Free Field Surface Motion at Different Site Types due to Near-Fault Ground Motions

Seismic hazards during many disastrous earthquakes are observed to be aggravating at the sites with the soft soil deposits due to amplification of ground motion. The characteristics of strong ground motion, the site category, depth of the soil column, type of rock strata, and the dynamic soil properties at a particular site significantly influence the free field motion during an earthquake. In this paper, free field surface motion is evaluated via seismic site response analysis that involves the propagation of earthquake ground motions from the bedrock through the overlying soil layers to the ground surface. These analyses are carried out for multiple near-fault seismic ground motions at 142 locations in Mumbai city categorized into different site classes. The free field surface motion is quantified in terms of amplification ratio, spectral relative velocity, and spectral acceleration. Seismic site coefficients at different time periods are also evaluated for each site category due to near-fault ground motions from the acceleration response spectra of free field surface motion at each site and the corresponding acceleration response spectra at a reference rock outcrop site. 1. Introduction Seismic response of a structure is dependent upon the nature of supporting soil. Severe structural damages to houses and manmade structures during many past earthquakes are observed to be concentrated in an area where the ground consisted of local alluvial deposits. Local soil deposits are found to have paramount influence on the characteristics of earthquake ground shaking and have played a major role in the damage and loss of life during many disastrous earthquakes such as the 1976 Tangshang, 1985 Mexico, 1989 Loma Prieta, 1994 Northridge, 1995 Kobe earthquakes, 2001 Bhuj earthquake, and 2005 Kashmir earthquake. The profound importance of the nature of the subsoil on the structural response of different types of structures has also been confirmed through several theoretical and experimental studies. The motion at the base of a structure founded on rock is identical to that occurring at the same point before the structure is built, but they are quite different if the structure is founded on soil. The motion that occurs in the soil or rock layers at some depth from the ground surface in the absence of any structure or excavation is defined as free-field motion. The motion at the base of a structure and the free field motion that would occur at the same point in the absence of the structure are different. The study of wave propagation in horizontal layered media is an