the unusual “seismic double” that has shaken Venezuela

The earth has given no respite in the north of Venezuela, where in the last few hours two earthquakes of magnitude 7.2 and 7.5accompanied by more than twenty aftershocks, have generated great chaos in much of the country with a growing number of people dead, injured and missing, and which has led to the activation of the state of emergency.

The anatomy of the earthquake. The temporal sequence and data returned by global monitoring systems, such as the United States Geological Survey, reveal a shock that defies usual seismic patterns. In this way, the first earthquake registered a magnitude of 7.2, a level that alone is capable of generating severe damage to critical infrastructure.

But the problem came just 39 seconds after the first tremor, when the earth broke again with an even more violent event, reaching a magnitude of 7.5. Two large earthquakes that have been located with an epicenter in the vicinity of Morón and the state of Yaracuy.

With doubts. At first, this is what happened with the data that was being generated by the automatic measurement equipment, since there was a possible overlap in the readings. But it was not like that, since confirmation ended up being imposed that the 7.2 tremor acted as an immediate precursor event to the main 7.5 earthquake and that it completely changes the perspective on the amount of mechanical energy that was dissipated in the Earth’s crust.

The magnitude of the problem. A ‘normal’ earthquake can basically be explained as a main rupture, which is the main tremor, followed by tens or hundreds of smaller earthquakes, which are known as aftershocks, which occur as the faults reset mechanically. But here in Venezuela we have lived a ‘seismic doublet‘, that is, there have been two earthquakes of large magnitude and comparable size that occur in the same tectonic region with an extremely short time interval, which in this case has been seconds.

Thus, when the first 7.2 earthquake fractured the rock, it did not relieve stress in the area, but instead dramatically altered the stress field on a segment of the adjacent fault. That is why a ‘trigger’ acted to push the neighboring fault beyond its rupture point, ‘triggering’ the 7.5 earthquake just 39 seconds later.

A problem. This type of chained ruptures subjects architectural structures to extreme punishment, since the buildings, whose bases have already been weakened and put into resonance by the first impact, receive a second blow of greater amplitude before having been able to dissipate the kinetic energy of the first.

That is why with this series of earthquakes we have seen major damage to civil constructions, including the main airports, which in some cases have been completely unusable.

The energy problem. Northern Venezuela sits on an area of ​​extremely high geodynamic complexity, marked by the interaction and continuous friction between the Caribbean plate and the South American plate. This is why the faults that cross this region accumulate elastic deformation at a constant rate of several millimeters per year.

Due to this complex configuration, it was perfectly known that this could happen at any time, and here the civil engineer, Gina Paola Villalobos, in statements for El País, collects these impressions and points out that the problem has been in the fragile constructions of Latin American cities that were not prepared for this.

Energy released. With all this information, we can see that literally a century of energy has been released that has been accumulating in the deepest layers of the planet. And this is not simply literary hyperbole, but, in seismological terms, it means that a specific segment of the fault had remained “locked” or mechanically coupled for decades.

During all that time, the movement of the tectonic plates continued its course, compressing and deforming the surrounding rock as if it were a gigantic spring. Now, the seismic doublet of 7.2 and 7.5 represents the exact moment in which the friction of the rock was overcome, violently unleashing in a matter of minutes the elastic energy that the Earth took more than 100 years to accumulate.

Images | Bona Lee Çağlar Oskay

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