I meant your actual CAESAR II input data. These spectra, alone, do not indicate the system response. Specific accelerations are selected for each system natural frequency. There could be a (slim) chance that these spectra are identical in magnitude for the system's natural frequencies.

A more plausible answer is there is an input error.

If you do not want to post your model here, please send your (dot C 2) model to caesarii@intergraph.com.