DETERMINISTIC VERSUS PROBABILISTIC SEISMIC HAZARD ANALYSIS FOR CRITICAL STRUCTURES

Probabilistic seismic hazard analysis has been practically unchallenge d since its inception three decades ago. However, information has been accumulating which shows convincingly that PSHA is a defective proced ure. Its greatest weakness is the dependence of the probability theory on the Gutenberg-Richter magnitude and a recurrence relation which ca n no longer be regarded as a power law. Remedies that rely on incorpor ating paleoseismic information and characteristic earthquakes into the probability calculation introduce other errors resulting from fragmen tary data and the known non-uniformity of earthquake occurrence in spa ce and time. The worst corrective for probability is the method develo ped by the Electric Power Research Institute and the Lawrence Livermor e National Laboratory that averages multiple expert opinions. Expert o pinions cannot be averaged meaningfully because the criteria for diffe rent models are nonequivalent. On the other hand, the deterministic pr ocedure for earthquake hazard evaluation avoids the above defects by e liminating the falsely precise time element in the probabilistic estim ation. Geologic time for recurrence is used, according to accepted cri teria such as a single movement in the past 12,000 years or multiple m ovements in 500,000 years. For a critical project, where the consequen ces of failure are intolerable and protection is needed against the wo rst that can be reasonably expected to occur (the maximum credible ear thquake), the deterministic method is strongly recommended.