1857 Fort Tejon Earthquake - Earthquake

Earthquake

The quake ruptured a substantial portion of the southern San Andreas, but not the entire length. Thomas 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 4.5 meters (15 feet), and a maximum offset of 9 meters (30 feet) was recorded in the Carrizo Plain area in southeastern San Luis Obispo County. The amount of fault slip gives this earthquake a moment magnitude of 7.9, comparable to that of the 1906 San Francisco earthquake. The extreme southernmost portion of the fault, which terminates near Bombay Beach at the Salton Sea, last ruptured in 1680.

Surface faulting may have extended beyond the boundaries of the regularly acknowledged slip length. Researchers recorded first and second-hand accounts of the ground crack, which was understood to be recent surface faulting and not just the topography of the existing rift. On the extreme northern end of the rupture zone, the surface cracking extended 80 kilometers (50 mi) north of Cholame into San Benito County. On the southern end, the population centers were not as close to the fault, and early observers were probably limited to the stretch of the fault between Fort Tejon and Elizabeth Lake, as that was close to the Stockton – Los Angeles Road. Evidence of uprooted and displaced trees south of Elizabeth Lake indicates surface faulting along a "mole track" that ran directly under three Jeffrey Pines. Two of the three trees examined were tilted in their lower extremity, while the upper portions remained relatively untilted. Tree ring dating confirmed that the trees had originated 10 and 25 years before 1857 and also that the rings began to grow twice as thick on the side in the direction of the tilt; this is a frequently noted compensation of tree tilt. Seismologist Kerry Sieh determined that fault slip and the associated ground disturbance was the source of the mole track and subsequent tree tilt.

The mainshock was followed by a series of aftershocks that continued for at least 3.75 years, although the total number of large aftershocks was less than would be expected for an earthquake of this size. The four largest aftershocks all had magnitudes greater than 6, although there are large uncertainties in both location and magnitude due to the limited number of data points available. On the night of 9 January there was a large aftershock with an estimated magnitude of about 6.25, with a possible epicenter near the Garlock Fault. The largest aftershock occurred on the afternoon of 16 January with an estimated magnitude of about 6.7, with a possible offshore location, and high felt intensities in Southern California communities. Santa Barbara and San Bernardino reported an MM intensity of V and Los Angeles reported V and VI. Two significant events occurred in the San Bernardino area on 15–16 December 1858, with the latter having an estimated magnitude of about 6. The last recorded major aftershock occurred on 16 April 1860, with an estimated magnitude of about 6.3, with an epicenter close to the Parkfield section of the San Andreas Fault.