Significant durations D_5-95 and D_5-75, which are the most frequently used duration parameters of a ground motion, are defined as 5-95% and 5-75% time intervals of the Husid plot, i.e., the time history of cumulative power of ground motion normalized to the total power. Therefore, the amplitude level of significant duration is not explicitly represented. In this study, the equivalent amplitude level of significant duration is defined so as to be compared with the maximum amplitude level. For this purpose, TD (Threshold-Duration) curve representing the uniform duration or bracketed duration for arbitrary threshold is calculated. Equivalent threshold is determined so that the significant duration becomes equal to uniform or bracketed duration. Strong motions records observed in major 35 earthquakes with M_w=5.0-9.0 during 1996-2013 in Japan are used for numerical analysis. On the average, D_5-95 and D_5-75 are found to correspond to approximately 17% and 35% of the maximum acceleration level, respectively.

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