The second earthquake that hit Venezuela on Wednesday started rumbling before the first had even concluded. Their onsets were separated by just 39 seconds.

The U.S. Geological Survey has described the pair of earthquakes — a 7.1-magnitude and a 7.5-magnitude — as a doublet sequence, a phenomenon in which two temblors of similar magnitude strike roughly the same area at around the same time.

“The seismic waves from the first one weren’t done yet when the second one happened,” said Harold Tobin, the director of the Pacific Northwest Seismic Network and a professor at the University of Washington.

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It’s not rare to have two earthquakes in relatively short succession. But in this case, the rapid, back-to-back shaking likely led to more collapsed buildings and other destruction. At least 188 people were killed and at least 1,520 were injured, with more than 150 still missing.

The first earthquake “will probably have weakened some buildings or structures,” Tobin said. “Then collapses would happen during the second earthquake, even if they made it through the first one.”

A 7.5-magnitude temblor is about three times bigger than a 7.1, since the Richter scale is logarithmic. (Each whole number is 10 times bigger than the previous.)

The doublet pattern took place on a tangled system of faults near San Felipe, Venezuela. The zone is so complex that it will take more time for researchers to understand precisely which faults within it ruptured Wednesday.

Maria Beatrice Magnani, a professor of seismology at Southern Methodist University, who mapped faults in Venezuela in the early 2000s, said the ruptures happened along a boundary between the South American and Caribbean tectonic plates, where the two are essentially sliding laterally past one another. In this area, the USGS estimates the Caribbean plate is moving eastward relative to the South American plate at a rate of about 20 millimeters per year (about three-quarters of an inch). The plates are also subject to compression — they’re being squeezed together as they move past one another.

Initial data indicates both of these earthquakes were strike-slip, Magnani said, meaning one plate lurched past the other along the boundary. But she added that much about how the quakes played out is still to be determined.

“It’s going to take some time to find a full history of the fault,” Magnani said. “It’s a very complex plate boundary.”

Tobin said it’s likely that the first earthquake triggered the second.

“The area is actually mapped as not a single fault line, but a kind of a complex of faults and fractures in the Earth’s crust, and there’s not just one single one, and that may be part of what contributes to the complexity of this pair of earthquakes,” he said.

According to a 1999 study in the Bulletin of Atmospheric Sciences, which reviewed seismic data from 70 events, doublets occurred in about 22% of earthquakes that were magnitude 7.5 or above. Venezuela experienced doublet earthquakes in 2025, with magnitudes of 6.2 and 6.3, but those were to the southwest of Wednesday’s event. The quakes killed one person and caused over 110 injuries, according to the USGS.

In 2023, doublet 7.8-magnitude and 7.5-magnitude earthquakes struck southern Turkey and Syria, killing more than 3,000 people.

In North America, two magnitude-6.2 earthquakes were reported near the border of northwestern British Columbia and Alaska within about two hours of one another in May 2017. And Klamath Falls, Oregon, saw doublet earthquakes in September 1993, when a magnitude-5.9 quake preceded a magnitude-6.0 by about two hours.

Tobin said scientists are still weighing whether Wednesday’s sequence was indeed a doublet, though the USGS described it as such.

“It’s actually a subject of lots of discussion right now among seismologists about whether to call it two earthquakes or one earthquake with multiple phases or pulses of energy,” Tobin said, adding that the debate was a little semantic.

Although Venezuela has had five earthquakes with magnitudes of 7.0 or higher since 1900, Tobin said none were along the same fault zone as Wednesday’s event. So the area had likely been accumulating stress for more than two centuries.

“There hasn’t been a magnitude 7 plus earthquake anywhere along that fault, possibly since about 1812,” he said.

Magnani said the twin earthquakes will offer new data for researchers to untangle the fault structure hidden below Earth’s surface.

“It tells us something about how the faults are moving. It’s a new tassel that can explain better how this region works,” Magnani said. “Every earthquake and the aftershocks that happen will help us unravel even better the complexity of this region.”