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<br />IogA = -1.02 + 0.249M - logr -0.00255r + 0.26P <br />where r = (d1 + 7.3z)~~ and 5.0 < M < 7.7 <br />A =peak horizontal acceleration in g's <br />M =magnitude <br />d =closest approach to the site <br />P = 0 for 50th percentile or P = 1 for 84th percentile <br />Because attenuation relationships aze based on observational data from <br />specific earthquakes, considerable variation can exist in accelerations <br />due to site dependent factors. The 50th percentile represents the <br />median value (P = 0) and was used in this study. In our judgment, the <br />84th percentile is strongly influenced by site conditions and is overly <br />conservative. <br />7.3 PEAK AND EFFECTIVE ACCELERATIONS <br />SEISRISK III calculates peak (horizontal) ground acceleration (PGA) <br />at grid points for various probabilities of non-exceedance (or risk <br />levels) selected by the analyst. In general, the project site lies within a <br />specified grid squaze; and thus, PGAs at each of the grid corners <br />defines the seismic hazard at the site lying within the grid. PGAs may <br />vary slightly at each of the grid corners due to varying distances from <br />the seismic source zones. These variations aze usually minor if the <br />grid size is suitably small. The program output (Appendix fI) is in the <br />form'of PGA with a 0.90 or 90% probability of non-exceedance in a <br />specified number of yeazs. The output may be presented in tabular <br />form as in Table III below or graphically in a seismic hazazd curve as <br />illustrated in Figure 8. Graphical presentation is especially useful <br />because PGAs for various probabilities of non-exceedance can be <br />derived from the seismic hazazd curve. <br />The probability of non-exceedance in a specific period of yeazs can be <br />related to annual probability of exceedance or return period (recurrence <br />interval) by the following formula (Reiter, 1990): <br />Return Period = -T <br />ln(] - P(Z?z)) <br />33 <br />