Hi Petra. As usual there will always be controversies about new discoveries. This is to be expected when a new study contradicts an old one. The following is only the preliminary results, but as I have told you once before, if I have to make an error I want it to be the one that costs me the least. That is one of the reasons I'm still around today.

The following has been taken in part from the most recent study available to me at this time. Take Care…Don in creepy town.

PRELIMINARY RESULTS

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 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).

In this study, we are re-investigating the onshore part of the Pinole fault in order to determine whether it has been active during Holocene time as suggested by the recent geophysical profiling. This one-year study was initiated in 1997 and the following describes our progress to date.

We have completed aerial photograph interpretation and compilation of existing geologic and geomorphic data. These investigations show the fault as a series of prominent topographic lineaments trending northwest and north-northwest from Wilson Point, through the City of Pinole and through Pinole Valley. The western most of these topographic lineaments faces to the east, while the eastern topographic lineament faces west forming a graben structure through the City of Pinole, north of Highway 4. This graben is filled with Quaternary alluvium, within which at least one extensive terrace surface has been developed. The topographic lineaments form the boundary between these alluvial deposits and the surrounding Miocene bedrock. These two topographic lineaments appear to converge somewhere in the region of where Highway 4 crosses the Pinole Creek, however this relationship is obscured. South of Highway 4 the Pinole fault continues as a southeast-trending, southwest-facing topographic lineament on the eastern side of the Pinole Valley. Considerable stream incision is noted on the eastern side to this lineament. Of note is a prominent tonal lineament crossing the alluvial terrace on the west side of the valley, immediately south of Highway 4. This lineament appears to be related to a prominent groundwater barrier and may even truncate a small alluvial fan that issued from the low hills on the west side of the valley (above the present Pinole High School). This lineament has no apparent topographic expression, and does not extend to the north of Highway 4. To the southeast it is obscured by residential housing development along the south side of Pinole Creek.

A preliminary analysis of digital elevation data suggests that the Pinole fault may not continue southeastward to join with the Moraga and Rheem faults as depicted by previous workers, but may follow a more north-south strike, crossing the San Pablo Valley to join the Hayward fault near Berkeley.

References

Anima, R.J., Williams, P.L., and McCarthy, J., 1992, High resolution marine seismic reflection profiles across East Bay faults: in Borchardt, G., and others (eds.), Proceedings of the Second Conference on Earthquake Hazards in the Eastern San Francisco Bay Area: California Department of Conservation, Division of Mines and Geology Special Publication 113, p. 18.

Bedrossian, T.L., 1980, Pinole fault and related faults, California Division of Mines and Geology Fault Evaluation Report, FER-97, 9 p.

Lienkaemper, J.J., Borchardt, G., and Lisowski, M., 1991, Historic creep rate and potential for seismic slip along the Hayward fault, California: Journal of Geophysical Research, v. 96, n. B11, p. 18,261-18,283.

Working Group on California Earthquake Probabilities, 1990, Probabilities of large earthquakes occurring in the San Francisco Bay region, California, U.S. Geological Circular, 1053, 51p.

Williams, P.L., and Ingram, L., 1994, using the shallow marine record to detect the geometry and recurrence behavior of active faults: U.S. Geological Survey Open File Report 94-568, p. 197-199.

Williams, P.L., Anima, R., McCarthy, J., Nakata, T., Okamura, M., and Shimazaki, K., 1993, Fault activity, geometry, and recurrence behavior at the junction of the Rodgers Creek and Pinole faults, EOS Transactions of the American Geophysical Union, v.74 , p. 411.

Williams, P.L., Dreger, D., Foxall, W., Loper, S., Pasyanos, M., and Uhrhammer, R., 1994, New data on the geometry of the Hayward-Rodgers Creek fault step, Seism. Res. Letts., v. 65, p. 62.

Wright, T.L, and Smith, N., 1992, Right step from the Hayward fault to the Rodgers Creek fault beneath San Pablo Bay, in Borchardt, G., and others (eds.), Proceedings of the Second Conference on Earthquake Hazards in the Eastern San Francisco Bay Area: California Department of Conservation, Division of Mines and Geology Special Publication 113, p. 401-417.

• Pinole Fault Study