Site response analysis is the calculation of how soil deposits modify earthquake ground motion as it propagates from bedrock to the ground surface. It is used to develop site-specific design spectra and to evaluate the effects of local soil conditions on ground shaking.
Site response analysis is the analytical or numerical procedure used to compute the modification of earthquake ground motion by the soil column between bedrock and the ground surface. The fundamental physical process is well understood: seismic waves propagating upward through soil deposits are amplified or attenuated depending on the impedance contrast between layers, the shear wave velocity profile, the thickness and damping of the soil, and the frequency content of the input motion. In a soft soil deposit over bedrock, the impedance contrast typically amplifies motions at periods near the site's fundamental period (Tβ = 4H/Vs for a uniform layer), producing a characteristic peak in the surface response spectrum.
Site response analysis is typically performed using one of three approaches. Linear analysis assumes that soil stiffness and damping are constant, appropriate for small-strain conditions. Equivalent-linear analysis (implemented in programs such as SHAKE, SHAKE91, and DEEPSOIL) iteratively adjusts the soil's shear modulus and damping to be consistent with the effective strain level induced by the motion β a widely used and computationally efficient approach suitable for moderate shaking. Nonlinear analysis (implemented in programs such as DEEPSOIL, OpenSees, and FLAC) uses advanced constitutive models that capture the actual stress-strain behavior of soil, including pore pressure generation and liquefaction. Nonlinear analysis is required for strong shaking, for sites with liquefiable soils, and for cases where the soil response is highly nonlinear. One-dimensional (1D) analysis is the standard for horizontally layered sites; two- and three-dimensional (2D, 3D) analyses are required for sites with irregular geometry (e.g., valleys, ridges, or near-surface discontinuities).
Site response analysis is required by modern codes for important structures on sites with soft soil or unusual conditions. For ordinary structures, code-based site factors (F_a, F_v in ASCE 7; or equivalent in Eurocode 8 and Standard 2800) provide approximate amplification factors based on site class. For critical facilities (hospitals, nuclear power plants, tall buildings, major bridges), a site-specific response analysis is required, using site-specific velocity profiles, measured or estimated damping characteristics, and ground motions selected to represent the design hazard. The output of the analysis is a site-specific response spectrum that serves as the basis for structural design. Challenges in site response analysis include uncertainty in the input motion (which ground motions to select, and how to scale them), uncertainty in the soil profile (Vs measurements, depth to bedrock), uncertainty in the constitutive model (which model to use, and how to calibrate it), and uncertainty in the nonlinear behavior (especially for liquefiable soils). Modern practice uses suites of ground motions and multiple soil profiles to characterize the uncertainty, and increasingly uses probabilistic methods to propagate uncertainty through the analysis.