An active fault is a fault which has had displacement or seismic activity during the geologically recent period. In the United States, an active fault is generally defined as a fault which displaced earth materials during the Holocene (Recent) Epoch (during the last 11,000 or so years before present). Active faults are the most common sources of earthquakes and tectonic movements. In California, the well known San Andreas Fault is the most active fault in that state and the source of the largest earthquakes historically and during the Holocene.