On December 10, 2023, eighteen SCEC community members convened for a special SCEC-sponsored field trip co-led by Kim Blisnuik (San Jose State University), Tim Dawson (California Geological Survey and SCEC Board of Directors Chair), Austin Elliot (USGS – Moffett Field), and Tyler Ladinsky (California Geological Survey). As part of SCEC’s Statewide focus, the trip was designed to highlight the unique geology, tectonic setting, and seismic hazards of the San Francisco Bay Area.  The trip was timed to take advantage of the critical mass of scientists gathered for the 2023 American Geophysical Union’s Fall Annual meeting, held in San Francisco for the first time in four years

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The trip’s first stop was at Corona Heights Park, an urban public park widely recognized for breath-taking views of San Francisco, fascinating geology, and immortalized in the “Streetcar 2 Subduction” field trip guide (the online version of this field trip guide can be found here).

From the top of Corona Heights (Figure 1) the group surveyed downtown San Francisco and discussed the geology and seismic hazards of San Francisco and the greater Bay Area.  The group then embarked on a short hike to the world famous (and not active) Corona Heights fault, a 15-meter-high exposure of a fault plane developed in Franciscan chert and expressed as well-developed slickensides coated by a thin, polished coating of relict silica gel formed during fault slip (Figure 2). 

The significance of the slickenlines and polished surface in this exposure with respect to fault strength during coseismic slip and fault rock rheology is discussed in two excellent papers by Kirkpatrick et al. (2013) and Kirkpatrick and Brodsky (2014).

The group then traveled south along the Highway 280, stopping at an overlook of the San Andreas fault and Kim Blisnuik provided an overview of the San Andreas lake and fault (Figure 3, top of page). Field trip participant Chris Madugo (Pacific Gas and Electric) kindly provided the perspective of the utility operators whose infrastructure of transmission and distribution lines cross numerous fault crossings throughout the Bay Area and the mitigation efforts led by geoscientists and engineers to prevent fault rupture from disrupting functionality of this essential infrastructure.

After an excellent lunch in the coastal town of Half Moon Bay, the group stopped at Pillar Point Bluff to view an exposure of an active strand of the San Gregorio fault zone (Figure 4). Kim Blisnuik and Tyler Ladinsky co-led this stop and discussed the long-term uplift of the marine terrace at this location, the faulting exposed in the head scarp of an active landslide, and the geochronological work that Blisnuik and colleagues have been developing to document the timing of deformation at this site.

For the final stop of the trip, the group journeyed eastwards to visit the Hayward fault and examine evidence of active fault creep. Within the City of Fremont, the Hayward fault cuts across Fremont Central Park, where active fault creep has right-laterally displaced cultural features within the park, including the former Fremont Community Center.  Thanks to the work of the educational non-profit group Math/Science Nucleus (msnucleus.org), the City of Fremont, and the U.S. Geological Survey, the community center has been preserved as the “Fremont Earthquake Exhibit”, where visitors can view the fault and observe the effects of fault creep as it cuts across the floor of the now 50+ year old building (Figure 5). Before heading back to the City, the group took a short walk to view curbs offset by fault creep (Figure 6) and the fault crossing of the Hetch Hetchy Aqueduct, which is the primary water supply for San Francisco.

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The field trip leaders would like to express their gratitude to SCEC for the opportunity to lead the first Statewide California Earthquake Center-sponsored field trip and introduce the group to the unique earthquake geology that northern California has to offer.

References

Kirkpatrick, J.D., C.D. Rowe, J.C. White, and E.E. Brodsky (2013) Silica gel formation during fault slip: Evidence from the rock record, Geology, v. 41; no. 9; p. 1015–1018; doi:10.1130/G34483.1

Kirkpatrick, J.D. and Brodsky, E.E. (2014) Slickenline orientations as a record of fault rock rheology, Earth and Planetary Science Letters, v. 408, p 24-34, https://doi.org/10.1016/j.epsl.2014.09.040

About the Author

Tim Dawson is the Manager of the CGS Seismic Hazards Program and is a specialist in active fault studies, paleoseismology, seismic source characterization and probabilistic fault displacement hazard analysis. He is a member of the Executive Committee on the Working Group on California Earthquake Probabilities (WGCEP). Tim has served as the California Geological Survey's representative on the SCEC Board of Directors since 2017, and has chaired the Board since 2023.