Faults | Suite101.com

As stated in my previous article on folds, the earth is a dynamic system, appearing stable but being stretched or compressed by strong forces throughout the earth’s interior. When these stresses are applied slowly over time, the rocks will experience plastic deformation and become folded and bent. Along many places the stresses are not released slowly but build up over time until they are released all at once. This sudden release of energy results in the rocks breaking apart in a brittle fashion, fracturing and faulting. Earthquakes are the result of this sudden release of energy and take place along faults that cross the earth’s surface and sub-surface. Before we can look at earthquakes in detail, we must first understand the different types of faults that can produce earthquakes.

Faults are the evidence of horizontal and vertical breaks in the earth’s surface. When different parts of the earth’s surface are subjected to different stresses, they will either compress or pull-apart. These compressional and extensional stresses are what create the different faults we see. Faults occur along areas of weakened or brittle rock. As discussed in my earlier article on metamorphic rocks, when rocks are subjected to changes in pressure, they can be altered to form different rocks. Closer to the surface, or when the deformation occurs at pressures or temperatures lower than the alteration point of the rock, then a fracture will occur along a weakened or brittle zone in the rock. As the fracture grows along this brittle zone, a fault forms.

There are four main types of faults. First, is what is called a normal fault. A normal fault occurs when tensional forces, or extension, are applied to the rocks. The rock pulls apart and a fault forms, one side dropping relative to the other side. When this movement is only vertical, with no side-to-side movement, it is referred to as a dip-slip fault; fault movement is parallel to the dip of the fault surface. More often, movement along a fault occurs in both the vertical and horizontal directions, this is referred to as an oblique-slip fault.

The second type of fault occurs when rocks are compressed together, the result is a reverse fault. In all appearances a reverse fault is similar to a normal fault except that in a reverse fault, one side moves upward relative to the other.

Probably the most famous fault, at least in the United States, is the San Andreas Fault. The San Andreas is an example of a strike-slip fault; movement along the fault is parallel to the strike of the fault surface. (See my article on Strike and Dip to learn how to measure the strike of a fault.) Strike-slip faults show mostly horizontal displacement, with very little vertical movement along the fault. Strike-slip faults are described based on which way the fault appears to have moved. If the movement is to the left, it is a left lateral fault, movement to the right indicates a right lateral fault. The San Andreas is a right lateral fault. If you stand on the east side of the fault and look west, it appears that Los Angeles is moving to the right. If you stand in Los Angeles, and look east, it appears that the rest of the country is moving to the right. (No political implications intended here.)

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