Department of Civil and Environmental Engineering, UCLA

Global sea levels are expected to increase up to five meters by the year 2150. Due to elevation differences, ocean dynamics, and the uneven redistribution of water from global ice melting, California will experience 1.25:1 of relative amounts of sea level rise (SLR). Ocean fluctuations such as storm surge and spring tides, as well as higher marine water levels can periodically elevate coastal groundwater. These fluctuations pose multiple interrelated coastal hazards, including groundwater salinization, contaminant mobilization and infrastructure damage. Specifically in California, over 2.5M people live along the coast making it the most populated coastline in the continental US. This densely populated area is also a critical infrastructure hub that serves not only California but other US states. The state has vital energy and water facilities, transportation lifelines, shipping ports, and countless residences – all under threat from rising sea levels. One meter of SLR could inflict economic damage of up to $100 billon US dollars to the state of California. However, there has been minimal attention paid to the effects of sea level rise on groundwater, specifically how rising groundwater levels interact with hazards such as earthquake-induced soil liquefaction. Therefore, we attempt to quantify the effects of sea level rise on earthquake-induced liquefaction in coastal California.

We propose to use a probabilistic liquefaction hazard assessment (PLHA) to obtain annual rates of nonexceedance of factor of safety of liquefaction. PLHA is an extension of probabilistic seismic hazard analysis (PSHA). It conceptually accounts for the rate of occurrence of all earthquake events that might shake a site of interest, including ground motion intensities, groundwater conditions at the time of the earthquake, and consequently the likelihood of liquefaction. We propose to integrate the liquefaction uncertainty into the hazard calculation. Existing codes for performing probabilistic liquefaction hazard analysis (PLHA) are decoupled as first a probabilistic seismic hazard analysis (PSHA) is required to obtain a hazard curve. Drawbacks of this method are it only accounts for the influence of magnitude on liquefaction because it uses a small number of magnitudes and selection of the magnitudes requires judgment. The new method integrates the PLHA directly with the PSHA without requiring a deaggregation. The liquefaction hazard is assessed probabilistically for all the rupture scenarios within any fault source model being used. Using traditional stress-based liquefaction evaluation methods, practitioners are unable to determine the return period associated with the computed factor of safety. This new methodology allows us to choose the factor of safety against liquefaction for a desired return period. With currently available computing power, running the probabilistic liquefaction hazard inside the hazard integral can be done efficiently (seconds). There is an added benefit that we can deaggregate the liquefaction hazard, which is not possible when the calculations are performed outside the hazard integral.

The Uniform California Earthquake Rupture Forecast (Field and Cornell, 2003) Fault Source model is used for both the PSHA and PLHA. Preliminary results have been calculated and calibrated for Cardiff State Beach in Southern California. Specifically, preliminary results of a 2-meter of sea level rise scenario using a calibrated groundwater model shows over a 400-year reduction in return period. Initial results suggest that for sites where more events contribute to the hazard the deterministic factor of safety and integrated PLHA factor of safety will differ.  This methodology will be used to analyze the effects of sea level rise on earthquake-induced soil liquefaction in coastal California for multiple sites along the coast. Input data from this analysis will come from the Coastal Geotechnical Database, housed at University of California, Los Angeles (https://www.uclageo.com/coastal_database/). This data was made available by the California Geological Survey. The database contains digitized geotechnical data (standard penetration tests and cone penetration tests) from hospital and school construction locations along the coast of California.