Fault lines are where faults in the Earth's crust intersect with the top surface of the Earth. A fault on the Earth's crust is generally a crack that makes up the boundary between the planet's various tectonic plates. Faults can be classified as either active or inactive.
An active fault line is defined as areas where the Earth's crust moves over a period of time. This movement can generate earthquakes. Fault lines that are inactive show signs of previous crust movement but are relatively static in nature. Fault lines on the Earth's crust can appear as a normal dip-slip fault, a reverse dip-slip fault or a strike-slip fault.