The current earthquake design spectra are based mainly on response spectra from recording stations located on alluvium deposits. A limited number of studies have shown that shape and the magnitude of response spectra for stations located on rock deposits are different from those located on alluvium deposits.
This study examines the effects of geological conditions on the response spectra and the ground-motion parameters such as peak ground acceleration, velocity, and displacement. Design spectra are presented for various sites such as alluvium deposits, rock deposits, and alluvium layers underlain by rock deposits. The study shows that the current design spectra are too conservative for most structures located on competent rock deposits.