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The mystery of earthquake wave amplification: Your home may be at the epicenter!
The damage caused by an earthquake often varies depending on geological conditions, especially the structure of some surface rocks and soils, and the amplification of seismic waves in upper geological layers. This phenomenon is called seismic site effect. If the geological conditions are poor (for example, the presence of sediments), surface motion may be strongly amplified. Therefore, the assessment of strong ground motions and the prediction of earthquake hazards require in-depth research on local site effects.
The amplification effect of earthquakes can cause greater damage in certain areas, as was the case with the 1985 Mexico City earthquake.
Defining Site Effects
When seismic waves propagate through different geological layers, they are usually reflected and refracted at the interfaces between different layers. For example, if there is a layer of sediment on a uniform plastic half-space, the thickness and material of these strata will directly affect the propagation of seismic waves and the final ground acceleration. If the stiffness of the upper layer of material is less than that of the lower layer, the amplitude of the surface movement can be significantly increased, resulting in the occurrence of amplification.
According to research, when there is a soft sedimentary soil layer, the amplitude of ground movement can be greater than the amplitude of seismic waves in the underlying layer, thereby triggering the amplification of seismic waves.
The Case of the 1985 Mexico City Earthquake
The 1985 Mexico City earthquake provides us with an intuitive example of earthquake site effects. Although the epicenter of this earthquake was hundreds of kilometers away from Mexico City, the shaking was extremely strong and caused huge losses. Research records show that locations close to the epicenter, such as Campos Station, had a maximum acceleration of up to 150 cm/s²; while the Teacalco Station, which was farther away from the epicenter, only recorded an acceleration of about 18 cm/s². The SCT station in Mexico City, also about 400 kilometers from the epicenter, recorded a strong earthquake acceleration of about 170 cm/s².
This phenomenon clearly shows the resonance effect caused by the seismic waves as they passed through the sedimentary basin in Mexico City, further intensifying the shaking on the ground.
Theoretical analysis: site effect of horizontal strata
When discussing the seismic site effects of horizontal soil layers, we can conduct a theoretical analysis. If we consider a shear wave that is reflected and refracted at the interface of different media, the behavior of the wave can be described by a series of mathematical models. At certain seismic wave frequencies, the amplification of the observed acceleration of motion can be very significant and varies depending on the physical properties of different geological layers.
When thickness, wave velocity and material properties do not match, earthquake site effects will show a higher degree of amplification, affecting the ground shaking and the stability of buildings.
Seismic site effects in the Caribbean
Site effects are also evident in the sedimentary basins of the Karakax region. Due to the region's geological structure and surface shape, seismic waves achieve higher amplification at the flooding epicenter. Studies have shown that the shape and structure of the basement significantly increase the degree of seismic wave amplification compared to horizontal stratification, strengthening the overall ground response.
The research results show that the amplification effect of up to 10 times can be confirmed in post-earthquake tests. This phenomenon is highly alarming, especially for building planning and risk assessment.
The amplification phenomenon of earthquakes has a direct impact on our lives. This not only concerns the safety of buildings, but also our daily lives. Understanding these phenomena may help us better cope with earthquake disasters and their potential threats in the future. Have you considered the geological conditions where you live and thought about how to improve the earthquake resistance of your hometown?
