Modal analysis is a technique by which one estimates the natural frequencies and mode shapes of a structure by measuring the FRF matrix . Experimentally, it is performed using either: (1) Roving hammer excitation and fixed measurements by an accelerometer, (2) Roving accelerometer and fixed excitation by a hammer, (3) shaker excitation, and (4) using ambient vibrations as excitations and employ Operational Modal Analysis (OMMA). 

On the other hand, response spectrum analysis is an effective method commonly used in earthquake engineering to estimate the peak response of a structure by utilizing the ground response spectrum. This method relies on considering the fundamental frequency (or the first frequency) of the structure as the dominant frequency. So, in this method one may need to used modal analysis to estimate the first frequency.