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603284-001 <br />accordance with Chapter 21 to determine the appropriate site-specific seismic design <br />parameters for the project. <br />Probabilistic Seismic Hazard Analysis (PSHA) <br />A PSHA is a mathematical process based on probability and statistics that is used to <br />estimate the mean number of events per year in which the level of some ground <br />parameter, Z (peak ground acceleration and/or spectral acceleration in the investigation), <br />exceeds a specified value z at the project site. This means number of events per year, <br />also referred to as "annual frequency of exceedance," is designated as "v(Zzz)." The <br />inverse of this number is called the "average return period" (ARP), which is expressed in <br />terms of years. Having the annual frequency of exceedance of a certain level of <br />acceleration, v(Zhz), the probability of exceeding that level, Pr(ZZz), within any time period <br />of interest, t, is then obtained assuming a Poisson Distribution as follows: <br />Pr(Z Z z) = 1- e-v(Ziz).t (D-1) <br />This procedure was originally proposed by Cornell (1968), which has been significantly <br />improved during the recent years and is described in more details by National Research <br />Council (1988) and Earthquake Engineering Research Institute (1989). <br />PSHA procedures require the specification of probability functions to describe the <br />uncertainty in both the time and location of future earthquake occurrences and the <br />uncertainty in the ground motion level that will be produced at the site. The basic key <br />elements of a PSHA are: <br />. Defining the location, geometry, and characteristics of earthquake sources relative to <br />the site; <br />Specifying an earthquake recurrence relationship for various magnitudes on each <br />source up to the maximum magnitude; <br />. Selecting appropriate attenuation relationships, which relate the variation of the <br />earthquake ground motion parameter with earthquake distance, directivity, <br />magnitude, site geology, and subsurface characterization; and <br />. Determining the probability of exceedance of peak ground accelerations and/or <br />response spectral levels (i.e., seismic hazards) utilizing the above input parameters. <br />D-2