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Link between two earthquake faults near San Francisco, California, revealed by detailed sub-seafloor mapping

News from USGS Pacific Coastal and Marine Science Center

Seismic-reflection profile shows sediment layers beneath a bay, and map shows variations in gravity under the bay.
Above, left: Seismic-reflection profile created by bouncing sound waves off sediment layers beneath San Pablo Bay. Note deformation of layers by Hayward fault (black arrow). Above, right: Variations in gravity caused by differences in rock density under the bay. Sharp change in gravity values (dashed white line) lies close to sites where seismic-reflection profiles imaged the Hayward fault (plus signs).

USGS scientists used a suite of complementary techniques to image sediment layers beneath San Pablo Bay, California, revealing an active fault beneath the bay that links two of the most urbanized and hazardous strike-slip faults in the San Francisco Bay area. The discovery, first announced December 2015 at the American Geophysical Union Fall Meeting, is detailed in “Missing Link between the Hayward and Rodgers Creek faults,” published October 19, 2016, in Science Advances. Longer fault ruptures produce larger earthquakes, so understanding how fault segments connect with one another is critical to assessing the earthquake hazards they pose. This study demonstrates the importance of integrated, high-resolution mapping of sub-seafloor layers for characterizing the often-subtle deformations (as small as a few centimeters) where fault segments meet.

Learn more about the USGS study, "Offshore Faults along Central and Northern California."

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