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. Although 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. Tectonic setting The 1857 earthquake ruptured about of the southern part of the San Andreas Fault, the structure that accommodates most of the displacement along the transform boundary between the North American Plate and the Pacific Plate. The Pacific Plate is moving north relative to the Sierra Nevada-Great Valley Block of the North American Plate at about per year. The displacement rate along the various sections that ruptured is per year along the Parkfield, Cholame, Carrizo and Big Bend sections and and per year along the Mojave north and Mojave south sections. Paleoseismic studies have found evidence for many prehistoric earthquakes in the last 3,000 years on this part of the San Andreas Fault. Earthquake The earthquake ruptured a substantial portion of the southern San Andreas fault, but not the entire length. Thomas H. Jordan, director of the Southern California Earthquake Center, stated that the slip likely stopped in the area near Cajon Pass, perhaps because the tectonic stresses on that part of the fault had been released several decades earlier during the 1812 Wrightwood earthquake. The average slip along the fault was , and a maximum offset of was