Any large earthquake can be dangerous, but the residents of Caracas and Venezuelan coastal cities like La Guaira and Catia La Mar were victims of several unlucky factors.
Two quakes in rapid succession, a fault that ruptured toward more populated areas, soft ground and the shallow depth of the temblors all combined to make the June 24 disaster especially severe, even for areas that were miles away from the epicenter.
Here is what made it so deadly.
The “doublet”
First, and most important: there was not one earthquake but two, separated by just 39 seconds. Seismologists call this a “doublet.”
The first was smaller measuring 7.2 in magnitude; the second was much more powerful, measuring 7.5.
The doublet meant that buildings had to withstand chaotic shaking for far longer than they would have in an ordinary earthquake.
“Almost certainly, what happened was that the first earthquake triggered the second one,” said David Oglesby, a professor of geophysics at the University of California, Riverside.
The earthquakes left over 2,200 people dead, according to the Venezuelan government. Over 400 buildings were destroyed, and hundreds more were damaged.
The direction
Seismic waves from the first quake radiated outward in all directions. But it was the waves traveling east that eventually reached a point on the San Sebastián fault that was ready to slip. That set off the second earthquake. The rupture from that continued east — more than 100 miles toward Caracas, the capital.
“There were two directions this fault could have gone,” said William Barnhart, a geophysicist in the earthquake hazards program of the United States Geological Survey. “It could have gone to the west, or the east toward Caracas. They would have been lucky if it had gone the other direction.”
The initial rupture occurred on what is known as the Boconó fault, part of the system that produced a tremendous earthquake in the 19th century, Dr. Barnhart said.
Satellite imagery shows that the second quake then traveled east along a well-known seam between two of Earth’s tectonic plates — the San Sebastián fault.
“We know that the fault extended as far as Caracas,” said Dr. Barnhart, “and that starts to explain, in part, why there’s so much shaking there.”
The ground shift
Scientists compared satellite imagery from before and after the quake to measure how much the ground shifted. In coastal regions such as La Guaira — where the fault runs only a couple of miles offshore or directly beneath the city — the ground moved as much as 1.5 feet to the west.
The San Sebastián is a strike-slip fault, meaning that the Earth moves primarily horizontally al
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