Hi All. My prediction for a quake in the Petaluma appears to be a bust. I suspect the M=2.6 quake that occurred to the NE of the predicted location was the quake I had predicted. It just didn’t occur where I thought it would. Had the quake occurred where I said it would it would have been on the Rodgers Creek fault in the area of the Dec. 1981 swarm.

Based on surface traces, the fault was zoned under the Alquist-Priolo Earthquake Fault Zoning Act from Windsor Creek on the north almost to San Pablo Bay (California Division of Mines and Geology, 1983 #5275; 1993 #5298).

Two paleoseismic investigations may have identified the last three earthquakes and determined a slip rate of 6.4-10.4 mm/yr for the south part of the fault (Schwartz and others, 1992 #5264) and several site investigations under the Alquist-Priolo Act have located or verified Holocene-active fault traces (Hart, 1982 #5267; 1992 #5268). High-precision theodolite surveys conducted since 1980 have not indicated any fault creep (Galehouse, 1995 #5283).

The lack of, or the very low number of quakes occurring on the southern segment of the Rodgers Creek fault is a pretty good indication that it is locked up pretty tight. I find it hard to believe that all the creep that is being seen on the northern Hayward fault stops in the San Pablo Bay. It may require a couple of quakes such as those that occurred before the Loma Prieta quake to occur to loosen up the area so the fault can move. That is just a guess on my part.

The quakes that have occurred in the area in the past are a pretty good indication that the stress is there. There were the two M=5.0+ quakes in Santa Rosa in 1969, then the Bennett Valley quake as well as the quakes in the Healdsburg area.

There are some faults just to the west of the Rodgers Creek fault that aren’t showing much in the way of quake activity either. These are the Tolay fault, the Petaluma Valley fault, and an unnamed thrust fault.

Petra drives parallel to the thrust fault when she goes to work. There could be a surprise there for her although I wouldn’t hold my breath waiting for anything to happen that could be associated with that fault. She also crosses the Tolay fault to get to her work place. Not much activity has been seen there either. This dextral fault significantly truncates Miocene and older bedrock and the northwestern part may offset Pliocene-Pleistocene deposits (Huffman and Armstrong, 1980 #4862; Fox, 1983 #5252). Southeast end may connect with the Hayward fault [55] or may merge with Rodgers Creek fault [32]. Based on geomorphology, the southeast part may be Quaternary (Huffman and Armstrong, 1980 #4862), but seems to lack evidence for significant Holocene activity (Hart, 1982 #5293). Dextral offset along the Tolay-Hayward fault during past 6 m.y. is suggested to be 45±10 km (Sarna-Wojcicki, 1992 #5265) and 45±15 km in the past 8 m.y. (Fox, 1983 #5252). A reverse-slip component (up to the southwest) of at least 1,100 m has been interpreted in several places by Morse and Bailey (1935 #5296), Weaver (1949 #5262), Travis (1952 #5290), and Wright and Smith (1992 #5258). The fault lacks evidence of historic seismicity (Wong, 1991 #5257; Goter and others, 1994 #4855). No detailed trench investigations have been reported.

All said an done the Santa Rosa/Petaluma area could be in for a surprise one day. The Rodgers Creek fault is the number one candidate for the next major quake in the Bay Area. I wonder how many people are prepared for it? Take Care…Don in creepy town

• Bay Area Faults