Seismic site response for surface structures

The presence of loose materials in subsurface geological layers modifies seismic movement. This modification is essentially a function of the nature of the materials making up the surface formations, their geometry and the amplitude of the seismic movement.

Evaluating the response of surface formations to seismic stress is fundamental to earthquake engineering, in order to determine the characteristics of seismic movement at the surface.

Knowledge of surface seismic motion is required for a number of practical applications, such as :

• dimensioning foundations under seismic loads ;

• assessment of the potential risks of liquefaction under the water table and induced settlement outside the water table;

• and structural justification.

Surface seismic motion is also required as input data for soil-structure interaction analyses.

The aim of this article is to present the influence of seismic site effects on free-field seismic motion, in terms of amplitude and frequency. It begins by detailing the phenomena involved, illustrating them with real-life examples. It then describes the behavior of soils when subjected to seismic loads and the geotechnical investigations required to study site effects.

Using simple analytical examples, this article also details the parameters involved in the amplification or damping (de-amplification) of seismic waves.

The consideration of seismic site effects in current building codes is then placed in the context of existing simplified or detailed approaches. This allows us to introduce potential evolutions in future generations of codes.

Finally, this work presents a method for seismic protection of structures by acting on the various components of seismic site effects.