Selection of Design Displacements Using Probabilistic Fault Displacement Hazard Analysis

ABSTRACT For structures that cross active faults, the seismic loading includes both the vibratory ground motion and the fault displacement. Design ground-motion values are commonly based on probabilistic seismic hazard analysis (PSHA), whereas design fault displacement values have often been estimated using deterministic seismic hazard analysis. To treat ground motion and fault displacement consistently, there has been a growing demand for probabilistic fault-displacement hazard analysis (PFDHA) for selecting design fault displacements; however, choosing the fault displacement from a PFDHA based on the same return period as used for the ground motion may not lead to consistent levels of risk for moderate-activity faults (slip rate between 0.2 and 2 mm/yr) if the hazard curve has a flat slope at the selected return period. In this case, the epistemic uncertainty in the slip rate on the fault will lead to a very large range for the fault displacement at the selected return period. For moderate activity rates, alternative approaches for selecting a design fault displacement are described: risk-targeted displacement approach, high-fractile hazard approach, and median displacement given surface rupture at the site approach. If it is not practical to design for the fault displacement, an alternative approach is described in which an emergency action plan is developed to respond to the expected damage from fault displacement rather than design the structure to withstand the fault displacement.