Hi All. I knew there were some foreshocks to the 1857 Fort Tejon quake, but wasn’t sure of the location. Did some looking around, sent out a couple of emails and in short order I received what I was looking for.

For those who don’t like to read this may not be for you, but for those who want to learn this is the ticket.

There are some indications that we are heading into a active period that is somewhat similar to the past before the 1906 earthquake. Earthquakes in California are a fact of life and do not mean the end for California. Look at this way. If it weren’t for the earthquakes the State wouldn’t be what it is today. We wouldn’t have the mountains that seem to hold up the sky, nor the rolling hills that make one think of a roller coaster. We wouldn’t have the peaceful valley’s although I will admit there are times when they aren’t all that peaceful.

I have learned to live with earthquakes simply because they are an absolute marvel. It never ceases to fascinate how one moment everything is peaceful and quiet the next moment you hear the deep rumbling and the ground feels as if it is alive.

I’m not afraid of earthquakes. When we have one I go straight to terrified. Saves time that way. My heart stops and I know that one-day it will not restart. That’s okay, as I have had one hell of time getting here. I wouldn’t change anything. Not one iota. Take Care…Don the ancient one

Now the 1857 quake.

Numerous foreshocks occurred during the several hours preceding the main shock. From historical accounts, the epicenters of the foreshocks were located in central California near the northwestern extent of the main shock fault rupture.

Epicenters of the last two foreshocks, which were estimated to be between M 5-6, were located within a 60 kilometer radius of a segment of the San Andreas fault that has historically be characterized by creep. The relatively constant slippage along this segment of the fault relieves much of the strain, but the reach of the San Andreas Fault from about Parkfield to San Bernardino is locked and thus not continually relieved. Stresses within this central segment of the fault continually build and every few hundred years become so great that the locked section ruptures suddenly and a great earthquake similar to that of 1857 occurs.

The foreshocks in the creeping segment may have triggered the main shock. Therefore, it appears that the 1857 earthquake occurred by propagating from the northwesterly creeping segment of the fault southeastward into and through the locked segment. If the 1857 earthquake did occur in this fashion, it implies that studies of the creeping segment of the San Andreas Fault north of Parkfield may aid in predicting the next great 1857-type earthquake in southern California.

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.

http://www.johnmartin.com/earthquakes/eqpapers/00000075.htm

http://www.johnmartin.com/earthquakes/eqpapers/00000039.htm

http://pangea.stanford.edu/GP/sanandreas2000/Toppozada.pdf