Chaos in Caracas : the role of sediments

The key factors determining the damage an earthquake is likely to cause are, of course, its magnitude—that is, the energy released at the source—and its depth. In the case of June 24, two earthquakes—with magnitudes of 7.2 and 7.5—occurred 39 seconds apart, with the second occurring at a depth of less than 20 kilometers. Nothing comparable had occurred in the region for more than 125 years. But that alone does not explain the extent of the damage and the number of casualties, particularly in Caracas, more than 150 kilometers from the epicenter.

The sedimentary basin on which Caracas was built is well known to Jean-François Semblat, a professor at ENSTA and deputy director of the Mechanics Training and Research Unit. In a scientific article published in the early 2000s in *Computers and Geotechnics*, based on data collected during a previous earthquake in 1967, he had already described the risks of seismic wave amplification that could reach a factor of 25. The cause, according to the researcher, is what he calls the “jelly bowl” effect .”

“The jelly bowl effect is simply a matter of contrasting properties between the rock that surrounds the sedimentary basin (the bowl in the analogy), which is very rigid, and the sediments themselves (the jelly), which consist of sand or clay. If we calculate the wave transmission coefficient as the wave passes from a rock layer to a sedimentary layer, it can be significantly greater than 1. And the more sedimentary layers the wave must pass through, the more the phenomenon amplifies, so that at the surface we can observe a very strong amplification of seismic waves. »

This phenomenon is linked to the resonance of these sedimentary layers at certain frequencies; if these frequencies coincide with the natural frequency of buildings, it can lead to damage or even their complete collapse.