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Entire families were buried under the rubble.
Entire families were buried under the rubble.
Such was the devastation left by the two earthquakes of magnitude 7.2 and 7.5 that hit Venezuela on June 24, that many wonder why so many buildings collapsed, leaving thousands dead and injured.
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The displacement of the South American tectonic plate and the Caribbean plate generated a gigantic release of energy from the depths of the Earth that reached its maximum point off the northern Venezuelan coast, in what was dubbed “ground zero” of the catastrophe: the state of La Guaira.
Although the epicenter of the two earthquakes was located in the state of Yaracuy – the first near San Felipe and the second near Yumare – the rupture of the Earth was so extensive that the seismic waves traveled to the coast of La Guaira, where the San Sebastián fault is located, located at the point of friction between the tectonic plates.
How many buildings fell in La Guaira and why? The answer is still a matter of analysis, but scientists have some clues.
“There can be more than 50 reasons for buildings to fall,” the president of the Venezuelan Society of Geologists, Feliciano de Santis, tells BBC Mundo.
Among them, the impact of seismic waves from two earthquakes – a double that had only been recorded in the country in 1812 – and the proximity of La Guaira to the area where a lot of seismic energy was released.
The type of soil and the resonance in the buildings also influenced, in addition to irregularities in the constructions, points out De Santis.
The government – which declared La Guaira a disaster area – reported this week that more than 800 buildings were damaged throughout the country, the majority in La Guaira, while other independent reports, such as that of the Environmental Systems Research Institute (ESRI), put the damage to buildings at around 900.
NASA, for its part, made a preliminary estimate of the probability of buildings that could have suffered damage throughout Venezuela, placing them at around 59,000, although this projection is referential and does not include verifications on the ground.
What could have caused the collapse of buildings on the northern Venezuelan coast?
The coastal edge of the state of La Guaira received the direct impact of seismic waves as it is located right in front of the San Sebastián fault, where two tectonic plates coincide, which move very slowly in opposite directions.
This fault runs along the submarine floor from west to east almost parallel to the coast.
It was precisely there, very close to the coastline, where the fault rupture caused by the seismic doublet caused the greatest impact.
“The double event had all the characteristics to be a disastrous earthquake anywhere in the world,” Rafael Abreu, geophysicist at the United States Geological Survey (USGS), tells BBC Mundo.
It had a high magnitude, long duration, shallow depth and a rupture, or horizontal slide, with characteristics that aggravated the phenomenon, the expert points out.
José María de Viana, civil engineer and professor at the Andrés Bello Catholic University, explains that the greatest impact of the second earthquake occurred off the coast of La Guaira, according to technical studies by the National Institute of Geophysics and Volcanology of Italy and the USGS.
“That helps us understand why the devastation was so exceptional there,” he explains. “The impact was at close range.”
The Italian investigation, adds the expert, showed that the landslide reached a maximum of 3.6 meters on the seabed just north of the city of Catia La Mar, in La Guaira, one of the most affected by the disaster.
And in other parts the fault rupture was just a few centimeters, explains Sergio Barrientos, director of the National Seismological Center of the University of Chile. “The fault does not move the same everywhere,” a phenomenon that explains why the effects are disparate.
In those places where the rupture is larger, he says, “the seismic waves are stronger and have greater amplitude even if the epicenter was somewhere else.”
In addition to the destructive power of the seismic doublet, did the buildings fall because the ground was soft or because they were poorly built? That is the big question that many experts ask themselves and, as is often the case, there is no single answer.
“Not all soils are the same in La Guaira,” warns Michael Schmitz, professor of geophysics at the Simón Bolívar University and the Central University of Venezuela.
Within the state, he states, there are specific points, such as the city of Caraballeda, where there is a deep basin of about 400 meters and the type of softer soil influenced the landslides.
But in parts like Catia La Mar, a tourist city that today looks like a war zone, the soils tend to be more of an intermediate rock.
There are sectors of La Guaira that are settled on river cones that have accumulated thin soft sediments, points out De Viana. These soils formed by sediments “acted as a filter that brutally amplified the movement of the ground.”
Ruth Quereguán, researcher at the School of Geology, Mines and Geophysics of the Central University of Venezuela, was touring Catia La Mar and the area surrounding the Maiquetía International Airport in La Guaira.
“I saw as much or more devastation than in the landslide,” he says, referring to the tragedy that occurred with the landslides on the El Ávila hill in December 1999.
These landslides directly affected La Guaira (which at that time was called the state of Vargas). Almost three decades after that tragedy, the same area was the scene of the seismic doublet. These two events, he argues, “are two overlapping phenomena.”
In La Guaira there are many partially consolidated soils as a result of landslides, that is, they are lands formed by sediments with an intermediate resistance. So, he explains, those sedimentary soils may have contributed to the collapse of the buildings. “Many answers will come when we have more data available.”
Even the possibility that the seismic doublet was actually a triplet is also being explored.
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