Answering to stakeholders: Sensitivity of liquefaction hazard during earthquakes to sea-level rise in the San Francisco Bay Area, California

Abstract:

The U.S. Geological Survey has engaged with San Francisco Bay Area (SFBA) stakeholders about two natural hazards; an earthquake scenario –HayWired; and a Coastal Storm Modeling System (CoSMoS) with sea-level rise (SLR). In the SFBA, 1 meter (m) of SLR is projected to occur in 40 to 80 years time and cause overland flooding and groundwater table shoaling. The liquefaction hazard caused by earthquake ground shaking increases with groundwater rise. Liquefaction surface effects, including sand boils, surface cracking, vertical settlement of the ground surface, and or lateral spreads, may damage foundations and buried infrastructure. SFBA stakeholders were invited to share their needs for information about the liquefaction hazard with SLR and to participate in usability testing of maps and figures showing the results of the analyses. Planners and engineers asked: where and how could the liquefaction hazard increase with SLR?

Scientists assessed the effects of SLR on the liquefaction hazard by computing changes in the liquefaction potential index (LPI) over 400 Cone Penetration Test (CPT) borings around the bay using two groundwater table models developed for current and SLR of up to 5 m. Ground shaking is represented by two earthquake scenarios, a M 6.5 earthquake with uniform 0.25 g peak ground acceleration (PGA) and the M7.0 Hayward fault (HayWired) scenario with a range of PGA between 0.20 and 0.95 g at boring locations. We find that while the majority of sites are insensitive to sea-level changes of less than 1 meter, some site conditions are highly sensitive to small changes in water levels, and the potential for liquefaction damage at most sites increase with large shifts in sea-level (> 3 m). At 1 m of SLR, most of the locations with the largest LPI changes are found in artificial fill over estuarine mud. We developed and tested two communication products for distinct audiences: (1) figures of LPI change at sites from rising sea levels using two groundwater models, two tidal datum and three hydraulic conductivity values for engineers and (2) maps of median LPI change for planners. This project illustrates how scientific investigation was conducted in parallel with stakeholder engagement and how the usability testing guided the development of scientific products more readily used by practitioners.