Even your source is harmonic, vibration propagates in soil by a number of wave types or "modes" which are often called Rayleigh waves of different order and Love waves. They are rather parallel to the surface, however in a layered soil structure with a stiff soil as substructure and in addition having a slope, the things are to a PhD thesis without a software. I have no idea about such software, by the other hand. I mean I'm not optimistic you'll solve by calculation.


About your question: in my opinion, your spectrum should be amplitude spectrum and/or velocity spectrum rather than acceleration spectrum, anyway. How to obtain them and how use such spectra in Caesar, this is another question...