Types of Faults in Geology

Tuesday, January 14, 2020

The Earth is a dynamic and constantly changing place. the tectonic plates which sit on the asthenosphere shift and move. This movement creates stress, and rocks at the surface may break in response to this. When the rock breaks, cracks are created which are called faults. On either side of the fault are blocks of rock called fault blocks. When these fault blocks move, earthquakes may occur.

There are three main types of faults. Strike-slip faults, normal faults,  and reverse faults

Let's first determine the difference between the hanging wall and the footwall. In all faults, except for vertical faults, the hanging wall is the block above the fault and the footwall is below.

Two simple methods to identify the foot and hanging wall is to place your finger on the fault surface and move it up. It will automatically be located in the hanging wall. The hanging wall will have an angle less than 90 degrees at the top and the footwall will have an angle greater than 90 degrees at the top.

In a strike-slip fault, the blocks move past each other horizontally. A strike-slip fault occurs due to shear stress which pushes the rock horizontally but in opposite directions.
Strike-slip faults are common along transform boundaries like the San Andreas fault.

During a normal fault, the hanging wall moves down relative to the footwall. These faults are called normal because this is what you would normally expect the faults to do. Normal faults are a result of tension that stretches or pulls the rock apart. Normal faults are common along divergent boundaries. The basin and range area is an example.

A reverse fault occurs when the hanging wall moves up the footwall. They are called reverse because this is the reverse of what you would expect as a result of gravity.
Reverse faults occur as a result of compression which is a stress that squeezes the rock together.

The San Gabriel mountains are caused by reverse faults.

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