Ground motion modification (or spectral matching) has been criticized, but has many appealing characteristics and is widely used in practice. Modification of ground motions can be performed in either the time domain or the frequency domain. Depending on the choice of modification technique, modified ground motions can be significantly different from each other as well as from the original ground motion. This paper studies the impact of these differences on seismic geotechnical analyses for two different site profiles using two earthquake scenarios and a total of 20 ground motions. This study shows that the final results are influenced by many factors such as the original (seed) ground motion, the target spectrum, and the local site conditions, in addition to the ground motion modification technique used. The results also show that while both techniques can significantly modify the original ground motion, neither technique is consistently more conservative than the other. Therefore, a general conclusion that a particular technique results in ground motions that yield the largest intensity parameters cannot be made a priori.

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