Hi All. I think I may have posted this once before so excuse the memory.

They say the first thing to go is the memory. That is not true. It’s the second. I don’t remember what the first one was though. Take Care…Don in creepy town.

31 March 1898. M=6.3 ( 95% confidence range 6.0-6.5) earthquake located near San Pablo Bay. This event caused a seiche in San Francisco Bay (Toppozada et at., 1992) so it is likely located near or beneath San Pablo Bay. The 1898 event was not seen in the USGS trench across the Rodgers Creek fault east of Petaluma, but slip during an M = 6.3 shock could easily be missed in the trench. Three scenarios: A) a 30-km long strike-slip rupture on the south end of the Rodgers Creek fault. Depth extent = 10 km and mean slip = 0.4 m. B) a 30-km long strike-slip rupture on the south end of the West Napa fault. Depth extent = 10 km and mean slip = 0.4 m. C) a 20- km long thrust on a blind-thrust fault east of San Pablo Bay. Depth extent = 12 km and mean slip =0.5m.

The Hayward fault accommodates the majority of fault slip in the East Bay and is considered to be the most likely candidate for the source of the next damaging earthquake in this region (Working Group on California Earthquake Probabilities, 1990). Slip along the Hayward fault appears to die out towards its northern end, somewhere north of Point Pinole in San Pablo Bay (Lienkaemper et al., 1991). Slip is then transferred over to the Rodgers Creek fault system through a right step or salient. The right step, about 6 km in width, between these two right-lateral, strike-slip fault systems, creates a region of tension resulting in the formation of a pull-apart basin beneath San Pablo Bay. Gravity data and stratigraphic relationships as revealed in oil company drill logs show that there has been at least 1100 m of down-dropping along the bounding faults of this basin since its formation (Wright and Smith, 1992).

The structure and geometry of this right step between the Hayward and Rodgers Creek faults has important implications for seismic hazards assessment in the Bay area. Whether this structure acts to facilitate or inhibit rupture propagation from the Hayward fault to the Rodgers Creek fault (or vice versa) is of great importance in determining the maximum credible earthquake (MCE) that is associated with these structures.

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 calls 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).

• Rodgers Creek Fault