Seismic site effects
[1] The surface ground motion may be strongly amplified if the geological conditions are unfavorable (e.g. sediments).
Therefore, the study of local site effects is an important part of the assessment of strong ground motions, seismic hazard and engineering seismology in general.
For alluvial basins, we may shake a bowl of jelly to model the phenomenon at a small scale.
This article defines site effects first, presents the 1985 Mexico City earthquake, describes the theoretical analysis of the phenomenon (through mechanical waves) and details several research results on seismic site effects in Caracas.
When propagating, the seismic waves are reflected and refracted at the interface between the various geological layers (Fig.1).
The example of Figure 1 depicts the seismic wave amplification in horizontal geological layers.
This latter wave will be reflected and refracted several times at the base and the top of the surficial layer.
If the layer is softer than the half-space, the surface motion amplitude can be larger than
When the geological interfaces are not horizontal, it also possible to study seismic site effects by considering the basin effects due to the complex geometry of the alluvial filling[2] For small inclinations of the subsurface layers and/or low impedance contrasts, the assumption of horizontal layering (i.e. the 1D assumption) can still be used to predict site response.
[3] In this article, we propose several examples of seismic site effects (observed or simulated during large earthquakes) as well as a theoretical analysis of the amplification phenomenon.
Seismic site effects have been first evidenced during the 1985 Mexico City earthquake.
[4] The earthquake epicenter was located along the Pacific Coast (several hundreds kilometers from Mexico-City), the seismic shaking was however extremely strong leading to very large damages.
Figure 2 displays the recordings performed at different distances from the epicenter during the earthquake sequence.
The acceleration amplitude measured at different distances changes drastically: the propagation process:[1] geometrical attenuation due to the expansion of the wavefront and material (or intrinsic) attenuation due to the energy dissipation within the medium (e.g. grains friction), We may notice that the acceleration amplitude strongly decreases first and then increases when the seismic waves reach the alluvial deposit on which Mexico City has been founded.
Figure 2a: Shows the effect of resonance: The thickness of the upper geological bed of the modern area of Mexico City is 40 m. The velocity of the shear waves through that layer is 80 m/sec.
[5] It means that the natural frequency of that formation is 0.5 Hz (period of 2 seconds).
[6] When the shear waves of the same frequency arrived to that area, the resonance was responsible for that huge seismic site effect.
In case of horizontal soil layering (constant thickness, cf Fig.1), we may analyze seismic site effects theoretically.
Assuming both media as linear elastic and writing the continuity conditions at the interface (displacement and traction) as well as the free surface conditions, we may determine the spectral ratio
and takes the following maximum values: The red curve corresponds to a large velocity contrast between the layer and the half-space (
As displayed in Fig.3, the maximum amplification is reached at certain frequencies corresponding to the resonance of the sedimentary layer.
The "quarter wavelength" approach can be used to estimate site amplifications due to the impedance contrast.
[7] When the sedimentary layers are not horizontal (e.g. sedimentary basin), the analysis is more complex since surface waves generated by the lateral heterogeneities (e.g. basin edges) should be accounted for.
The aggravation of the amplification level when compared to the case of horizontal layering may be up to a factor of 5 or 10.
[9] Such phenomena are named basin effects and we may consider the analogy with the vibrations in a bowl of jelly.
The theoretical analysis of site effects in canyons or semi-circular sedimentary basins has been performed through semi-analytical methods in the early 80's.
[8] Recent numerical simulations [10] allowed the analysis of site effects in ellipsoidal sedimentary basins.
Depending on the basin geometry, the aggravation of site effects is different from that of the horizontally layered case.
When the mechanical properties of the sedimentary basin are known, we may simulate site effects numerically.
: Numerous geological sites have been investigated by various researchers for weak earthquakes as well as for strong ones (cf synthesis[1]).
