1857 Fort Tejon Earthquake
Relatively little is known about the seismic activity near Parkfield at the time of the great Fort Tejon earthquake on January 9, 1857. What we do know is that several meters of sudden right-lateral slip on the San Andreas fault produced shaking that lasted 1 to 3 minutes and that was felt over more than 350,000 square kilometers of central and southern California. Also, the maximum fault movement of about 9 meters occurred in the Carrizo Plain section, 90 to 130 kilometers southeast of Parkfield, and slip was certainly less near the north and south ends of the rupture zone. The limit of slip at the north end of the zone is not known exactly but definitely includes slip on the Cholame section estimated at 3 to 7 meters. Clearly less slip occurred along the Cholame section than in the Carrizo Plain.
Although more than 130 years have passed since the great Fort Tejon earthquake, there is little reason to anticipate a repeat of that even in the next several decades. Crustal deformation measurements along the San Andreas fault southeast of Parkfield indicate that plate movement is straining the region at a rate corresponding to 3 centimeters per year of right-lateral slip. Thus movement since 1857 has not been sufficient to repeat the 9 meters of slip that occurred on the Carrizo Plain section in 1857. However, it is uncertain whether the potential for the 3 to 7 meters of slip that apparently occurred along the Cholame section in 1857 has been recovered by crustal straining along the fault since then. Thus it was possible, though not likely, that the anticipated magnitude 6 Parkfield earthquake could trigger, or "grow into", a shock of about magnitude 7 on the Parkfield and Cholame sections.
Accounts of the 1857 earthquake indicate that several shocks indicate that several small to moderate size California shocks preceded it by 1 to 9 hours. In particular, two foreshocks were widely felt. A study of felt areas and intensities of these two foreshocks by Professor Kerry Sieh of the California Institute of Technology indicates they were similar to the Parkfield main shocks of 1901, 1922, 1934, and 1966. Sieh concluded that the 1857 foreshocks were magnitude 5 to 6 earthquakes located within an area that includes the Parkfield section. Because foreshocks generally occur near the epicenter of the ensuing larger main shock. Sieh believed the slip in the great Fort Tejon earthquake actually began near Parkfield at the northwest end of the 1857 rupture zone and extended along the fault to the southeast through the Carrizo Plain to the vicinity of San Bernardino, east of Los Angeles.