Department of Civil and Environmental Engineering, UCLA

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. Our approach integrates the liquefaction uncertainty into the hazard calculation by performing a PLHA for every event in the PSHA. We adopt the UCERF3 source model, which uses over a million events. For each event, ground motion at the site is a random variable that is used to compute cyclic stress ratio (CSR). Cyclic resistance ratio (CRR) is also a random variable obtained from a liquefaction triggering model. Because CSR and CRR are log normally distributed, we can use a closed-form solution to obtain a distribution for factor of safety (FoS). The rate of the event is then multiplied by the cumulative distribution function (CDF) of FoS and the rate-weighted CDF’s are summed over all events to compute the liquefaction hazard curve. Using efficient vectorized operations, the PLHA runs within seconds for a single site despite the large number of calculations. Streamlining the code required developing computationally efficient data storage structures for the UCERF3 model, and a vectorized source-to-site distance calculation. The code is used to compute the PLHA for an example soil profile at Cardiff State Beach in Southern California for current groundwater conditions and for a 1-meter sea level rise scenario.