Influence of Soil Conditions on Ground Motions During Earthquakes

Data are presented to show that in many cases the characteristics of ground surface motions during earthquakes, and the corresponding forms of the response spectra, vary with soil conditions in a reasonably predictable way. A method of analysis, based on a knowledge of the shear moduli and damping factors of soil strata is presented. It is shown that appropriate values of both the shear modulus and the damping factor of a soil are strain dependent. Thus behavior observed during small earthquakes, inducing small strains, is not necessarily indicative of the behavior of the same soil deposit during a large earthquake with the associated large strains. Furthermore in any analysis it is important to ensure compatibility of the soil characteristics with the magnitude of the induced strains. It is also shown that the characteristics of the ground motions at the surface of a soil deposit are greatly influenced by the magnitude and epicentral distance of the earthquake inducing them. It is concluded that for many sites, appropriate analyses can provide a useful guide in evaluating the characteristics of the surface motions likely to be induced by earthquakes.