Influence of local geology or site effects

When an earthquake occurs, seismic waves that propagate from the focus to the surface pass through different media. Depending on the nature of these geological layers, the impact of the earthquake will not be the same, as their mechanical characteristics are different. This is referred to as Site Effect to refer to this change in seismic tremor according to local geology.

To highlight a site effect, it is often possible to detect an increase in the amplitude of the seismic tremor recordings and its duration.

But it is not easy to know which of the rocks or sediments, for example, are the most critical soils for the earthquake. Some rocky landforms show worrying amplifications as well as some sedimentary fillings: it is the nature of the geological formations but also the geometry of the landforms and basins that determine the amplification of seismic movement, with direct consequences for the resistance of the structures. It is therefore necessary to be able to evaluate them in order to protect against them.

Each building has its own period of oscillation. When the period of ground oscillation corresponds to the period of building oscillation, the building gets carried away and amplifies the movement: we speak of resonance. It is this phenomenon that can cause major material damage in the event of an earthquake.

In the graph below, seismic motion is represented as a function of the motion of an oscillator with different vibration periods. The oscillation period of the sedimentary layer shows a strong amplification at 2 seconds, where all the seismic energy is focused. It is therefore the buildings with this period that will be the most shaken.