Verónica Cañas barely had time to grab her 6-year-old son and put on her shoes before running out of her apartment in Caracas. As she ran down the stairs, the walls began to crack and part of the facade started to crumble. A few kilometers away in Altamira, 50-year-old Eduardo Burger watched as one building swayed while another fell apart.

Neither of them knew that this was not just a single terrible earthquake but instead a rare phenomenon. On June 24, Venezuela experienced a seismic doublet that saw earthquakes of magnitude 7.2 and 7.5 occur just 39 seconds apart. The first tremor occurred with its epicenter in Yaracuy. Just a few seconds later, an even more intense earthquake shook the same region again.

Both occurred at a shallow depth of between 10 and 20 kilometers (6 and 12 miles), which caused the energy to reach the surface with greater intensity and allowed the seismic waves to be felt as far away as Colombia, northern Brazil, and several Caribbean islands such as Aruba, Bonaire, and Curaçao. While one alone would've caused damaged, it was the one-two punch that created the conditions that brought down so many buildings and have made it hard to rescue survivors as the death toll mounts.

The Technical Explanation: Tectonic Plates, Damage, and Resonance

“The dining room table started to shake … We thought it was a tremor; then it started shaking much more violently. The walls were cracking, and pieces of the ceiling were falling. We thought it was going to collapse on top of us,” Cañas says.

She and her family managed to make it to a sports field across from the building, where other neighbors were beginning to gather. There, they were hit by another tremor.

“We all hugged each other, terrified, because we’re not used to this. In Mexico and Chile, there’s an earthquake-preparedness culture, and people are already prepared when an alarm goes off or they feel certain movements, but we aren’t,” she says.

Cañas’ experience highlights one of the main differences between Venezuela and other countries with higher seismic activity. Although the country lies at the boundary between the Caribbean Plate and the South American Plate, earthquakes of this magnitude are relatively rare.

Alan Damián Sánchez Pulido, a civil engineer from Mexico’s Ibero-American University and a specialist in structural damage assessment, explains that the plates’ positions and movements are why earthquakes aren't as common as they are in other regions—and why they're so powerful when they do occur.

“In Venezuela, the interaction between the Caribbean and South American plates involves parallel movement; that is what may have caused two earthquakes of considerable magnitude to occur in such quick succession,” he notes.

Unlike Mexico, where the Cocos Plate subducts beneath the North American Plate, in Venezuela, lateral movement leads to different outcomes. “It’s a very rare phenomenon, but the probability isn’t zero. It can occur anywhere in the world where there is interaction between tectonic plates,” Sánchez Pulido says.

What was surprising was not only that two major earthquakes occurred but that the second struck just 39 seconds after the first. To Sánchez Pulido, that short interval is what made this set of quakes so destructive.

“Many structures sustained some kind of damage from the first earthquake. That doesn’t mean the damage was extensive, but any damage alters the original behavior for which they were designed. When another earthquake of similar magnitude strikes immediately afterward, there is no longer any opportunity to reinforce, inspect, or repair the structure. As a result, it no longer performs as intended,” he says.

That helps explain scenes like the one Eduardo Burger witnessed in Altamira: “The San Miguel building began to crumble,” he recalls. “The walls on the first and second floors broke away and fell right where I was standing.” As he and another person ran toward the center of the street to get to safety, he watched as a building swayed “like a pendulum.”

The Same Street, Different Damage

Yet not all buildings were affected equally.

Sánchez Pulido explains that every building has a natural period of vibration—that is, a frequency at which it tends to oscillate. When that frequency matches part of the energy released by an earthquake, a phenomenon known as resonance occurs, which can significantly amplify the structure’s movement. The engineer compares this to a glass that can shatter when a voice reaches exactly the right frequency.

“When a building happens to resonate with the earthquake—regardless of its structural strength or whether it was well- or poorly designed—this resonance phenomenon sets in, and then a building that might have moved only slightly due to the earthquake ends up moving much more,” he says.

Sánchez Pulido emphasizes that quality of construction isn't the only reason some buildings collapsed while others stayed standing. Another variable is the type of soil on which they were built.

Each type of soil—whether rocky, soft, or claylike—alters the way seismic waves reach the surface and, consequently, the response of the buildings. But Venezuelan regulations do not incorporate the level of detail that other countries do, Sánchez Pulido says.

“In Mexico City, there are tools developed by the UNAM Institute of Engineering that allow us to determine with great precision how the soil will behave in different areas … When we build our structures, we tailor them specifically to the soil in which they will be built. Unfortunately, upon diving deep into Venezuela’s regulations, I found that they aren’t nearly as specific,” he notes.

That difference can influence how buildings behave during a high-intensity earthquake, especially when it comes to a phenomenon as unusual as a double earthquake.

To Return or Not to Return: Uncertainty in the Wake of the Earthquake

While experts try to understand what happened, thousands of families are uncertain of whether they’ll be able to return to their homes due to concerns about their structural integrity. That was the case for Cañas of Caracas.

“Thank God we’re OK, but we’re out of our home because both the apartment and the building’s structure have been damaged. We’re still waiting for an agency to come and assess it,” she says.

The situation is no different for 55-year-old Carolina Armas. After leaving her building with her family, she found refuge at a relative’s house while she waits for the Civil Protection inspection.

“There are many deep cracks in my building,” she says. “I personally called Civil Protection to assess the damage to the building, and it’s been more than 12 hours now, and nothing has happened.”

Although Burger acknowledges the efforts of rescue workers, neighbors, and emergency responders, he believes that coordination was insufficient during the first few hours. The lack of equipment also complicated rescue efforts. Burger says that some volunteers explained to him that they lacked basic tools to clear collapsed structures.

Humanitarian organizations also noted the shortage of supplies. According to the international humanitarian organization Project Hope, among the most urgent needs are bandages, suture materials, medications, and specialized tools to free people trapped in the rubble.

The Response to the Emergency

Cañas, Armas, and Burger all note that citizens have begun stepping up with their own efforts, from delivering aid to setting up digital platforms to find missing people.

Other countries are also providing support. The US announced that it has committed $150 million to address the emergency: $50 million will be distributed through organizations such as World Vision and Samaritan’s Purse, and another $100 million will be contributed to a United Nations fund for Venezuela. For its part, Mexico sent 250 personnel from the Ministry of National Defense, five specialized search dogs, four aircraft, and a search-and-rescue drone, while Spain deployed teams from the Military Emergency Unit and aid workers to strengthen search, rescue, and humanitarian relief efforts.

For Sánchez Pulido, the engineer, the most important lesson for planning for the future is building a culture of prevention.

“Often, we only worry about an earthquake after it has already happened, and if a long time passes without a major one, we stop conducting drills, stop following Civil Protection guidelines, and forget that it can happen at any moment,” he warns.

Because they are impossible to predict far ahead of time, Sánchez Pulido says, “The important thing is to know how to react.”

This story originally appeared on WIRED en Español and has been translated from Spanish.