Faults are fractures in rock where displacement has occurred. A fault plane is the surface where the fracture and displacement took place. There are several key terms to describe faults, including footwall, hanging wall, fault trace, downthrown side, and upthrown side. Faults can be classified based on the direction of slip, attitude relative to adjacent rock, fault pattern, dip, and apparent movement. Common types include strike-slip faults, dip-slip faults, normal faults, reverse faults, and rotational faults.

2. FAULTS
 Faults are fractures with displacements.
 A fault is a planar discontinuity between blocks of rocks that have been
displaced or passed one another in a direction parallel to the
discontinuity.
 The magnitude of this displacement may vary within wide limits.
 Some faults are only a few cms long and the total displacement is
measured in fractions of cms. On the other hand, there are fault from
hundreds of kms long with displacement measured in even tens of kms.

3. Terminologies
 Foot wall and hanging walls -
These two terms are used to indicate the walls or country rocks
laterally bounding a fault on both sides of the two blocks lying on either side
of the fault planes. One appear to rest on the other. The former is known as
hanging wall side while the lateral which supports the hanging wall is known
as footwall.
 Fault surface / fault plane –
The surface along which fracture and displacement had taken
place is known as the fault surface. The term fault plane is used for a fault
surface when it is more or less planar.
 Fault scrap -
It is an upstanding stucture with a steepside formed by the
relative displacement on either side of the fault plane.

5.  Fault line –
The intersection of the fault with the surface of the earth is
known as fault line /fault trace/ fault outcrops. Fault line is a fracture along
which crust has moved.
 Downthrown side and upthrown side -
In the case of a fault, one of the dislocated block appears to
have been shifted downwards in comparison with the adjoining block lying
on the other side of the fault plane.The former is known upthrown side and
that shifted downwads are called downthrown side.
 Throw and Heave -
Throw of a fault is the vertical component of apparent
displacement measured along the direction of dip of the the fault. Heave of
a fault is the horizontal component of apparant displacement.
 Strike -
It is the trend of a horizontal line in a fault plane.

6.  Dip (Ɵ) -
It is the angle between horizontal plane and the fault plane
which is measured in a vertical plane that strike at right angle to the fault.
 Hade (Ф) -
It is the complementary angle of dip that in the angle
between the fault plane and the vertical plane.
Ф = 90°-Ɵ
Hade = 90°-dip
 Net slip -
The total displacement due to a fault is described as net slip.

8. GEOMETRICAL CLASSIFICATION OF FAULTS
 The geometrical classification of faults are based on –
1. Rake of net slip
2. Attitude of fault relative to the attitude of adjacent rock
3. Pattern of fault
4. Angle at which fault dips
5. Apparent movement on the fault surface

9. 1. Based on the rake of net slip –
The term rake is defined as the angle that a line in a plane
makes with a horizontal line in that plane. Based on the nature of slip
observed along the fault surface in terms of the rake of slip, 3 types of faults,
namely:-
(a) Strike slip fault: - Strike slip fault in which the netslip is parallel to the strike
of a fault. There is no dip slip component for net slip. A strike slip fault can
also be defined as one in which the rake of net slip is zero.
(b) Dip slip fault:- A dip slip fault is one in which net slip is up or down in the dip
direction of fault surface. In other words, the net slip of such fault equals the
dip slip and there will not be strike slip component. Here the rake of net slip
is 90%.
(c) Oblique or diagonal slip:- This fault is one in which the netslip in a direction
diagonally up or down the fault surface. Diagonal slip fault have both dip slip
and strike slip component. And rake of net slip will be in between 0 and 90°.

11. 2. Based on attitude of fault plane relative to the attitude of adjacent rock:-
a) Strike fault:- A fault that strikes parallel to the strike of the strata disrupted is
termed as strike fault.
b) Dip fault:- A fault that strikes parallel with dip of strata involved in faulting is
termed as dip fault.
c) Oblique or diagonal fault:- A fault can be termed oblique fault when it strikes
oblique to the strike and dip of the constituent rocks.
d) Bedding fault:- It is a variety of strike fault that has a fault surface/fault plane
parallel to the bedding surface of disrupted strata.
e) Longitudnal fault:- A fault striking parallel to the trend of regional structure is
known as longitudinal fault.

12. f) Transverse fault:- A fault that strikes perpendicular to the strike of regional
structure is termed as transverse fault. It is also known as cross fault.

13. 3. Based on pattern of fault-
 Fault that are classified on the basis of their pattern are :-
1. Parallel fault-
When a group of fault have essentialy the same attitude
they are considered as parallel fault. When a group of parallel fault are
closely spaced the term sheet fault may be used.
In a series of parallel fault whether vertical or inclined the
sucessive blocks are downthrown more and more towards a particular
direction and the resulting structure more or less like a step in staircase
and is know as step fault.
A series of normal faults having more or less parallel strikes
may sometimes cause uplift of alternating blocks with the intervening
ones downthrown. The elevated rocks are known as horst while those
forming depression are called graben.

14. 2. Peripheral fault (arcuate fault) – peripheral faults are circular or arcuate faults
that bound a circular area or a part of a circular area either elevated or
depressed.
3. En-echelon fault – these are parallel sheet faults that overlap each other.
4. Radial fault – these are a system of fault that radiates from a central point.
 Horst / ridge fault – a fault system consisting of a
pair of faults when fault plane is inclined away
from each other with a common upthrown side in
between. The upthrown side of a ridge fault
system form a conspicous wedge shaped ridge
parallel to the fault plane generally long. This is
called horst.
 graben / rift valley – a fault system consisting of a
pair of normal faults whose fault plane is inclined
towards each other with a common downthrown
side in between. The downthrown side of a trough
fault system forms a long depression parallel to
the fault plane. This is called rift valley/ graben.

16. 4. Based on the dip of the fault plane –
It is of two types;
1. High angle fault – in this fault, the dip amount is more than
45 °.
2. Low angle fault - in this fault, the dip amount is less than 45°.

17. 5. Based on the apparent movement of fault surface –
Based on apparent movement of the faultblocks, fault are
classified into;
 Normal fault – in normal fault, the actual or apparent movement of the
hanging wall is downward with respect to the footwall. A normal fault dips
towards the block that has been relatively lowered. A vertical fault is also
considered as a normal fault.
 Reverse fault – the fault in which the actual or apparent displacement of
the hanging wall is upward relative to the footwall. Reverse fault is also
known as thrust fault.
 Hinge fault/ pivot fault/ rotational fault – in this case, one block appears to
have rotated about a point on a fault plane, displacement increases with
distance from the hinge.