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Problems with Probabilistic Seismic Hazard Analysis (PSHA)

Although probabilistic seismic hazard analysis (PSHA) is the most widely used method for seismic hazard assessment, it has been found that PSHA is not consistent with modern earthquake science (see a. Seismic Hazard Assessment: Issues and Alternatives; and b. Comment on “Why Do Modern Probabilistic Seismic-Hazard Analyses Often Lead to Increased Hazard Estimates?” by Julian J. Bommer and Norman A. Abrahamson; Comment on “Sigma: Issues, Insights, and Challenges” by F. O. Strasser, N. A. Abrahamson, and J. J. Bommer). PSHA was developed in the early 1970’s. An earthquake was generally considered a point source in the early 1970’s. Today, however, an earthquake is considered a complex finite source.  For example, the great Sumatra earthquake of December 26, 2004 had a rupture length of about 1,200 km with a width of about 200 km. The different earthquake source models (point vs. finite fault) will result in different statistical models, the ground motion attenuation relationship in particular. This can be seen in the following figure. For a single point source, earthquake could occur at any point on the fault and a probability function was introduced to account for this in PSHA (i.e., hypocentral distance varies over the fault). However, for a finite fault, whole fault will break during an earthquake and the probability function is not needed (i.e., only one single fault distance).  Therefore, the basic mathematical formulation of PSHA is not valid. The use of PSHA has caused so many problems (see a. Communicating with Uncertainty: A Critical Issue with Probabilistic Seismic Hazard Analysis; b. Comparison Between Probabilistic Seismic Hazard Analysis and Flood Frequency Analysis; c. Response to Comment by T.L. Holzer; d. Comment on J.U. Klügel’s: Problems in the Application of the SSHAC Probability Method for Assessing Earthquake Hazards at Swiss Nuclear Power Plants; e. Summary of USGS-KGS Meeting on Seismic Hazard Assessment in Western Kentucky; f. and Comment on “How Can Seismic Hazard around the New Madrid Seismic Zone Be Similar to that in California?” by Arthur Frankel).

Problems with Probabilistic Seismic Hazard Analysis (PSHA)

In a simple term, what is wrong with PSHA is that it mixes temporal and spatial measures of earthquakes and their consequences (i.e., ground motions) at a site. Temporal and spatial measures of earthquakes are two fundamental and intrinsic parameters and should not be mixed one way or the others. This is similar to time and location that are two critical elements in our every day activities. The world will be in chaos if the time and location are mixed up one way or the others.

Schematic geometry of an earthquake fault. Hypocenter – where the fault rupture starts, Epicenter – the surface projection of hypocenter. The whole fault will break and the ground motion at a site will be resulted from a dynamic process during an earthquake.