Offshore Faulting In The Gulf Of The Farallones

ABSTRACT Extensive marine geophysical surveys were conducted in the Gulf of the Farallones as part of the predesign studies for the Southwest Ocean Outfall, a major element of San Francisco's wastewater master plan Interpretation of the geophysical data was aided by geologic studies and the results of an offshore boring and vibratory coring program. The Outfall will cross the San Andreas fault, which was located and delineated as a zone of en echelon breaks separating horizontally-bedded deposits from a thinly-bedded anticlinal structure. The Seal Cove and Pilarcitos faults were also located, but were found to be beyond the extent of the planned Outfall structure. Special joints have been designed for the San Andreas fault zone portion of the Outfall conduits. INTRODUCTION The wastewater Management Master Plan of the City and County of San Francisco, when completed in the 1980's, will provide for the collection, transportation treatment, and disposal of both sanitary and storm wastewater flows. The peak design capacity of the master plan is 670 million gallons per day. After passing through a system of transport lines, treatment plants, and tunnels, these flows will reach the proposed Southwest Ocean Outfall, where they will be dispersed at diffuser sections in the Pacific Ocean approximately 2 and 4 miles southwest of Lake Merced as shown on Figure 1. Since early 1977, the Southwest Ocean Outfall Project (SWOOP) design team has conducted various data acquisition and analysis programs (1,2, 3,4) in the project area. The results of these studies have been incorporated in the planning, preliminary design, and final design efforts which were completed in late 1979. It is presently expected that construction of the Outfall system will be completed in 1984. The proposed Outfall will cross the. San Andreas fault zone in the Gulf of the Farallones, a relatively shallow portion of the Pacific Ocean immediately seaward of the Golden Gate between Point Reyes to the or the and Pillar Point to the south. Because of this, the objectives of many of the studies previously mentioned were to better define the regional and site specific geological and seismological environment of he Outfall, to determine the location at which the fault zone crosses the Outfall alignment, and to aid the development of appropriate criteria for Outfall design. The extent of the marine geophysical surveys performed and the results of the interpretation of the geophysical data obtained for the Southwest Ocean outfall Project are summarized in this paper. It is believed that the geophysical survey interpretation shown on Figure 2 represents the first detailed information on offshore faulting in the Gulf of the Farallones. MARINE GEOPHYSICAL SURVEYS Two separate marine geophysical and bathymetric survey efforts were made as part of the predesign studies for the Outfall; the first was a general survey over the entire area shown on Figure 1; the second was site-specific, concentrating primarily on the Outfall alignment shown on Figure 2. Over 400 nautical miles of geophysical data were collected, resulting in more than 4000 feet of analog records.