A fault may be defined as a well-defined crack along which the rock masses on either side have relative displacement. The displacement along a fault may be less than a meter, several meters, or many kilometers. Faults results from tensional as well as compressional forces
TERMINOLOGY OF A FAULT
Fault Plane: The fracture surface of a fault, along which relative movement has taken place, is called a “fault plane”. A fault plane is generally inclined but in some other cases, the fault plane may be markedly curved or undulating. In a fault, there may be a number of parallel shear fractures along which the fault movement is distributed. Such a fracture zone is called a “shear Zone”.
Hanging Wall and Foot Wall: The block of rock lying above the fault plane such block of rock is called a hanging wall and if it is lying below the fault plane, then it is known as a footwall. Vertical faults have neither hanging wall nor footwall.
Fault Scarp: A fault scarp is a cliff formed initially along the up-throw side of a fault.
Strike: Strike of a fault in the direction of its continuity on the ground surface. It may be defined as the trend of the line formed by the intersection of a fault plane and a horizontal plane, such as the ground surface of the map surface.
Dip: Dip is the angle between the horizontal surface and the plane of fault.
Hade: It is the compliment angle of the dip that is the angle between the fault plane and the vertical plane or (90-dip = Hade)
Throw: The vertical component of the displacement of the fractured rock blocks is called the throw of fault. The sides on which the strata appear to have thrown down is called the “down throw side” while the other side on which strata appear to have gone up is called the “up throw side”.
Heave: The horizontal component of the displacement of a fault is called the “Heave”
Net Slip: The total displacement measured along the fault plane is called the “net slip”. It is measured between two points that were originally in contact. The movement along the fault plane can be resolved into two component directions, dip-slip, and strike-slip.
a) Dip slip: it is the movement, which is parallel to the direction of dip of the fault plane.
b) Strike-slip: It is the movement, which is parallel to the strike of the fault plane
Types of Fault
Classification of fault: 1
On the basis of apparent movement, the faults are classified into two types-
1) Normal fault: A normal fault is one in which the hanging wall appears to have moved down relative to the footwall. In this case, the fault plane dips towards the down-throw side. Generally, the normal faults are formed due to tensional force and they are also known as gravity faults. The normal fault has a high degree dip. The subdivision of the crust into blocks by normal fault is called block fault.
2) Reverse fault: A reverse fault is one, in which the hanging wall block appears to have moved upward relative to the footwall. In this case, the fault plane dips towards the up-throw side. Generally, the reverse faults are formed due to the compressional force.
Classification of fault: 2
On the basis of the relationship between the strike of a fault and the attitude of strata, the faults are divided into four types-
1) Dip fault: A fault, which runs perpendicular to the strike of strata, is known as a dip fault.
2) Strike fault: A fault, which runs parallel to the strike of strata, is called strike fault.
3) Bedding fault: A fault, in which the fault plane is parallel to the bedding plane of the adjacent rocks.
4) Oblique fault: a fault, in which the strike of the fault is oblique or diagonal to the strike of the adjacent rocks.
Classification of fault: 3
On the basis of the degree of dip, the faults are divided into two types-
1) High angle fault: The high angle faults are those, which have a dip greater than 45 degrees. Normal faults are commonly high-angle faults.
2) low angle fault: The low-angle faults are those, which have a dip less than 45 degrees. Thrust faults are the commonly low angles of fault.
Classification of fault: 4
On the basis of the relationship between the direction of slip and the attitude of the fault the plane, the faults are classified into three types:
1) Strike-slip fault: In a strike-slip fault, the direction of movement of rocks is essentially horizontal along the strike of the fault. It is also known as a transcurrent fault.
If the direction of movement of fault is the left side of the fault and parallel to the strike of the fault, then the fault is called sinistral fault Dip slip fault: a fault, which direction of movement is essentially downward along with the dip of the fault, is called a dip-slip fault.
If the direction of movement of fault is the right side of the fault and parallel to the strike of the fault, then the fault is called a dextral fault.
2) dip-slip fault: A fault, in which the movement of rocks is essentially downward along with the dip of fault.
3) Oblique slip fault: A fault, which direction of movement is essentially diagonal such the fault is called an oblique-slip fault.
Classification of fault: 5
On the basis of the force responsible for the formation of the fault, the faults are classified into the following types:
1) Gravity fault or tensional fault: These faults are formed by the tensional forces, which pull the earth’s crust apart. Normal faults are an example of it.
2) Compressional fault: These kinds of faults are formed by the compressional force. The reverse fault and the thrust faults are examples of it.
3) Pivotal fault: A fault, in which the displacement of rocks at a point downward and upward at another point along the strike due to rotational stress, such fault is called a pivotal fault.
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