Forward and Inverse Ground Response Analysis: An Introduction and Need

Devastating effects of earthquakes (EQs) are usually witnessed in terms of excessive ground shaking, liquefaction, ground subsidence, etc. Such effects are primarily the result of system’s response to surface ground motion at the site of interest. Bedrock motions during an EQ in general are amplified by the local soil when reaching the surface. Depending upon the subsoil characteristics and bedrock motion properties, no to high amplification can be witnessed at a site during different EQs. Forward ground response analysis (FGRA) is numerical approach to quantify how local soil is responsible for amplifying the ground motion at the site of interest. Numerical methods of FGRA are functions of input bedrock motions and dynamic properties of subsoil. Globally, it has been observed that though numerous studies exist, attempting GRA of local soil during specific ground motion scenario in majority of such studies are usually restricted to dynamic soil properties obtained from standard curves and not region/ site specific curves. In such a case, whether the outcomes should be considered as region-specific findings, is a matter of debate. Inverse ground response analysis (IGRA) methodology, uses ground motions recorded at various depths, during various EQs to find dynamic soil properties in in-situ medium. In this paper, a detailed discussion in terms of the role of local soil during earthquake, as experienced globally during different EQs along with governing framework for FGRA and IGRA is discussed. Further the influence of input motion amplitude, frequency content and duration on local site effect are discussed. Additionally, the need for IGRA outlining the nuances of approximating dynamic soil properties is provided with appropriate case studies.