Why Venezuela's 39-second double earthquake stunned scientists

A magnitude 7.2 earthquake hit Venezuela's northern coast. Thirty-nine seconds later, a stronger 7.5 struck the same fault, a rare "doublet" that has killed at least 235 people and has seismologists debating whether it was even two earthquakes at all.

Venezuela was hit not once but twice. At 6:04 pm local time on June 24, a magnitude 7.2 earthquake ruptured the ground west of Caracas. Thirty-nine seconds later, before anyone could reach a doorway, a magnitude 7.5 quake tore through the same stretch of coast. It released nearly three times as much energy as the first, according to the US Geological Survey.

The back-to-back shocks killed at least 589 people, injured more than 2,900, and left thousands missing, Venezuelan officials said. By June 26, international rescue teams had begun arriving to search the rubble. It was the strongest seismic event to strike Venezuela in more than a century. And it has unsettled the scientists who study these faults for a living.

The pairing has a name. A “doublet” is two earthquakes of similar size striking the same place within a short window. In plain terms, it’s one earthquake setting off the next. When a fault slips, it dumps stress onto the surrounding rock. If a neighbouring fault is already close to breaking, that nudge can be enough to push it over the edge.

"It's highly likely that the first event triggered the second," said Harold Tobin, director of the Pacific Northwest Seismic Network at the University of Washington. Raffaele De Risi, an associate professor in civil engineering at the University of Bristol, described the mechanism: “Static stress transfer onto the adjacent fault, combined with passing dynamic waves, pushed an already-loaded segment past its failure threshold.”

What surprised researchers was the timing. “Most double earthquakes don't occur so close together in time. A gap of a few hours to a few days is more common,” said Brandon Bishop, a seismologist at Saint Louis University. Thirty-nine seconds was almost no gap at all.

The damage was so swift, partly because the two blows landed back to back. “The first weakens buildings, the second destroys them,” said Matthew Blackett, an associate professor in natural hazards at Coventry University. De Risi added a human detail: "People are also still in shock and disoriented before the next arrives."

The buildings themselves made it worse. Christian Malaga-Chuquitaype, a seismic engineering expert at Imperial College London, said most of what came down were “non-ductile concrete buildings”, the kind that “tend to explode when they fail”. A structure cracked by the first quake, he said, “would have had no time to recover before the second blow.”

FAULT LINE WITH A VIOLENT HISTORY

The quakes struck on the boundary where two of the planet's tectonic plates grind past each other. The Caribbean plate is sliding east relative to the South American plate at about two centimetres a year, fast in geological terms — and comparable to California's San Andreas Fault.

In northern Venezuela, that seam is expressed through the Bocono-Moron-El Pilar fault system, a tangle of strike-slip faults where blocks of rock slide horizontally past one another. These ruptures tend to be shallow, and this one was no exception. “Around 10 to 20 km deep, which gives no time for dissipation of energy,” Blackett said. Less depth means more violence at the surface.

Two other factors exacerbated the destruction. The rupture itself was long, around 90 km (possibly anywhere between 50 and 150 km), according to Roberto Gentile, an assistant professor in catastrophe risk modelling at University College London. And it travelled in the worst possible direction.

“The earthquake started in the west and ruptured to the east, towards Caracas,” said Karen Lythgoe, a research fellow at the University of Edinburgh. “This explains why the shaking is so large in Caracas.” Seismic energy, in effect, was funnelled straight at the capital.

TWO EARTHQUAKES OR ONE?

Here, the science gets unsettled. Some researchers question whether this was a doublet at all. "It takes about 30 seconds for a magnitude 7.2 to rupture, so it is possible that this is a single bigger event," Lythgoe said.

Stephen Hicks, a seismologist at University College London, made the same case. Rather than two quakes split by a pause, he argued, the event may be better understood as a near-continuous rupture lasting about 50 seconds, “where one rupture triggers another in rapid succession.” The distinction matters: a single cascading rupture and a triggered doublet load the surrounding faults differently, and that shapes what comes next.

What is not in doubt is the rarity. Northern Venezuela has recorded only seven earthquakes above magnitude 6 in the past 100 years, according to Richard Luckett, a seismologist at the British Geological Survey. The deadliest in the country's history struck the same fault system in 1812, killing an estimated 30,000 people. That disaster, too, may have been a doublet: a pair of magnitude 7.1 and 7.4 quakes, according to Laura Gregory, an associate professor in earthquake geology at the University of Leeds.

DANGER HASN’T PASSED

The ground is still moving. The USGS estimates a 99 per cent chance of at least one magnitude 4 aftershock this week and a 24 per cent chance of a magnitude 6, a major earthquake in its own right. Slopes loosened but not toppled by the first shock are now primed to fail. Domniki Asimaki, chair in geotechnical earthquake engineering at Imperial College London, said landslides “not triggered during the first event may have come much closer to failure” and could be set off by later shaking.

Venezuela has no earthquake early-warning system. Such sensor networks can give residents seconds of notice before the shaking arrives. On Wednesday, even seconds were not on offer. The 39-second gap that defined the disaster was the only warning anyone got.

“It's distressing that there was basically no time to evacuate,” said Christine Goulet, director of the USGS Earthquake Science Centre. “That's extremely unfortunate.”

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