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A new study from United States Geological Survey (USGS) researchers and industry partners uses advanced sub-surface analyses and sediment dating to reassess the slip rate of the Hosgri fault off the central California coast. The research was led by Dr. Jared Kluesner, USGS Research Geophysicist at the Pacific Coastal and Marine Science Center. 

Assessing earthquake hazards demands a comprehensive understanding of active faults, including their location, length, connectivity, slip rate, and rupture history. For densely populated areas like the central California coast, this knowledge can help safeguard both lives and critical infrastructure.

“The work yielded a slip rate similar to what we previously estimated, but the level of documentation and our confidence in the result are substantially higher." - Sam Johnson, USGS Emeritus and study co-author

Offshore field data obtained for this study include seismic-reflection profiles and sediment cores. State-of-the-art processing of seismic profiles yielded fault-zone imagery with exceptional resolution. High-precision ages of sediments offset by the fault were obtained using multiple techniques, including radiocarbon dating of organic material such as shells and wood and optically stimulated luminescence dating of quartz grains in the sediment, which determined when the quartz was last exposed to daylight.

Previous recent work—which used offsets of coastal terraces and offshore slopes and channels with estimated ages—led to variable Hosgri Fault slip-rate estimates, ranging from less than one millimeter to several millimeters per year. This new dataset yielded a slip rate of 2.6 ± 0.8 millimeters per year, similar to a slip rate published in 2014 that used slope maps from high-resolution bathymetry to measure offset and an age model based on global sea-level curves.  

“This new study yielded unprecedented high-resolution imagery of the fault zone and the adjacent shallow stratigraphy beneath the seafloor, providing a framework for meticulous sampling and dating of sediment offset by the fault,” said Sam Johnson, USGS Emeritus and author of the 2014 work. “The work yielded a slip rate similar to what we previously estimated, but the level of documentation and our confidence in the result are substantially higher. We need comprehensive studies like this on other fault zones offshore of California.” 

In discussing the new work, Kluesner noted, “This multi-pronged approach to characterizing the seafloor and below allowed us to attain high-resolution data in coarse-grained sediment layers, which are typically challenging to image. The methodology developed in this study could serve as a blueprint for reevaluating other offshore fault zones in high-energy coastal environments, which should lead to more confident earthquake hazard and risk assessments.”

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