The 1857 Fort Tejon earthquake occurred at about 8:20 a.m. (Pacific time) on January 9 in central and Southern California. One of the largest recorded earthquakes in the United States, with an estimated moment magnitude of 7.9, it ruptured the southern part of the San Andreas Fault for a length of about 225 miles (350 km), between Parkfield and Wrightwood.

Though the shock was centered near Parkfield, the event is referred to as the Fort Tejon earthquake, because that was the location of the greatest damage. Fort Tejon is just north of the junction of the San Andreas and Garlock Faults, where the Tehachapi, San Emigdio, and Sierra Pelona Transverse Ranges come together.

The earthquake is the most recent large event to occur along that portion of the San Andreas Fault, and is estimated to have had a maximum perceived intensity of IX (Violent) on the Modified Mercalli scale (MMI) near Fort Tejon in the Tehachapi Mountains, and along the San Andreas Fault from Mil Potrero (near Pine Mountain Club) in the San Emigdio Mountains to Lake Hughes in the Sierra Pelona Mountains. Accounts of the events' effects varied widely, including the time of the main shock as well as foreshocks that occurred at several locations earlier in the morning.