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How can we calculate the distributed mass of structure for SDOF?

Hello all, This might be a minor question to ask .But then I have some queries on the design spectrum plot as per Chopra."Dynamics of Structure"(3rd edition), response spectrum chapter 6.After the plot is made by amplifying given peak acceleration,velocity and displacement.The same is plotted in the tripartite graph ,from where the values corresponding to T=1/33 sec,1/8 sec ,10 sec and 33 sec.why are those coordinate points and the value corresponding to it are noted for obtaining pseudo acceleration or displacement from the relation given ,ie A=omegaVelocity,V=omegaDisplacement?I know how to plot but I am not getting any physical idea about the science of plotting at these points.After this,for each intervals and in place where acceleration governs ,velocity governs and time governs,an equation ,Acceleration(A)=xT^y(where,time is time of consideration of the occurence of acceleration 'A') is used ..Please kindly give me thoughtful insights on why this equation would work to determine the variance of acceleration with the time period of actual structure.