Comment on "How can seismic hazard around the New Madrid seismic zone be similar to that in California?" by Arthur Frankel

A recent paper in the September/October 2004 issue of Seismological Research Letters, “How Can Seismic Hazard around the New Madrid Seismic Zone Be Similar to That in California?”, by Arthur Frankel, concludes “I have demonstrated how probabilistic seismic hazard for New Madrid can be greater than that at San Francisco at low probabilities for PGA and similar at low probabilities for 5 Hz S.A. By low probabilities, I mean annual probabilities less than the reciprocal of the return time of the New Madrid characteristic source, that is, 1/500. This is a consequence of the higher ground motions, for PGA and 5 Hz S.A. (and other high-frequency measures), expected for large New Madrid characteristic earthquakes compared to San Andreas earthquakes with similar magnitudes.” His conclusion is confusing and not necessarily true. The fundamental differences in assessing seismic hazard between San Francisco and the New Madrid seismic zone are the uncertainties, especially the uncertainties associated with the ground-motion attenuation relationship. The larger ground-motion uncertainties in both the median and its standard deviation derive a higher hazard in the New Madrid area. The larger standard deviation, in particular, derives a higher hazard at low annual probabilities of exceedance. Moreover, Frankel also equated the hazard (annual probability of exceedance) defined in probabilistic seismic hazard analysis (PSHA) with the average recurrence rate defined in engineering risk analysis. There is a fundamental difference in defining risk between PSHA and the engineering risk analysis. The risks—10%, 5%, and 2% probability of exceedance (PE) in 50 years—defined in PSHA are not interchangeable with those defined in engineering risk analysis. Hence, though Frankel's stated intention was to improve understanding of PSHA among nonpractitioners, the paper may cause more confusion and problems for users of the U.S. Geological Survey national seismic hazard maps.