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How did the 1985 Mexico City earthquake change our understanding of earthquakes?
On September 19, 1985, a massive earthquake occurred in Mexico City, with the epicenter located on the Pacific coast hundreds of kilometers away from the city. Despite its distant epicenter, the earthquake caused massive damage within the city and prompted a rethinking of seismic wave propagation and the seismic design of buildings. With the occurrence of disasters, scientists have gained new understanding of the propagation behavior of seismic waves under different geological conditions, especially the amplification effect of shallow geology on earthquake acceleration.
Study finds: Seismic motions in the surface ground may be strongly amplified if geological conditions are not favorable (e.g. sediments).
Definition of earthquake location effect
The earthquake location effect refers to the phenomenon that seismic waves are amplified in surface geological layers. During the propagation process, when seismic waves reach the interface of different geological layers, reflection and refraction will occur, resulting in changes in wave amplitude. Especially in situations such as flow sedimentation basins, this phenomenon is more significant, further affecting the seismic performance of the building.
Example of the 1985 Mexico City Earthquake
In the case of Mexico City, although the epicenter of the earthquake was located far away, when the seismic waves reached the city, they caused unprecedented damage due to the amplification of the waves by sediments under the surface. The investigation showed that the Campos station close to the epicenter recorded an acceleration of 150 centimeters per second, while the Teacalco station 200 kilometers away from the epicenter recorded only 18 centimeters, which clearly reflects the attenuation during seismic wave propagation.
After passing through the sedimentary basin, the seismic waves in Mexico City rebounded and triggered a resonance effect, which significantly increased the seismic acceleration.
Theoretical analysis: horizontal layer theory
When discussing the earthquake location effect, we can also conduct theoretical analysis based on the layered structure of the earth's crust. Assuming a special sedimentary layer above a uniform elastic half-space, seismic waves are reflected and refracted at these interfaces. In this case, we are able to mathematically estimate changes in wave amplitude at different frequencies, especially when it comes to the thickness and wave speed of the overlying sedimentary layer.
Mud basin effect in Caracas
Similar earthquake location effects have prompted extensive research in other cities such as Caracas. In the city, the edges of sedimentary basins enhance the propagation of surface waves during earthquakes. Analysis shows that such a gyration effect can cause seismic waves to increase in amplitude by five to ten times what would normally occur.
When the velocity contrast between the sedimentary layer and the mid-air zone is large enough, the amplification effect of seismic waves will become more obvious.
Summary
The 1985 Mexico City earthquake not only revealed the differences in the propagation of seismic waves under different geological conditions, but also changed the way of thinking about seismic design in construction projects. Engineers and scientists began to pay more attention to the impact of geological conditions on the amplification effect of seismic waves, which also promoted the progress of earthquake engineering and disaster prevention research. Can we still ignore the profound impact of geological properties on earthquakes when designing buildings and urban infrastructure today?
