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48 Figure 5-14. Comparison between WUS-Rock and CEUS-Rock correlations for PGV = 30 kmax. noted that the correlations for other regions (that is, CEUS- 5.2.9 Confidence Level Soil, WUS-Rock, and WUS-Soil) result in relatively similar The displacement correlations discussed in previous sec- displacement levels slightly greater than the Martin-Qiu tions were based on a mean regression curve on the observed correlation. data. For design purposes a higher confidence level than the Consequently correlations were combined for these data mean curve (the mean curve corresponds to 50 percent con- leading to a mean displacement correlation given by: fidence level) is often selected. A common practice is to use All data except CEUS-Rock: the mean curve plus one standard deviation, which approxi- log ( d ) = -1.51 - 0.74 log ( k y kmax ) + 3.27 log (1 - k y kmax ) mately corresponds to a confidence level of 84 percent. Fig- ures 5-18 and 5-19 show the 84 percent confidence intervals -0.80 log ( kmax ) + 1.59 log ( PGV ) (5-8) for permanent displacement based on site-adjusted peak with a standard error of 0.23 log10 units. ground acceleration coefficient of 0.3 and PGV = 30 kmax and Figure 5-15. Comparison between WUS-Rock and CEUS-Rock correlations for PGV = 60 kmax.