Seismic site effects are related to the amplification of seismic waves in superficial geological layers. 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. Damage due to an earthquake may thus be aggravated as in the case of the 1985 Mexico City earthquake. 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.
Definition of the phenomenon
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. We consider a homogeneous elastic half-space (in green) over which an elastic alluvial layer of constant thickness h {\displaystyle h} is located (in gray). A shear wave ( S H {\displaystyle SH} ) of amplitude A 2 {\displaystyle A_{2}} reaches the interface between the half-space and the alluvial layer with an incidence θ 2 {\displaystyle \theta _{2}} . It thus generates:
a reflected wave in the half-space with amplitude A 2 ′ {\displaystyle A_{2}^{'}} and incidence θ 2 {\displaystyle \theta _{2}}
a refracted wave in the superficial layer with amplitude A 1 {\displaystyle A_{1}} and incidence θ 1 {\displaystyle \theta _{1}}
The refracted wave originates a reflected wave when reaching the free surface; its amplitude and incidence are denoted A 1 ′ {\displaystyle A_{1}^{'}} and θ 1 {\displaystyle \theta _{1}} respectively. 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 A 2 {\displaystyle A_{2}} thus leading to the amplification of seismic waves or seismic site effects. 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 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. 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.
Example: site effects in Mexico City (1985)
Seismic site effects have been first evidenced during the 1985 Mexico City earthquake. 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:
Campos station: this station is located very close to the epicenter and recorded a maximum acceleration of 150 c m / s 2 {\displaystyle 150~cm/s^{2}} , Teacalco station: this station is located at more than 200 km from the epicenter and recorded a much lower acceleration (about 18 c m / s 2 {\displaystyle 18~cm/s^{2}} ). This amplitude decay is due to the wave attenuation during the propagation process: 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),
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