Entire families were buried under the rubble.

The devastation left by the two earthquakes of magnitude 7.2 and 7.5 that hit Venezuela on June 24 was such that many wonder why so many buildings collapsed, leaving thousands dead and injured.

The displacement of the South American tectonic plate in relation to 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, baptized as “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?

1.The direct impact of seismic waves in La Guaira

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 coastal edge, where the rupture of the fault system 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.”

2. Soil characteristics

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.

3. The quality of the constructions

Regarding the quality of the buildings, Quereguán points out that, after the 1999 landslides in El Ávila, some of the houses that were partially damaged were rebuilt. The problem, he says, is that experts do not know if the habitability permits, construction permits or the type of materials that were used were verified. “There was surely negligence” in some cases, he says.

Then there are the buildings that were built after the landslides. There is also no certainty that, for example, the corresponding soil studies have been carried out for new buildings, he points out.

“Was the standard we have after the 1967 earthquake met? We don't know,” adds Queraguán. The standard that regulates earthquake-resistant constructions, which has been updated several times, came into force in the early 1970s.

It is likely that some of the construction in La Guaira did not follow the norm, he says. “That's when the negligence and corruption part comes in.” And in that sense, “you can't cover the sun with a finger.” In any case, he adds, it is necessary to wait for qualified professionals to provide answers.

For now, some members of emergency teams who have traveled from other countries to collaborate in the rescue and debris removal efforts have been surprised by the use of construction materials that they describe as “deficient.”

One of them, who preferred not to make his name known, reported that in a building in La Guaira he found beams made with a light material that breaks with the hands (technically expanded polystyrene) and that were covered with a few centimeters of concrete. He also found pillars without iron inside.

The technical studies published following the tragedy in La Guaira will probably provide evidence that will clarify more precisely what happened to the soils and buildings.

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