Hi All. As usual Petra the harsh taskmaster has been keeping my nose to the grindstone doing some research for her and in doing so I found something that might prove to be interesting. This concerns the Rohnert Park quake on September 22, in the northern San Francisco Bay Area near Santa Rosa as well as two other earthquakes.

The focal depth was placed at a depth of 7 km and the fault plane solution indicated reverse-slip motion along a generally east-west trending plane. The quake was centered about 4 miles southeast of Santa Rosa in Bennett Valley east of Taylor Mountain and near the intersection of Bennett Valley and Grange Roads.
The quake was centered close to the surface trace of the Rodgers Creek Fault, one of the prominent faults in the northern San Francisco Bay Area. But because the fault dips slightly to the west and the focal point of the quake was at 7 km beneath the surface, it is likely the quake occurred in the footwall to the Rodgers Creek Fault and probably along some adjacent or sub parallel fault or fracture. Moreover, the typical sense of motion of the Rodgers Creek Fault is strike-slip and not reverse, and the orientation of the fault is more northwest than east-west, adding further to the possibility that the earthquake did not occur along the Rodgers Creek Fault. Recent geophysical surveys in San Pablo Bay involving conventional and very high resolution (VHR) seismic imaging have suggested that the Rodgers Creek fault may be linked with the Pinole fault, therefore increasing its potential rupture length by 10-15 km (Anima et al., 1992). In addition, a structure, thought to be the offshore continuation of the Pinole fault, shows repeated movement during Holocene time (Williams et al., 1993; Williams and Ingram, 1994; Williams et al., 1994). Previous studies (summarized in Bedrossian, 1980) have indicated that the onshore part of the Pinole fault is inactive. The recent geophysical survey results now call into question this assumed inactivity. Age-dating of sediments offset in San Pablo Bay shows that the Pinole fault has a recurrence interval of about 900 years; a recurrence rate that is about a factor-of-five smaller than either the Hayward or the Rodgers Creek faults (Williams and Ingram, 1994).

Another interesting note was found and that is that this is the second moderate earthquake recorded in the North Bay in about a month and follows the Bolinas M5.0 earthquake on August 17 , which also had a reverse-slip motion. The last M4+ quake this close to Santa Rosa was a M4.4 event on June 4, 1996, and was centered a few miles north of Santa Rosa.

The more recent 5.2M Yountville quake on 09/03/2000 also had a reverse slip motion even though the faults in that area are right lateral strike slip. It is now believed that the quake did not occur on West Napa Fault, but on a previous unknown fault, which by the way Jim Berkland had identified a few years back, but was not accepted.

Now the reason I find this interesting is that before the Rohnert Park and Yountville quake there was an indication of acceleration in creep for the Bay Area. No big deal except that in both cases it was reverse creep as is the case now.

The most recent quake for the Bay Area is the 3.0M near Vallejo on 11/08/00. I haven’t been able to find any data for this quake (don’t really expect to) as it was not spectacular enough to warrant a study. This quake along with the recent acceleration in creep makes me wonder if something a little larger is in the offering. Guess we will just have to wait and see. Take Care…Don in creepy town.