Site amplification is the phenomenon by which local soil conditions increase the amplitude of ground shaking compared to a reference rock site. Soft soils amplify low-frequency motions, while stiff soils and rock amplify high-frequency motions, and this effect is a primary reason why damage patterns vary dramatically across short distances during earthquakes.
Site amplification refers to the modification of ground motion by local soil and geological conditions as seismic waves travel from bedrock to the ground surface. When seismic waves pass from stiffer bedrock into softer overlying soil, their amplitude increases due to the conservation of energy and the impedance contrast between layers. The degree of amplification depends on the shear wave velocity profile, the thickness of soil layers, the impedance contrast, and the frequency content of the incoming motion.
The phenomenon has been recognized since the early days of earthquake engineering, but it was dramatically demonstrated in the 1985 Mexico City earthquake, where soft lakebed deposits amplified motions by a factor of 5 to 20 at periods around 2 seconds, causing catastrophic damage to mid-rise buildings while nearby rock sites experienced only moderate shaking. Similar effects were observed in the 1989 Loma Prieta earthquake (soft soils in San Francisco and Oakland) and the 1995 Kobe earthquake. These observations drove the development of modern site classification systems and site coefficients in building codes.
Modern building codes account for site amplification through site classes (A through F in ASCE 7, I through IV in Eurocode 8, and 1 through 4 in Standard 2800) and site coefficients (F_a, F_v) that modify the design spectrum based on the soil profile. Site amplification can be estimated through 1D equivalent-linear site response analysis (SHAKE-type programs), 2D or 3D numerical modeling, or empirical methods using Vs30 and other site parameters. For sites with unusual conditions β deep soft deposits, steep topography, or strong impedance contrasts β a site-specific analysis is generally required.