Future design procedures for civil structures, especially those to be protected from extreme and blast related
loads, will need to account for temporal evolution of their frequency content. There are, however, several instances
where acceleration time histories are required as seismic input. For example, to determine the ultimate resistance and
to identify modes of structures’ failure, a nonlinear time history analysis is needed. In other cases, acceleration time
histories are required for linear analyses. Many seismic codes require this type of analysis for buildings which have
irregularities. The process of time-frequency analysis made possible by the wavelet transform provides insight into the
character of transient signals through time-frequency maps of the time variant spectral decomposition that traditional
approaches miss. In this paper an approach is examined and a new method for processing the ground motion which is
modeled as a non-stationary process (both in amplitude and frequency), is proposed. This method uses the best basis
search algorithm with wavelet packets. In this approach, the signal is expressed as a linear combination of timefrequency
atoms which are obtained by dilations of the analyzing functions, and are organized into dictionaries as
wavelet packets. Several numerical examples are given to verify the developed models.