Hi All. This is a follow up on the previous post I made about the Yountville quake in Sept. of 2000.

This is pure speculation on my part, which for the most part gets me into trouble.

The Yountville quake was supposedly caused by an unknown fault, but an individual has claimed to have identified that fault about 24 years ago.

This unknown fault is approximately midway between the West Napa Fault (the northern extension of the Calaveras Fault) and the Rodgers Creek Fault (the northern extension of the Hayward Fault) and parallel to them. The earthquake did not produce any ground surface rupture. According to USGS the fault rupture mechanism is likely to be right-lateral strike slip. Because the seismic waves traveled prevalently towards south-southeast, it is expected that the rupture started at the epicenter and propagated in that direction. The rupture length, according to USGS seismologists, is expected to be in the order of 3.5km. Note that it is not infrequent that earthquakes occur in previously unknown fault. For example, a 5.7M earthquake occurred on March 3, 1986 in the East Bay hills, about 10 miles northeast of San Jose, near MT Lewis. This earthquake generated nearly 2000 aftershocks over a period of a year and a half, yet geologists have been unable to identify any fault at the surface that can be related to these earthquakes at depth.

It is interesting to note that no major earthquakes have been generated by the West Napa Fault and or by this unknown fault in more than a century. The last event felt by the population in this area was a M3.6 that occurred 8 miles Northwest of Napa in June 1990. The two stronger (M5.6 and M5.7) 10/02/1969 earthquakes that occurred in Santa Rosa were caused by the Rodgers Creek Fault.

What should be noted here is the seismic wave generated by this quake is in the same direction as the Bolinas, and Rohnert Park quakes giving the impression that the quakes occurred on a reverse fault. However it has now been discovered that the Rohnert Park quake occurred on a thrust fault located not to far from the Rodgers Creek fault. As with the Bolinas and Rohnert Park quakes there were very little aftershocks.

As of right now there is no indication of movement (small quakes) on the southern section of the Rodgers Creek fault. For the most part this is the section of the fault located between Rohnert Park and the San Pablo Bay. There were 4 small quakes in Dec. of 1981 that were located south of Rohnert Park and east of Petaluma, but can find no history of quakes south of there.

My way of thinking is that this section of the Rodgers Creek fault is locked tight and this is causing the quakes parallel to the fault. They are sort of the weak link in the fault system in the North Bay. When there are no more weak links that will be when there will be a major quake in the Bay Area. The question is when do we run out of weak links? How many more are there and where are they? Take Care…Don in creepy town