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 recorded in the
Carrizo Plain area in southeastern
San Luis Obispo County. 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 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, the primary inland north−south route then. 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 were the source of the mole track and subsequent tree tilt.
Foreshocks for the main shock Various accounts of the event indicate the presence of
foreshocks between one and nine hours before the main event, and based on the (uncertain) distribution of those shocks, it is assumed that the beginning of the fault rupture (the
epicenter) was in the area between
Parkfield and
Cholame, about northwest. The lack of standardized
timekeeping during this period of California's history contributed to some of the inaccurate reports of when the pre-shocks occurred. Local
solar time was being used in 1857 and San Francisco would have been the locality with the most accurate time kept as it was a center of commerce and other activity.
Standard time was not followed until the 1880s, with the
Pacific Time Zone being aligned with the 120th
meridian. The differences in local times were substantial, with
San Francisco at 122.43 W and San Diego at 117.10 W, the difference between the two would be around 22 minutes (4 minutes per
degree). At least one individual reported foreshock times that varied by as much as half an hour when speaking to two different newspapers. The firsthand reports were most abundant for the shocks felt at 1, 2, and 4 hours before the main shock which were later labeled the predawn, dawn, and sunrise shocks. The predawn event shook residents of San Francisco (MMI II – III), San Jose (MMI IV), and
Santa Cruz (MMI IV). The dawn shock was felt in those locales plus Fort Tejon and possibly the
Carrizo Plain. The sunrise shock was felt in San Francisco (MMI III),
Monterey (MMI IV), and
Visalia (MMI II – III).
Sacramento and
Los Angeles did not report any of these events. Several mid-twentieth-century earthquakes had similar felt reports to the dawn and sunrise shocks, and with close inspection, Sieh theorizes that both events were local to coastal
central California, probably between
Point Conception and Monterey. Also, during that period, no central California earthquake with a magnitude of less than five had a felt area as large as the two foreshocks, while events larger than magnitude six have had "somewhat larger" felt areas, so it could be said that the foreshocks most likely were between magnitude five and six.
Parkfield earthquakes occurred with exceptionally regular intervals (between 20 and 30 years) between 1857 and 1966. Sieh studied four of these events (1901, 1922, 1934, and 1966) and found that they helped to determine the southeast boundary for the origination of the dawn foreshock. The coverage and intensities of felt reports for that earthquake show a solid resemblance to the Parkfield events. though the San Andreas is not the only possible source for the dawn event. For example, the November 22, 1952, magnitude six
Bryson earthquake "nearly duplicates" the felt reports. That event may have occurred west of the San Andreas on the Nacimiento fault, though the highest felt reports were along the Rinconada fault around southwest of the San Andreas.
Damage Most of the
adobe buildings at
Fort Tejon were badly damaged and several people were injured there. More buildings were destroyed along a twenty-mile stretch between Fort Tejon and southeast to
Elizabeth Lake, a
sag pond that was formed directly on the San Andreas fault. Streams and springs experienced disturbances in San Diego and Santa Barbara Counties, while the
Kern River,
Kern Lake, and
Los Angeles River all spilled over their banks. Farther north in
Santa Clara County the flow of well water was affected. Ground cracks from
liquefaction of swampy ground were observed near the
Pueblo de Los Angeles and in the
Oxnard Plain, and ground fissures were reported near the
Los Angeles,
Santa Ana, and
Santa Clara Rivers.
Aftershocks The main shock 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 January 9 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 January 16 with an estimated magnitude of about 6.7, a possible offshore location, ==Future threat==