Hi All. I sure hope none of you are getting tired of me writing articles about the Rodgers Creek fault, but being as how it thee number one fault most seismologists are looking at for the next major quake in the Bay Area to occur on I thought I would see what more I could out about it.

The following is from the Bulletin of the Seismological Society of America, June 1, 2005.

“The Rodgers Creek fault (RCF) is a principal component of the San Andreas fault system north of San Francisco. No evidence appears in the historical record of a large earthquake on the RCF, implying that the most recent earthquake (MRE) occurred before 1824, when a Franciscan mission was built near the fault at Sonoma, and probably before 1776, when a mission and presidio were built in San Francisco.”

“The first appearance of nonnative pollen in the stratigraphic record at the Triangle G Ranch study site on the south-central reach of the RCF confirms that the MRE occurred before local settlement and the beginning of livestock grazing.”

“Chronological modeling of earthquake age using radiocarbon-dated charcoal from near the top of a faulted alluvial sequence at the site indicates that the MRE occurred no earlier than A.D. 1690 and most likely occurred after A.D. 1715. With these age constraints, we know that the elapsed time since the MRE on the RCF is more than 181 years and less than 315 years and is probably between 229 and 290 years. This elapsed time is similar to published recurrence-interval estimates of 131 to 370 years (preferred value of 230 years) and 136 to 345 years (mean of 205 years), calculated from geologic data and a regional earthquake model, respectively. Importantly, then, the elapsed time may have reached or exceeded the average recurrence time for the fault.”

“The age of the MRE on the RCF is similar to the age of prehistoric surface rupture on the northern and southern sections of the Hayward fault to the south. This suggests possible rupture scenarios that involve simultaneous rupture of the Rodgers Creek and Hayward faults.”

“A buried channel is offset 2.2 (+1.2, –0.8) m along one side of a pressure ridge at the Triangle G Ranch site. This provides a minimum estimate of right-lateral slip during the MRE at this location. Total slip at the site may be similar to, but is probably greater than, the 2 (+0.3, –0.2) m measured previously at the nearby Beebe Ranch site.”

The exact location of the quake isn’t really know, but is felt to have been at the northern end of the Rodgers Creek fault. The rupture was about 40 miles long and that being the case it could be said that it ruptured from north of Santa Rosa and possibly all the way to San Pablo Bay. Of course it can’t be ruled out that the rupture didn’t stop there as a paragraph above indicates. Not a pleasant thought if that were the case. The magnitude of the quake wouldn’t be all that much larger, but the area of damage would be almost two and a half times greater plus the shaking would most likely last longer. Petra would really have something to go WaHooooo about. Of course there is the chance that it could leave her speechless. Nah. No chance of that happening.

When and if you open the link go all the way to the bottom of the page to see the maps. Take Care…Don in creepy town

http://erp-web.er.usgs.gov/reports/annsum/vol45/nc/03hqgr0083ann.pdf

• Rodgers Creek Fault