Mechanism of the Generation of Abnormally High Accelerations >1g on Soft Soils during Earthquakes

Physical mechanisms that contribute to the generation of abnormally high accelerations >1g in earthquakes at soft soil sites, where nonlinear behavior reducing accelerations is considered typical, are discussed. The models constructed of soil behavior (vertical distributions of stress and strain in soil layers) have shown that the nonlinearity of the soil response at the stations where the recorded accelerations exceeded 1g was usually insignificant, because of the rather low motion imposed on the soil layers: PGA ~ 120–270 cm/s². The generation of abnormally high accelerations at sites with soft subsurface soils was obviously related to (1) the response of subsurface soils that effectively amplified strong motions due to the character of the stress–strain relations; (2) resonant phenomena in soft subsurface layers; and (3) the effects of directivity (during strong earthquakes with extended sources), i.e., constructive interference of seismic waves in the directions of crack propagation along the fault planes.