The geometry of deformation and the evolving
shape of a fold can reveal useful information
about the nature of the subsurface faulting and
the way in which the rocks adjacent to the fault
respond to fault motions.
In order to take full advantage of the
information to be gleaned from geomorphologi-
cal surfaces, an understanding of the typical ways
in which rocks deform due to both seismic and
aseismic movements is helpful. A primary goal of
this chapter is to examine the nature of faulting
and folding and to discuss concepts related to
scales of faulting and similarities between succes-
sive earthquakes on the same fault, the displace-
ments of the ground surface that are expected for
different types of faults, and the geomorphic
imprint of faults of different types.
Stress, strain, and faults
Stress
All rocks in the crust are subjected to forces due
to gravitational acceleration, plate tectonic
motions, and the mass of rocks, water, and air
around them. We typically identify two kinds
of forces:
body forces
and
surface forces
. Body
forces act equally on every element throughout
a volume. The magnitude of a body force is pro-
portional to the mass of the element on which it
is acting, or its volume times its density. An
example of a body force would be the weight of
an object, which is the mass of the object times
the acceleration due to gravity. Surface forces
are forces that act across or along surfaces, and
they include forces such as friction or pressure.
Strictly speaking, these stresses are termed
tractions
, which are defined as the force per
unit area acting on a surface.
A stress or traction acting on a plane with any
orientation can be resolved into two compo-
nents: a normal stress
s
n
, acting perpendicular
to the surface; and a shear stress
s
s
, acting
parallel to the surface. The unit of stress in the
SI (meter-kilogram-second) system is the
pascal. One pascal (1 Pa) represents the stress
produced by one newton (1 N
=
1 kg m/s
2
) acting
across or along a surface of one square
meter (1 m
2
); thus 1 Pa
=
1 N/m
2
=
(1 kg m/s
2
)/m
2
.
Pressure can also be expressed in bars, where
