Hi Petra. Since 1987, scientists from LBL and UCB have run a precision high-resolution seismic network at Parkfield. It involves 10 boreholes, each 200 to 300 meters deep, and containing a set of three sensors cemented at the bottom. With these sensors, several thousand seismic events have been recorded over the years. The results have been analyzed at LBL's Center for Computational Seismology.

They observed that only a small fraction of the fault zone was responding to the motion of the plates; almost two-thirds of the several thousand earthquakes recorded occurred in less than one percent of the active fault zone. The earthquakes were clustered in approximately 300 small cells, each about 20 meters across, unevenly distributed along the fault. When one cell was activated, another would go off as far away as about 200 meters, implying some sort of communication process between the two cells, such as fluid migration in the rocks.

The small earthquakes measure less than magnitude one on the Richter scale and recur with distinct regularity. "These micro-earthquakes are indistinguishable from one to the next--they're identical," McEvilly says. "Most of them occur with a very regular periodicity, of the order of one year. They're like seismic pulsars."

In the course of the study, conducted during what is thought to be the final portion of the interval between the last big earthquake at Parkfield and the one due to come, the frequency of small earthquakes has increased steadily, from fewer than 200 earthquakes per year to 500. At the same time, they are happening deeper in the earth, and fewer of them reside in the cluster cells. According to the scientists, these changes in seismic activity may be signals of an impending magnitude- 6.0 quake.

Here is a short list of the quakes that occurred after the 1857 quake. I threw in the Coalinga quake, as there are some who say it was the quake that Parkfield was expecting. There are also those who say that the Coalinga quake relived enough of the stress at Parkfield that the expected quake will occur on a later date.

9 Jan 1857 Great Fort Tejon quake [San Buenaventura; Santa Cruz] 8.25
2 Feb 1881 Parkfield 5.75
3 Mar 1901 Parkfield 6.4
23 Oct 1916 Tejon Pass Area 5.3
10 Mar 1922 Parkfield 6.3
8 Jun 1934 Parkfield 6.0
28 Jun 1966 Parkfield 6.0
2 May 1983 Coalinga 6.5

My feeling is that the next 6.0 quake will be in the area of Middle Mountain. As to when this will happen. I haven’t got the foggiest idea—not at this time anyway.

As to “deep, dark and ugly.” I still think it will be on the Great Valley fault south of Coalinga. That is about 16 miles from Parkfield. Take Care…Don in creepy town