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

Department of Civil and Environmental Engineering, Imperial College London, South Kensington campus, London SW7 2AZ, United Kingdom j.bommer@imperial.ac.uk

Norman A. Abrahamson

Geosciences Department, Pacific Gas and Electricity Co., San Francisco, CA 94177

The first 300 words of the full text of this article appear below.

The lengthy submission by Klügel (2007b) extends considerably beyond a discussion of our review article (Bommer and Abrahamson, 2006), and we resist the temptation to respond to all the points raised. The author has already challenged the validity of the results of the PEGASOS project (Klügel, 2005) and more recently has presented his general arguments against probabilistic seismic-hazard analysis (PSHA) and for a deterministic approach (Klügel, 2007a). Very little of this is directly relevant to the article on which the author has commented, and we limit ourselves to responding on the key issues of the nature of ground-motion variability and how it should be incorporated into PSHA.

Regarding Klügel’s "Refutation of the Assumptions of Bommer and Abrahamson," the discussion does not address the assumptions that we made. The only assumptions we made are that the ground-motion residuals follow a lognormal distribution and that the ground-motion model fits the magnitude and distance dependence of the data from which it is was derived. In ground-motion studies the assumption of a lognormal distribution is nearly universal. The results of ground-motion studies are typically shown in ground-motion units (g) rather than log(g). For a lognormal distribution, the mean value in log units corresponds to the median value in units of g. Therefore, it is common practice for ground-motion studies to call the expected ground motion, in units of g, the median ground motion. Klügel (2007b) seems to be unaware of this common practice, and so he says that we have used the term median when we should have used the term mean.

In passing, we do acknowledge that, although it was not a typographical error because the complete sentence is correct, we should have referred to as the normalized residual in introducing . . . [Full Text of this Article]

Related articles in Bulletin of the Seismological Society of America:

Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates?

Julian J. Bommer and Norman A. Abrahamson
Bulletin of the Seismological Society of America 2006 96: 1967-1977. [Abstract] [Full Text]  

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

Jens-Uwe Klügel
Bulletin of the Seismological Society of America 2007 97: 2198-2207. [Abstract] [Full Text]