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Surveying Faults and Sediment Outside the Entrance to San Francisco Bay
A small group of U.S. Geological Survey (USGS) personnel from the Western Coastal and Marine Geology Team ventured out of San Francisco Bay's Golden Gate for 10 days in late September to survey offshore faults near the epicenter of the great San Francisco earthquake of 1906. They used high-resolution seismic-reflection profiling systems to image several faults considered capable of producing damaging earthquakes, including the San Gregorio, San Andreas, and Golden Gate faults. The USGS and the California Geological Survey have recently focused on updating fault maps to be used by the Working Group on California Earthquake Probabilities in developing a new time-dependent seismic-shaking-hazard model for the State of California. Among other things, this model will be used to set State-wide earthquake-insurance rates.
The scientific crew, headed by Holly Ryan, included Larry Kooker, Anne Gartner, and Peter Triezenberg, joined at various times by Patrick Hart, Ray Sliter, Gerry Hatcher, and Mike Boyle. The 54-foot boat Lakota, recently transplanted to the West Coast from treasure hunting off Florida, was used as the research vessel on its maiden geophysical voyage, with captain Tim Fleming at the helm. Although temperatures were a bit chilly, the seas were incredibly calm, almost glassy, for the entire trip. (September in northern California sometimes cooperates.)
Data were collected from Fort Funston, south of the Golden Gate, to as far northward as Bolinas, near where the San Andreas fault crosses onto shore. One of the main goals was to study the transfer of seismic slip between the San Gregorio, San Andreas, and Golden Gate faults. Another part of the mission was to determine the subsurface structure of gigantic (as much as 10 m high and 220 m from crest to crest) sand waves imaged by USGS scientists Patrick Barnard and Dan Hanes just outside the Golden Gate (see related Sound Waves article, "Giant Underwater Sand Waves Seaward of the Golden Gate Bridge") and to provide shelf-sediment information for Barnard and Hanes' coastal-erosion study off Ocean Beach, located between the Golden Gate and Fort Funston (see related Sound Waves article, "USGS Scientists Investigate Surf-Zone Hydrodynamics at San Francisco's Ocean Beach"). This effort included collecting data to calculate the volume of sediment in an ebb-tide delta immediately seaward of the Golden Gate and in Holocene deposits on the continental shelf. The scientists gathered data with two high-resolution seismic-reflection systems, both of which produce pulses of sound and record the return of their echoes from layers of sediment and rock beneath the sea floor. Such data are used to produce cross-sectional images, or profiles, of the sub-sea-floor layers. For studies of fault geometry, the scientists used a 50-tip minisparker as the sound source and a short, single-channel hydrophone streamer to receive the echoes. The minisparker is towed just beneath the water 5 to 10 m behind the vessel, where, at regular intervals, it produces an electric spark that vaporizes a small volume of water. Rapid expansion of the vapor bubble generates a sharp pulse of sound that radiates outward through the water. The process is similar to that which produces lightning and thunder, though on a much smaller scale. Wherever the sound energy encounters a change in acoustic impedance, which is the product of density and acoustic velocity (the speed of sound in a given material), some of the sound energy is reflected. Thus, some sound energy is reflected at the boundary between seawater and the sea floor, while sound energy that penetrates the sea floor is reflected from boundaries between sub-sea-floor layers of differing acoustic impedance. The returning echoes are picked up by underwater microphones, called hydrophones, spaced at regular intervals in a clear, oil-filled hose, or "streamer," towed behind the vessel. To image the sand waves, the scientists used an Edgetech 512i subbottom-profiling system composed of a high-frequency "chirp" source and a small hydrophone array, all housed within a 190-kg "fish" that is towed 3 to 5 m below the sea surface. The chirp system uses a piezoelectric transducer to convert electrical pulses into mechanical vibrations, or sound energy. The hydrophones—like those in the minisparker system—reverse the process, converting the mechanical energy of returned echoes into electrical signals that are digitized, displayed, and stored in a computer. (To learn more about seismic mapping, see WHSC Seismic Profiling systems.) The minisparker system provided deeper imaging penetration below the sea floor but slightly less resolution than the chirp system. Data for both systems were digitally recorded for postcruise processing and interpretation. Several coincident profiles were collected with both systems for data comparison. Results of the seismic-reflection surveys were mixed. The sediment in the area of the shallowest part of the ebb-tide delta—where thick, sandy deposits are present—obscured faulting. In addition, gas from natural seeps scattered the sound energy, making it difficult to map faults in some areas. Evidence of gas in the seismic-reflection data includes bright spots caused by pockets of trapped gas, data "washouts" caused by gas dispersed through sediment, and fuzzy clouds of reflectivity caused by gas bubbles in the water column. (Cruise participant Pat Hart notes that gas "has a dramatic effect on data quality—usually bad.") In many areas, however, especially away from the ebb-tide delta, the data were spectacular, thanks in part to relatively calm seas. The data are currently being processed and will be used not only in the new seismic-shaking-hazard model for California but also in studies of tectonics and coastal erosion. Another important aspect of the cruise involved using USGS staff rather than paid consultants as marine-mammal observers. Several of the Western Coastal and Marine Geology Team research staff have been trained to scan for and identify marine mammals in order to determine whether operations should be shut down to prevent the acoustic equipment from adversely affecting whales, dolphins, or porpoises. Possibly owing to a lack of food sources in the area, only a few sea lions and harbor seals were spotted, to the relief of the scientists and the boredom of the observers.
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in this issue:
Submarine Ground-Water Discharge at Dor Beach, Israel
Chinese Delegation Briefed on USGS Science Restore America's Estuaries Conference
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U. S. Department of the Interior | U.S. Geological Survey
