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According to the US Geological Survey, 39 out of the 50 American states are vulnerable to seismic activity of some or the other degree.

Fault lines play a significant role when it comes to identification of the regions vulnerable to seismic activities. Basically, a fault is the fracture or discontinuity in a rock structure, typically characterized by significant displacement.

The line of intersection which marks the separation of this fault plane and the surface of the Earth is known as the 'fault line'. There are several such earthquake fault lines in the United States, which are used by the U.S. Geological Survey for earthquake forecasting as they help in determining the vulnerability of various regions to seismic movements.