Geology Reference
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form in the concave regions. Anticlinal grabens,
such as those seen at El Asnam (Figs 4.31 and
4.32), and out-of-syncline thrusts are examples
of bending-moment faulting.
Complex combinations of normal, thrust,
bending-moment, and flexural-slip faults are
exhibited by ruptures during the El Asnam
earthquake (Fig. 4.34) in areas where a strong
coupling existed between bedding orientations
and fault geometries (Philip and Meghraoui,
1983). Folding of strata caused flexural-slip
faulting, which is expressed at the surface as a
succession of high-angle reverse faults. Bending-
moment stresses caused out-of-syncline thrusts
to develop due to compression along the con-
cave surface of folds. As the main thrust plane
ramped toward the surface, it encountered
steeply tilted strata and was diverted along weak
bedding planes. In places as the fault followed
bedding, it passed through a vertical orientation
and became overturned. This overturning caused
the displaced ground surface along the fault
plane to appear analogous to a normal fault,
rather than a thrust fault (Fig. 4.34). The diversity
of structures associated with the El Asnam
earthquake and the contrasts in their orientations
clearly emphasize the importance of examining
the full spatial array of rupture geometries and
investigating the role of underlying bedding
during faulting before using coseismic rupture
patterns to specify the stress field at the time of
the earthquake.
Bending-Moment Faults
σ 1
σ 3
σ 3
urface
σ 1
flexed
plate
σ 1
σ 3
σ 3
σ 1
Folds
Relationships of folds to faults
Fig. 4.33 Bending-moment faults resulting from
flexure of an elastic plate.
Folding causes stretching along the outer, convex
surface of the warped layer, whereas shortening is
induced along the inner, concave surface.
Many faults do not rupture the Earth's surface.
Typically, faults nucleate within the brittle crust
at depths of several kilometers, and, as they
Complex Coseismic
Thrust Deformation
overturned thrust, deflected
along bedding, showing
"normal" sense of
displacement
flexural-slip
thrusts
out-of-syncline
(bending-moment)
thrust
Fig. 4.34 Complex fault geometries in folded, stratified bedrock.
Multiple rupture patterns result from thrust faulting in folded strata, as exemplified by the 1980 El Asnam rupture.
Coseismic slip along bedding planes creates flexural-slip faults and overturning of the thrust plane in places. Footwall
folding leads to out-of-syncline thrusting. Modified after Philip and Meghraoui (1983).
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