The twin earthquakes in and around Venezuela’s capital Caracas are feared to have killed tens of thousands. The first quake, of magnitude 7.2, hit about 160 km west of Caracas late on Wednesday night. Just 39 seconds later, it was followed by a 7.5 magnitude quake.
The US Geological Survey (USGS) described the disaster as a “seismic doublet”. The second quake was the largest to strike Venezuela or just off its coast since 1900, according to the USGS.
The agency’s preliminary death toll estimate ranges from 10,000 to 100,000. The back-to-back quakes flattened buildings and triggered a major rescue operation as officials warned the disaster could become one of the country’s deadliest natural disasters.
What we know about the two earthquakes in Venezuela
The magnitude 7.5 mainshock (right) produced stronger and more widespread ground shaking than the magnitude 7.2 foreshock (left). (USGS)
The first earthquake struck about 24 km from San Felipe in Yaracuy state. The second hit near Morón in neighbouring Carabobo state, about 168 km west of Caracas, according to the USGS.
Given that both earthquakes occurred within a short time and distance from each other, the USGS classified the event as a doublet.
In a typical sequence, the larger quake is followed by a series of smaller aftershocks. Doublets, however, are generally defined as earthquakes of similar magnitude that originate from distinct but closely related ruptures and occur in quick succession.
The shocks from the disaster were felt as far away as neighbouring Colombia, with residents in the capital, Bogotá, reporting feeling the tremors.
Associated Press also reported that buildings were evacuated in Manaus, Belem and Macapa in Brazil’s Amazon region. Venezuela President Delcy Rodríguez said the worst-affected areas included Caracas and the north-central states of Miranda, La Guaira, Aragua, Carabobo and Falcón. Despite the widespread damage, Venezuela’s oil infrastructure did not appear to be affected, according to a Reuters report.
What caused the earthquakes?
Venezuela sits in one of the most seismically active regions in northern South America, along the boundary between the Caribbean and South American tectonic plates. These two plates can slide horizontally past each other along faults, in a phenomenon called a strike-slip fault (see box below), causing earthquakes.
Different kinds of faults
According to the USGS, Wednesday night’s second, stronger earthquake resulted from shallow strike-slip faulting near the boundary between the two plates.
Earthquakes often occur in sequences. The USGS classified the magnitude 7.2 earthquake as a foreshock, or a smaller earthquake that occurs before a larger one, and the magnitude 7.5 event as the mainshock, the largest earthquake in a sequence. The agency said the sequence “likely indicates a complex rupture interaction process” that formed a seismic doublet.
The USGS also noted that earthquakes of this size should not be viewed as a single point on a map, but as the rupture of a much larger section of fault.
So why were the earthquakes so destructive?
The mainshock occurred at a relatively shallow depth of about 10 km. This meant that the shockwaves reached the surface with high intensity. Worsening the situation was the fact that this was a doublet, distinct from routine aftershocks.
“Doublets or twin earthquakes are slightly different from aftershocks that are routine after any major earthquake. Aftershocks are largely a result of disturbances or ruptures caused by the main earthquake. Some stress is already present, and it is then amplified by the main earthquake, resulting in another quake. Aftershocks are usually one or more magnitude smaller than the main earthquake, although these are not very tight definitions, and have to be assessed on a case-by-case basis,” Vineet Gehlot, director of Dehradun-based Wadia Institute of Himalayan Geology, told The Indian Express.
“Doublets are two independent events happening within a very short span of time, a few seconds or a few minutes, or maybe a couple of hours. They are roughly of the same magnitude and happen in close proximity to each other,” he said.
Earthquake doublets can also be especially destructive because they prolong the strong ground shaking. A 2016 study in Nature Geoscience, analysing Pakistan’s 1997 Harnai earthquake, found that a second major shock, which struck 19 seconds after the first, doubled both the duration of ground shaking and the area affected by the strongest shaking.
Thus, buildings already weakened by the first earthquake remain exposed to another major shock almost immediately, increasing the risk of collapse and complicating rescue efforts.
Experts say the scale of destruction depended not only on the intensity of the shaking but also on how well buildings and infrastructure were designed to withstand earthquakes.
Professor Virendra Kumar Paul, Director of the School of Planning and Architecture, New Delhi, told The Indian Express that earthquake resilience requires architectural and structural design to work together from the earliest stages of planning. Rather than treating seismic safety as an engineering fix added later, he said seismic considerations should shape a building’s design from the outset.
Referring to the widespread damage in Venezuela, Paul said preliminary reports suggested many buildings were not designed for shaking of this intensity. That could reflect shortcomings in structural design, poor construction quality, or both, particularly as many low-rise buildings had also been damaged.
“On both counts Japan fares many notches higher,” he said, adding that Venezuela’s economic constraints may have affected the earthquake resilience of its building stock. He also noted that because it was the country’s strongest earthquake in a century, the perceived seismic risk may have been lower. By contrast, Japan’s frequent exposure to major earthquakes has led to better preparedness in design, construction and emergency response.
Is Venezuela vulnerable to earthquakes?
The country has witnessed about a thousand earthquakes of magnitude 4.0 and higher over the past decade. Before the doublet on Wednesday, the country experienced a series of earthquakes in Zulia state, with the largest reaching magnitudes of 6.2 and 6.3 near Mene Grande.
The strongest event it experienced prior to this was the 1900 magnitude 7.7 San Narciso earthquake in 1900, the previous strongest recorded event, killed at least 21 people and devastated parts of Caracas. According to the USGS, Wednesday’s doublet earthquakes could prove to be far deadlier than the one in 1900.
Severe earthquakes are common in neighbouring Mexico and Chile. The two countries lie along the 40,000 km-long Pacific Ring of Fire, home to about 75 per cent of the world’s volcanoes, and prone to 90 per cent of the world’s earthquakes.
With inputs from Amitabh Sinha
Fanny Nasri waits at the site of a collapsed building as emergency workers search for her son, alongside Noemi Cordoba, who awaits news of her granddaughter, after an earthquake in Caracas, Venezuela. (Reuters) 
The magnitude 7.5 mainshock (right) produced stronger and more widespread ground shaking than the magnitude 7.2 foreshock (left). (USGS)
Different kinds of faults
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