Average and standard deviation of group delay time in frequency domain mean the centroid and the duration of wave series in time domain. We show fundamental study for envelope characteristics of long period strong motions by using this relation. In this study we use JMA 87 type records at 26 sites during 1990 Izu-Oshima Kinkai earthquake in a period range of 1 to 15 sec. First we show proper ways to calculate group delay time and its average and standard deviation. Then it is confirmed that the dispersion characteristics of observed surface waves are simulated well by average and standard deviation of group delay time calculated in a narrow frequency band. From these results it is found that the average and the standard deviation of group delay time can be used to represent envelope characteristics of strong motions if they are calculated properly as described in this paper. By using observed records after correction the start time of recording to origin time we found that epicentral distance is one of the influential parameters to both the average and the standard deviation of group delay time and that the relation between them can be physically interpreted.
