Stress and Deformation
Anderson's theory of faulting
In 1951, Anderson recognized that since the principal stress directions are directions of zero
shear stress, we can place faults in the context of principal stress. All faults have a common
function, to extend the crust in one direction and shorten it in another. The directions of
shortening and stretching are at right angles to one another.
The basis of the theory is the assumption that the surface of the Earth must be a principal plane
of stress, containing two of the principal stress directions. Hence the third principal stress
direction must be oriented normal to the Earth's surface. Anderson combined this idea with the
Coulomb law of failure to explain the orientations of conjugate sets of faults within the Earth. In
each case s2 is always parallel to the line of intersection of the two faults.
Since the principal stress directions are directions of zero shear stress, the surface of the earth
must be a principal stress plane containing two of the three principal stress directions. The
average dip and sense of slip at or near the surface of the earth can be explained by Coulomb law
of failure.
Principal stress axes
σ1 bisects the acute angle between the conjugate shear fractures.
σ defined by the intersection of a pair of conjugate shear fractures.
σ3 bisects the obtuse angle between the conjugate shear fractures.
Stress axes and faulting
Thrust Faults: σ1 and σ2 are horizontal and σ3 is vertical.
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Normal Faults: σ1 is vertical and σ2 and σ3 are horizontal.
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Strike-Slip Faults: σ1 and σ3 are horizontal and σ2 is vertical.