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Structure and mechanics of the San Andreas–San Gregorio
fault junction, San Francisco, California
G3 (Geochemistry, Geophysics, Geosystems), Volume 6, Number 1,
ISSN: 1525-2027, Q01009, doi:10.1029/2004GC000838, 2005
[Printable article (1.4 Mb)]
Tom Parsons, Terry R. Bruns, and Ray Sliter
U.S. Geological Survey, Menlo Park, CA
Summary. Just offshore of the Golden Gate Bridge, two major faults come together, the San Andreas and the San Gregorio. This fault junction is also where the 1906 earthquake is thought to have nucleated. In this study we developed a subsurface image of the Earth in a section across the major faults offshore of San Francisco. This image shows us which faults are active, where the most recent deformation has happened, and where the youngest sediments have been deposited. With that information we developed a 3-D fault map, and with the aid of computer simulations, we explored the ramifications of the fault geometry at the Golden Gate. We found that the San Gregorio and San Andreas faults join with each other in such a way that the crust between them gets stretched and deformed each time the faults slip. In addition, our modeling suggests that this deformation has caused the San Andreas fault to keep jumping eastward over time. This type of jump is thought to have happened during the great 1906 earthquake.
Figure caption. The San Gregorio fault approaches the more northwest trending San Andreas fault in the offshore Golden Gate region. The red line shows the location of a seismic refraction and reflection cross-section model. The white dots are a subset of relocated earthquake epicenters [Waldhauser and Ellsworth, 2000] that occurred near the seismic cross section. The black box around the epicenters shows the area of hypocenters collapsed onto the 2-D cross section. The yellow star shows the approximate epicenter of the 1906 earthquake [Geist and Zoback, 1999].
