1857 Fort Tejon Earthquake - Foreshocks

Foreshocks

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 97 kilometers (60 mi) northwest. Nevertheless, it is usually called the Fort Tejon earthquake because this was the location of the greatest damage.

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 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 was 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.

Kerry Sieh, an American geologist and seismologist, in a 1978 report published in the Bulletin of the Seismological Society of America, stated that the firsthand reports were most abundant for the shocks felt at 1, 2, and 4 hours before the main shock which were then labeled the predawn, dawn, and sunrise shocks. The predawn event shook residents of San Francisco (MM II — III), San Jose (MM IV), and Santa Cruz (MM 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 (MM III), Monterey (MM IV), and Visalia (MM 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 time 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.

Prior to the twenty-first century, six moderately sized Parkfield earthquakes occurred with exceptionally regular intervals (with an average of 22 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. Only one of the four Parkfield events was not felt further southeast than the dawn shock, so Sieh concluded that if the San Andreas was the source of the event, then it was reasonable to assume that the Parkfield to Cholame stretch of the fault was responsible for producing the dawn felt intensities, 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. The 1952 event may have occurred west of the San Andreas on the Nacimiento fault, though the highest felt reports were along the Rinconada fault around 56 kilometers (35 mi) miles southwest of the San Andreas.