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<br />where (P(Z>z) is the desired probability of non-exceedance <br />during the time, T <br />Peak ground accelerations typically occur at high frequencies and aze <br />of short duration. High frequency, short duration ground motions <br />generally have little engineering significance. As a result, the concept <br />of effective acceleration has been employed to provide more meaning- <br />ful ground motion values in engineering analyses. In general, effective <br />accelerations aze smaller than peak values and more representative of <br />longer duration, lower frequency ground motions than commonly cited <br />PGAs. In our experience, effective accelerations should average about <br />85% of peak values. The use of peak or effective accelerations is a <br />matter of engineering judgment. We present peak and effective accel- <br />eration values derived from SEISRISK III in Table III and on Figure 8. <br />Ground motions aze tabulated for the grid comer at 106.084° W. <br />Longitude, 38.806° N. Latitude, which represents the highest values <br />for the grid surrounding the site. Complete program output for the site <br />is presented in Appendix II. <br />TABLE III <br />PEAK AND EFFECTIVE ACCELERATIONS <br />TROUT CREEK DAM SITE <br />Grid Coordinate 106.084° W. Long., 38.806° N. Lat. <br />90% PROBABII.,ITY OF <br />NON-EXCEEDANCE IN <br />YEARS PEAK GROUND <br />ACCELERATION <br />(g) EFFECTIVE GROUND <br />ACCELERATION, g (85% <br />of PGA) RETURN <br />PERIOD IN <br />YEARS <br />50 0.037 0.031 475 <br />100 0.052 0.044 949 <br />250 0.079 0.067 2372 <br />1000 0.143 0.122 9491 <br />10,000 0.344 0.292 94912 <br />99,999 (100,000) 0.668 0.568 949112 <br />34 <br />