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Geological Studies of Active Faulting in the Bay Region

Pat Williams

Subsections


This year has seen the start-up of two major geological fault study initiatives in the Bay Area. These initiatives are the "Probability'99" Working Group, and BAPEX (Bay Area Paleoseismic EXperiment), both administered by the U.S. Geological Survey. The Working Group, a cross section of approximately 50 regional geologists and geophysicists, has met this summer in several day-long meetings for the purpose of producing a new statistical evaluation of Bay Area fault hazards. Three sub-groups are working to develop expert opinion consensus statements as inputs to an updated statistical ordering of regional fault-rupture forecasts. I am participating in two panels: a North and East Bay study of transform faulting (Professor Roland Burgmann is also participating in this working group) and a compressional fault survey of the entire region. The BAPEX program results from an infusion of three years of Pacific Gas and Electric research funding (1,000,000/yr) dedicated to recovery of a substantial body of new paleoseismic data from the largest Bay Area faults. This program will enable continued work at three Hayward fault research sites: Mira Vista golf course paleoseismic study (collaboration with USGS and William Lettis); a long-term slip-rate study at Strawberry Creek, and paleoseismic studies in the City of Fremont.

Tectonics of the Sacramento Delta

Work on the active fault record at the Pittsburg townsite has advance considerably this year. I have developed a new topographic map to recover original tectonic landforms across a key area of man-altered land. This was accomplished using digital "soft copy" photogrammetric methods to model sub-meter landform features from stereographic photography. The primary application of this work is to evaluate evidence of punctuated uplift of the Pittsburg pressure ridge. Complementary work is coming to a completion on determination of the radiocarbon ages of subsided and buried peat surfaces on the fault's downgoing side.

Application of Microstratigraphic Techniques to Fault Studies

I have continued to involve UC Berkeley micro-fossil experts in paleoseismic research. Previously Doris Sloan, Lynn Ingram and I collaborated on analysis of microstratigraphy of a 70-meter core from the down-dropped side of the Pittsburg fault. Presently I am continuing that study, and have set out on a new study of the palynology of freshwater sites in the Bay Basin. This work is used to evaluate two key historical sediment horizons: introduction of Erodium and Eucalyptus Pollen (and diminishment of redwood pollen) is used to constrain 19th century levels at which the resolution of radiocarbon is very poor, and across which earthquake history remains problematic (i.e. how big was the 1868 Hayward earthquake? were there other 19th century ruptures?). This data has turned out to be critical at several other sites on this and other faults. I am working closely with Roger Byrne and his students in these studies.

Slip Rate of the Hayward fault

Analysis of the morphology and ages of the Strawberry Creek channels enables measurement of the total lateral slip rates of the Hayward fault at Berkeley. Prior to development, the fault was strongly expressed by fault scarps and stream offsets at UC Berkeley. Active and former channels of Strawberry Creek are offset large distances across the fault. The modern stream course is offset 335+/-30 m (1100 ft) in a right-lateral sense, and prominent beheaded channels of Strawberry Creek are offset 580 m (1900 ft) and 730 m (2400 ft).

The radiocarbon age of beheaded stream deposits, obtained from a construction exposure across the youngest paleochannel, is 32,000 radiocarbon years. This is a minimum initiation age for the modern 335 m stream offset, and provides a direct record of the long term-average slip rate of the Hayward fault, of about 10+/-1 mm/yr. This long term slip rate enables more robust forecasts of future rupture hazard. The rate is approximately 10 percent higher than previous geological determinations of slip rate, indicating an accordant increase in probabilistic earthquake hazard estimates for the northern Hayward fault.

A new phase of fault mapping and study has been undertaken at the direction of the Campus Seismic Review Committee, to inform the campus seismic safety retrofit program (SAFER). This work complements work associated with the slip rate determination. Both projects require a good knowledge of the fault's location.

The resulting map data are being used to evaluate retrofit design and decisions impacting future use of University buildings in or near the Hayward fault zone. These studies have produced three reports to the University, and interpretations in these studies are being tested in a program of trench investigations across suspect fault features.


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Next: About this document ... Up: Ongoing Research Previous: Research Projects under Lane

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