A fault is a thin zone of crushed rock between two tectonic plates which are being forced to move relative to one another. A fault plane can be at any angle to the surface of the earth and it may extend from a few centimeters to thousands of kilometers. When an earthquake occurs on a fault plane, the rock on one side of the fault slips with respect to the other. This slip could occur at any angle and in any directions.
Faults are classified into two basic types depending on the nature of relative motion between adjoining rocks viz. strike-slip & dip-slip motion. The various faults types are shown in image below:
Courtesy - Gujarat Ambuja Cements Ltd.
In a strike-slip motion, movement occurs on a near vertical plane when plates on either side of a fault move horizontally with respect to each other.
In a dip-slip motion, the fault is at an angle to the earth’s surface and relative movement is up-down.
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When the upper rock tries to move down, it is known as a normal fault. When the crust is extended, a normal fault occurs. It is also called as extensional fault. A downward block between two normal faults dipping towards each other is called a Graben. An upward block between two normal faults dipping away from each other is called a Horst.
When the fault is opposite to normal fault, it is called reverse fault. The dip is greater than 45°. A thrust fault is same as reverse fault but the dip of fault plane is less than 45°.
In the latter case, when fault angle is near horizontal it is termed a thrust fault where pressure generated could be large and may give rise to hills.