Fault lines are the places where two tectonic plates meet. Since these plates are moving very slowly, fault lines are places where two plates are either drifting away from each other or moving toward each other. Seismic faults are faults that are particularly active and usually produce earthquakes. There are many earthquake fault lines. A few of the most active earthquake producing fault lines in the U.S. are as follows:
San Andreas Fault, running nearly 800 miles North-South through California
Denali/Tintina Fault, running from British Columbia, Canada, through Alaska and the Aleutian Islands
Wasatch Fault, running almost 250 miles from Idaho, through Utah, and into Arizona
