For a Fourier Spectra (Fourier amplitude spectrum), its Y-axis is the velocity unit which related to the energy of a earthquake. From a fourier spectra, you can find during which period range the most energy are distributed.
With respect to the response spectra, its Y-axis is the accleration, which related to the force a building will take. we only use response spectra to design the buildings.
A Fourier spectrum is the decomposition of a wave into weighted and phase-shifted sine waves. The amplitudes in the frequency domain are not directly related to amplitudes in the time domain. An inverse Fourier spectrum takes you back into the time domain. Fourier analysis is not structure-dependent, and deals ONLY with the properties of the wave itself.
A response spectrum is the response of a single-degree-of-freedom to a given input waveform. A damping (typically 5%, though this values is not a requirement) of the SDOF system is needed for realistic evaluation. The response spectrum is calculated by determining the maximum relative displacement of the single-degree-of-freedom system for each frequency calculated, yielding a spectrum. This is commonly related to an equivalent sine wave for each frequency that would produce the observed response. The velocity or acceleration of the equivalent sinewave is termed the pseudo-velocity or pseudo-acceleration. A response spectrum can NOT be inverted to get a waveform back -- a response spectrum is non-unique, i.e., many waveforms could generate the same response spectrum.
