After reading this article you will learn about:- 1. Definition of a Fault 2. Parts of a Fault 3. Types 4. Field Evidence 5. Effects 6. Engineering Considerations.
Definition of a Fault:
Faults are fractures along which movement of one block with respect to others has taken place. This movement may vary from a few centimetres to many kilometres depending on the nature and magnitude of the stresses and resistance offered by the rocks.
Parts of a Fault:
The following are important from the subject point of view:
1. Fault plane.
2. Hanging wall and Footwall.
1. Fault plane:
A plane along which the rupture has actually taken place or where one block is moved with respect to other is known as ‘Fault Plane’. It may be noted that such a plane is generally formed along the line of least resistance.
2. Hanging wall and Footwall:
The upper block or, in other words, the block above the fault plane is called ‘Hanging wall’. The block below the fault plane or, in other words, beneath the fault plane is called the Footwall.
It is the inclination of the fault plane that is vertical.
It is the vertical displacement between the Hanging wall and Footwall.
It is the horizontal displacement between the Hanging wall and Footwall.
Types of Faults:
Depending upon the inclination of the fault number of types of faults are recognized.
1. Normal Fault:
A fault in which Hanging wall (HW) has apparently come down with respect to the Footwall (FW) is termed as ‘Normal Fault’.
A fault in which hanging wall has apparently gone up with respect to the Footwall is termed as ‘Reverse Fault’.
The only difference between the Normal Fault and Reverse Fault is that, in Normal Fault the Hanging wall is downward with respect to the Footwall whereas in a Reverse Fault the apparent movement of the Hanging wall is upwards with respect to the Footwall.
3. Thrust Fault:
A fault which is a very small angle of hade (i.e. the inclination of fault plane with the vertical plane is very small) and the Hanging wall that apparently goes up with respect to the Footwall is called ‘Thrust Fault’.
4. Vertical Fault:
A fault in which the fault plane is vertical (having an angle of hade up to 5 degrees) and either of the walls has moved upwards or downwards.
5. Horst Fault:
Horst (German, Horst = upthrow) Fault is one in which wedge shaped block has gone up with respect to the side blocks.
Graben (German, Graden= Trench)Fault is one in which wedge shaped block has down with respect to side block.
7. Dip Fault or Transverse Fault
A Dip Fault whose strike is parallel to the dip of the strata is called ‘Transverse Fault’.
8. Strike Fault:
A Strike Fault is one whose strike is parallel to the strike of the strata.
9. Parallel Fault:
A series of faults running more or less parallel to one another and all handing in the same direction is called ‘Parallel Fault’.
10. Step Fault:
The term ‘Step Fault’ is applied to that parallel fault where downthrown of all is in the same direction and it gives a step-like arrangement.
Field Evidence of Faulting:
Field evidence of faulting can be divided into two groups.
1. Lithological Evidence:
This evidence includes:
ii. Fault Breccia and Gouge,
v. Repetition and Omission of Beds,
vi. Abrupt Termination of Structures, and
vii. Silicification and Mineralization.
The movements of one wall against another along fault results in polishing and grooving of one or both surfaces. These are known as ‘Slickensides’. The direction of the movement is indicated by the trend of the striations or grooves.
ii. Fault Breccia and Gouge:
Along some faults the rocks are found highly fractured or even crushed to angular fragments. Such crushed rocks are called ‘Fault Breccia’. When the dislocation forces are very severe, as is frequently the case in thrusting, the rock may be ground to fine clay like powder called ‘Gouge’.
Drag is the minor folding of strata along the walls of a fault. It is caused by fault displacement.
The displacement of beds, igneous dykes; veins, etc. along a fault may be seen in either plan or section.
v. Repetition or Omission of Beds:
The repetition and omission of beds often establish the fault.
vi. Abrupt Termination of Structures:
An abrupt termination of structures such as folds, beds or dykes along a common line or zone suggests faulting.
vii. Silicification and Mineralization:
Action water while percolation through a fault zone may deposit fine-grained Quartz causing Silicification. Many mineral deposits have also been localized along faults.
2. Physiographic Evidence:
This evidence includes Fault Scrap, Fault Line Scrap and Fault Control of Streams:
i. Fault Scrap:
An actual surface of fault displacement may stand up unmodified by erosion as in escarpment or cliff. It is called a ‘Fault Scrap’. In this case the escarpment faces towards the down throw side.
ii. Fait Line Scrap:
Fault frequently brings together resistant and non-resistant rocks. The resistant rock will stand out prominently as ridge along a fault zone. Such ridges that will generally face the upthrow side of the faults are called ‘Fault Scrap Side’.
iii. Fault Control or Streams:
Streams may be guided in the direction and course of their flow by faulting such stream, which may follow a straight line or make approximately right angle turns.
Effects of Faults on Outcrop:
1. Effect of Dip Fault:
The effects of Dip Fault is to cause lateral displacement in the outcrops as shown in Fig. 4.34(a).
The amount of displacement becomes less with increase in the dip of rocks and in vertical strata the displacement of outcrops will be nil.
2. Effect of Strike Fault:
The effects of Strike Fault are either to cause a repetition of the outcrops of the beds or to eliminate the outcrops of some of the beds altogether. Repetition of the outcrops occur when a Strike Fault Hades in the opposite direction to the dip of strata as shown in Fig. 4.34(b).
Engineering Considerations of Faults:
1. Strictly speaking on site should be selected on a fault for any major project because movements along the existing fault plane is much easier than any other planes.
2. For major projects like a dam, tunnel, etc., a site that is highly faulted should be avoided because the engineer may have to face much troubles sooner or later. The structure constructed on fault may collapse at any moment even due to slight disturbance.
3. If the project is of a scattered nature like electric or telegraphic poles the work can be carried out without much of a risk.
4. Safety factor building modification should bear the initial shocks of an earthquake of low intensity.
5. Faults are responsible for lakes, swamps and marshy places.
6. Fault traces often provide potential springheads.
7. Fault zones often form oil traps.