Seismic Ground Motion - Dynamic Behavior of Concrete and Seismic Engineering - page 105

Figure 3.1. Generic shapes of acceleration Fourier spectrum, for increasing
magnitudes ranging from 3 to 7 (these curves were established on the
basis of scale laws specified in section 3.4.1)
Beyond f max , the acceleration Fourier spectrum modulus decreases in f D , but
with an exponent D that can vary according to the site and the event.
As shown in Figure 3.2, the acceleration response spectrum, represented as a
function of the period, has a characteristic shape, with an ordinate equal to the PGA
at the origin. It has a plateau between two periods depending on the site and the size
of the event and a level typically between 2 and 3 PGA, followed by a branch
decreasing first as 1/T, and then as 1/T².
Figure 3.2. Example of a real response spectrum for a 5.5 magnitude earthquake
(San Salvador 1986), and generic shape of the pseudo-acceleration response spectrum,
as defined in most para-seismic regulations (EC8 in this case)