Comment on "Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates?" by Julian J. Bommer and Norman A. Abrahamson

Abstract

In a recent article, “Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates?,” Bommer and Abrahamson (2006) provided an excellent review on probabilistic seismic-hazard analysis (PSHA) and its key issue: how the ground-motion variability is treated. Bommer and Abrahamson (2006) stated that “although several factors may contribute to the higher estimates of seismic hazard in modern studies, the main reason for these increases is that in the earlier studies the ground-motion variability was either completely neglected or treated in a way that artificially reduced its influence on the hazard estimated.” In other words, Bommer and Abrahamson (2006) argued that “the main reason for the increases in the modern estimates of seismic hazard is that the ground-motion variability in early application (and indeed formulations) of PSHA was not treated properly,” and concluded that “the increased hazard estimates resulting from modern probabilistic studies are entirely appropriate.” We argue, however, that ground-motion variability may not be treated correctly in modern PSHA. This incorrect treatment of ground-motion variability perhaps leads to increased hazard estimates, at low annual frequencies of exceedance (10-4 or lower) in particular. As shown by Bommer and Abrahamson (2006), modern PSHA is often referred to as the Cornell–McGuire method (Cornell, 1968, 1971; McGuire, 1976). According to Cornell (1968), Cornell et al. (1971), and McGuire (1976, 2004), modern PSHA is based on the following equation: ![Graphic][1] (1)where ν is the activity rate, f M ( m ) and f R ( r ) are the probability density function (PDF) of earthquake magnitude M and epicentral or focal distance R , respectively, and y mr and σ ln, y are the median and standard deviation at m and r . f M ( m ) and f R … [1]: /embed/inline-graphic-1.gif