Hi BW. After seeing the small quake near Taff I thought you might be interested in some history of the 1857 Fort Tejon quake as it is near here that you keep expecting a large quake. While it may be possible for a large quake to occur along this section of the San Andreas fault the probability is on the low side.

The location of the epicenter of the Fort Tejon earthquake is not known. As the name suggests, one idea is to locate it near the area of strongest reported shaking -- Fort Tejon. However, because there is evidence that foreshocks to the 1857 earthquake may have occurred in the Parkfield area, it is located on the map near the northwestern end of the surface rupture, just southeast of Parkfield, near Cholame. Cholame is a watering stop and if you blink your eyes you will miss it as Petra and I found out.

Not much is known about the seismic activity at Parkfield at the time of the great Fort Tejon earthquake that occurred on January 9, 1857. Several meters of sudden right-lateral slip on the San Andreas fault produced shaking that lasted 1-3 minutes and that was felt over more than 350,000 square kilometers of southern and central California. The maximum fault movement of about 9 meters occurred in the Carrizo Plain 70-100 km southeast of Parkfield. Slip on the fault clearly was less near the ends of the 1857 rupture. The northwest end of slip along the fault in 1857 is not known and estimates of the amount of slip on the Cholame section between the Parkfield section and the Carrizo Plain range from 3 to 7 meters. Clearly less slip occurred along the Cholame section than in the Carrizo Plain in 1857.

Crustal deformation measurements along the San Andreas fault southeast of Parkfield indicate that relative motion of the Pacific and North American plates is straining the regions at a rate corresponding to 3 cm per year of right-lateral slip on the San Andreas fault. Clearly the plate motion 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 not at all clear whether or not the potential for the 3-7 meters of slip that apparently occurred along the Cholame section in 1857 has been recovered by crustal straining along the fault since 1857. Thus it is possible, though not likely, that the anticipated magnitude 6 Parkfield shock might trigger, or grow into, a shock of about magnitude 7 on the Parkfield and Cholame sections.

Accounts of the 1857 shocks indicate that several small to moderate size central California earthquakes preceded the great 1857 earthquake by 1 to 9 hours. Two large foreshocks were widely felt. A comparison of the felt areas and intensity distributions of these two felt foreshocks by Professor Kerry Sieh of the California Institute of Technology (CIT) suggests that the foreshocks 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 of about 60 km radius that includes the Parkfield section. Because foreshocks generally occur near the epicenter of the ensuing larger main shock, Sieh suggested that the great Fort Tejon earthquake in 1857 began near Parkfield at the northwest end of the rupture zone.

As you can see the epicenter of that quake was quite a ways from the area you have been looking at. Hope you enjoy. Take Care…Don in creepy town

• Fort Tejon Quake