Flow, deformation, and transport - Physical Processes in Earth and Environmental Sciences - page 160

(a) Dip-slip

(b) Strike-slip

PV

Normal Thrust or reverse

CS

Sinistral

(c) Oblique-slip

PV

CS

Normal-sinistral Reverse-sinistral

Dextral

(d) Rotational

Normal-dextral Reversal-dextral

Fig. 4.98 Fault classification in relation to the relative movement of blocks along the fault surface. (a) Dip-slip faults include normal and thrust
or reverse depending on the relative movement of the blocks up or down the fault surface; (b) strike-slip faults can be sinistral or dextral
according to shear: in plan view (PV), if the left block of a strike-slip fault moves toward an observer straddling the fault trace (no matter
which end of the fault) the fault is sinistral, whereas if the right block moves toward the observer, the fault is dextral. The notation used for
shear sense in cross section, in both sinistral and dextral cases is also shown (CS). (c) Faults can show oblique-slip displacements, allowing for
different combinations and, finally, (d) faults can be rotational, when the hangingwall block rotates over the footwall block.

4.15.3 Anderson's theory of faulting

stresses are too weak to form fractures, topographic relief
is negligible, and the Earth's surface is considered perfectly
spherical. If the surface is a principal stress surface then the
principal stress axes have to be either horizontal or vertical
and two of them have to be parallel to the Earth's surface.
Anderson supposed that a hydrostatic state of stress at
any point below the Earth's surface should be the com-
mon condition, such that the horizontal stresses in any
direction will have the same magnitude to the vertical
stress due to gravitational forces or lithostatic loading.
When the horizontal stresses become different from the
vertical load and a regional triaxial stress system develops,
faults will form if the magnitude of the stresses is big
enough. In order to have a triaxial state of stress, and con-
sidering that the vertical load remains initially constant,
the horizontal stresses have to be altered in three possible
ways: first, decreasing the stress magnitude by different

In Section 4.14 we showed that for a particular stress state
under certain values of confining pressure and where
Coulomb's criterion applies, two conjugate fractures form
at about 30 1 . Faults are shear

fractures in which there is a prominent displacement of
blocks along the fault surface. Consider again the nature of
the stress tensor (described in Section 3.13) and remem-
ber that the principal stress surfaces containing two of the
principal stresses are directions in which there are no shear
stresses. Taking into consideration these facts Anderson
concluded in his paper of 1905, that the Earth's surface,
envisioned as the boundary layer between the atmosphere
and the lithosphere, is a free surface in which no shear
stresses are developed, that is, there is no possibility of slid-
ing parallel to the surface. In this approach, atmospheric

from the principal stress