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.

Please visit ( for more presentations)

www.slideshare.net/RAHULSINHA1993