For aseismic design of structures, the ground motion data is assumed either in the form of an accelerogram or indirectly in the form of response spectra. The usual measures of response are maximum relative displacement whicl1 is a measure of the strain in the spring element of the system, maximum relative velocity which is a measure of the energy absorption in the spring of the system and maximum acceleration which is a measure of the maximum force in the spring system. The response spectra approach is generally used for design of ordinary structures and also for preliminary analysis of t important ones where a number of trials may be required to arrive at the final result. Engineers associat~d with the problems of earthquake resistant design have been concerned about the vital problem of choosing suitable response spectra for a particular site. This is a difficult problem because of the difficult nature of parameters involved in it.