In general, the amplitude of ground motions decreases with the distance from the source as the results of a geometrical energy distribution and material damping. Due to the material damping of soil the high frequency component of propagating waves also decreases with an increasing distance. Far-source earthquakes therefore have generally smaller amplitude and low frequencies while near-source earthquakes have larger amplitude and have also high frequency contents. Local soil conditions may also have a substantial influence on the characteristic of ground motions. Indeed, the response spectra of ground motions recorded on soft soil deposits exhibit a substantial amplification at frequencies corresponding to the resonant frequencies of the soil structure below the station, whereas the spectra of those on firm ground are broadly banded. In addition, inclined propagating waves and soil layering may produce significant amplification and long duration of the surface ground motion. In order to investigate the response of structures in regions with expected near-source earthquakes it is therefore importance to consider the effect of the distance from the source and the soil condition.
In this work, the effect of the distance and soil on vibrations of soil surface due to incident earthquake waves is addressed. These ground vibrations are then used as an input motion for the investigation of the behavior of above ground structures in near field. The response of soil surface is obtained by using a coupled finite element and boundary element method (FE-BEM). The bounded near surface part of the soil that may be soft and non homogenous soil is modeled by FEM. The unbounded far part is considered to be elastic homogenous medium and is described by BEM. The compatibility and equilibrium conditions are satisfied along the interface boundary. Ricker wavelets and earthquake records are considered for P- and SV-wave incidence in order to obtain the overall response characteristics and in order to investigate the response under real seismic excitation conditions, respectively. The considered above ground structure is a steel frame structure which is described by FEM.
The investigation confirms that the amplitude and frequency content of ground motions are strongly influenced by the distance of the considered location from the source and the soil condition as well
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