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What is fault? Fault terminology Fault plane: Hanging wall Foot wall Slip and separation: Separation Classification of faults Apparent movement as basis Normal faults Graben Reverse faults: Strike – slip faults On the basis of altitude (dip and strike) Mode of occurrences as basis Parallel faults Enechelon faults Peripheral faults Radial faults On the basis of slip Engineering consideration of faults
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by
RAHUL SINHA
What is fault?
• It may be defined as a fracture surface
in rock across which there is relative
motion parallel to the surface between
the adjacent blocks of the rock.
Fault terminology
• Fault plane: it is the planer surface
along which relative displacement of
blocks takes place during the process
of faulting.
• The walls:
1. Hanging wall: it is the term used for
the faulted block which lies on the
upper surface of the fault plane.
2. Foot wall: it is the term used for the
faulted block which lies on the under
surface of the fault plane.
• Slip and separation: slip may be defined as the
relative motion of rock blocks along fault
planes.
1. Strike slip: the slip that occurs along the
direction of the strike of fault plane.
2. Dip slip : the slip that occurs along the
direction of the dip of the fault plane.
3. Oblique slip: the slip that occurs both in dip
and strike direction i.e. it is combination of
strike and dip direction.
• Separation: The amount of apparent offset
of a faulted surface, measured in specified
direction. There are strike separation, dip
separation, and net separation.
1. Heave: The horizontal component of dip
separation measured perpendicular to
strike of the fault.
2. Throw: The vertical component measured in
vertical plane containing the dip.
Classification of faults
• Following are the factors commonly
considered important :
1. The apparent movement of the rock blocks
along the fault plane.
2. The direction of the slip.
3. The relation of the fault altitude with the
altitude of the displaced bed
Apparent movement as basis
a. Normal faults: faults in
which the hanging wall is
moved down with respect
to the foot wall.
Horst : when two normal faults are on the either
side of a central wedge shaped block such that
it appears high up with respect to either blocks.
• Graben: when two normal faults are on the either side of a wedge shaped block such that it appears downwards with respect to either blocks. b. Reverse faults:
this type of fault in
which the hanging
wall appears to have
moved up with
respect to the foot
wall and dips at the
angle more than 45
degrees.
• Thrust fault: these are variety of reverse faults in which
the hanging wall has moved up with respect to footwall
and the fault dip at the angle below 45 degrees.
c. Strike – slip faults: it may
be defined faults in which
the faulted blocks have
been moved against each in
the horizontal direction
along strikes.
There are two types of
strike – slip faults:
1. Right lateral strike-slip
fault (dextral): Where the
side opposite the observer
moves to the right.
2.Left lateral strike-slip fault
(sinistral): Where the side
opposite the observer
moves to the left.
On the basis of altitude (dip and strike)
• Strike faults: faults that develop parallel to the strike
of strata. In other words, the strike of the fault and
that of disrupted layers are essentially
parallel.(figure a)
• Dip faults: faults that develop parallel to the dip of
the strata. In other words, the fault strike is parallel
to the dip of layers disrupted by faults. (figure b)
• Oblique faults: faults whose strike makes an oblique
angle with the strike of the rock in which it has
caused the displacement. (figure c)
Mode of occurrences as basis a. Parallel faults: a group of normal faults occurring
in the close proximities having same dips and
strikes. when this group gives step like stricture in
the structure then it is called step faults. (figure a)
b. Enechelon faults: Faults that are approximately parallel one another but occur in short unconnected segments, and sometimes overlapping. (figure b)
c. Peripheral faults: when in any region the majority of faults are concentrated along the border or margin of the area. (figure c)
d. Radial faults: the group of fault that appear emerging outward from a common point is called radial faults. (figure d)
On the basis of slip
1. Strike slip fault: it may be defined faults in which the faulted blocks have been moved against each in the horizontal direction along strikes.
• They are called by other names under which they occur:
a) Wrench fault: it is the strike slip faults in origin in which fault plane has developed transverse to the regional structure and even the net slip is in the same manner. The dip of the fault is very steep, nearly vertical.
b) Transform faults: these are the strike slip faults that occur in the oceanic ridges.
c) Tear faults: these are strike slip faults occurring in groups in continental regions that divide an originally extensive block into blocks of smaller and convenient dimensions.
There are three types of transform faults :
•Ridge-Ridge transforms link two segments of a
constructive plate boundary.
•Trench-Trench transforms link two segments of a
destructive plate boundary.
•Ridge-Trench transforms link a constructive plate
boundary to a destructive one.
2. Dip slip faults: may be defined as the type of fault in
which slip has taken place in parallel to the dip of
the fault.
3. Oblique slip fault: may be defined as the faults in
which the direction of net slip has taken place
neither to the dip nor to the strike of the fault but is
inclined to both of these direction.
Engineering consideration of faults
• Faults cause very much shearing and crushing of rocks
located in the fault zone making the rocks weak on one
hand and porous, permeable on the other hand.
• So following general conclusions can be drawn:
I. The faulted rocks will form weak foundations for the
dams.
II. The fault zones will provide easy pathways for the
water and causes leakage when left untreated in the
dams.
III. Once the fault zone becomes lubricated with water, the
probability of further slipping becomes high. This may
create critical condition within the foundation.
IV. Faulting products like gouge and breccia create
additional problems.
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