Understanding Seismic Hazard and Risk Assessments: an Example in the New Madrid Seismic Zone of the Central United States

Seismic hazard and risk are fundamentally different concepts. Seismic hazard describes phenomena generated by earthquakes that have potential to cause harm, but seismic risk is the likelihood (chance) of experiencing a specified level of seismic hazard in a given time exposure. Seismic hazard occurs naturally and can be evaluated from instrumental, historical, and geological observations. Seismic risk depends not only on the hazard and exposure, but also on models (i.e., time-independent [Poisson] and time-dependent ones) used to describe the occurrence of earthquakes. High seismic hazard does not necessarily mean high seismic risk, and vice versa. Probabilistic seismic hazard analysis (PSHA) is a commonly used method to derive seismic hazard curve – a relationship between a ground motion parameter and its return period. The so-called return period in PSHA is a modification of the recurrence intervals of earthquakes using the probabilities of ground motions. The return period is not an independent temporal parameter, but it has been inappropriately treated as the mean recurrence interval of an independent event (ground motion) and used in seismic risk analysis. In the New Madrid Seismic Zone of the central United States, the mean recurrence interval of large earthquakes (~M7.5) is about 500 years, and the risk posed by such events or their ground motions (consequences) is about 10 percent probability of exceedance (PE) in 50 years. However, PSHA could predict ground motions with a range of return periods, up to 10 to 10 years, for the same earthquakes. In other words, use of PSHA could derive a range of risk estimates for a single earthquake. Thus, the use of PSHA for seismic risk analysis is not appropriate and confusing. An alternative method, seismic hazard assessment (SHA), is presented in this paper. SHA is comparable to flood and wind hazard analyses and can be used for risk analysis in a similar way. 1 Seismologist and civil engineer, Kentucky Geological Survey, 228 Mining and Mineral Resources Building, University of Kentucky, Lexington, Kentucky 40506, [email protected] Paper No. 416