How to combine deterministic and probabilistic methods for assessing earthquake hazards

Abstract

Deterministic seismic hazard analysis (DSHA) and probabilistic seismic hazard analysis (PSHA) are incompatible methods. DSHA is based on geology and is attuned to physical reality in nature; PSHA is based on earthquake statistics and theory-guided numerical calculations. PSHA is less reliable than DSHA because PSHA is full of grave uncertainties that are created by the method itself; DSHA is more reliable because it deals principally with observed geological facts and is logical and transparent. DSHA and PSHA can be combined to advantage if one accepts that they are not equally valid. To combine them, one must recognize the weaknesses in PSHA and not use PSHA for design of critical structures. Only DSHA is suitable for that purpose. However, PSHA can be used for (1) preliminary evaluations, (2) for an operating basis earthquake (OBE), (3) for risk analysis when unrelated to design decisions for a critical project, and (4) for design of non-critical construction. Probabilistic methods that should never be used are (1) multiple expert opinion, (2) logic trees, and (3) deaggregation. No unrepentant seismic probabilist will agree to this diminishment of PSHA. Consequently, DSHA and PSHA will remain irreconcilable. The solution is for policymakers in regulatory agencies and owners of major engineering projects to determine which of the methods to use or in what combination. They need to (1) engage open-minded advisors, (2) ask hard questions, and (3) choose wisely. Meanwhile, Krinitzsky [Eng. Geol. 65 (2002) 1] provides procedures to obtain earthquake ground motions for engineering design that combine DSHA and PSHA according to the above criteria.