In structural dynamic mode analysis, one would have to carry out modal analysis in order to get different modes that contribute to the response of the seismic ( or wind) force. Here is a nice summary.
In a large structural model, there are so many modes, and we can't take them all. So we can only take the first few prominent modes ( with low frequencies), assuming that the modes will grow less and less important as the frequency becomes higher and higher.
Then the issue arises: how to judge the relative importance of different modes ( whatever the term 'importance' may mean?) I find that the often-cited mode participation factor
$\Gamma=\frac{\sum M \phi}{\sum\phi^T M \phi}$
doesn't work, because it depends on how we normalize the eigenvector, as one can readily see from the above formula.
Is there any other factor that allows us to judge whether a mode is more important or not?