Rescue teams are racing against time across Venezuela after two powerful earthquakes ripped through the country, leaving hundreds of people dead, thousands injured and many others trapped beneath collapsed buildings.

The country’s health minister said at least 235 people have been killed and more than 4,300 wounded, with the death toll expected to rise amid the devastation, which is concentrated in the northern coastal state of La Guaira and in the capital city, Caracas.

The back-to-back earthquakes – measuring magnitude 7.2 and 7.5 – were felt across much of the country and as far away as parts of Brazil, prompting international offers of humanitarian assistance and the dispatch of search and rescue teams from around the world.

Acting President Delcy Rodriguez has declared a nationwide state of emergency, while Simon Bolivar International Airport in Caracas has been closed after it was damaged.

As rescuers comb through the rubble, every passing hour matters. The first 72 hours after a major earthquake are widely regarded as the most critical period for saving lives. During that window, emergency responders race to free survivors before dehydration, crush injuries and collapsing buildings dramatically reduce the chances of survival.

Here’s what typically happens – and all the factors which must be considered during rescue efforts – during those crucial first three days after a major earthquake.

Why was this earthquake so destructive?

Venezuela lies on the boundary between the Caribbean and South American tectonic plates, which grind past one another at roughly 22 millimetres a year, making the country particularly vulnerable to powerful earthquakes. About 80 percent of Venezuela’s population lives in seismically active areas.

According to the United States Geological Survey (USGS), the first magnitude 7.2 earthquake struck near San Felipe, about 284km (176 miles) west of Caracas, at 18:04 local time (22:04 GMT). It was followed almost immediately by a second, stronger magnitude 7.5 earthquake near Yumare, roughly 293km (182 miles) west of the capital.

The earthquakes were also relatively shallow, occurring less than 20 kilometres (12 miles) beneath the Earth’s surface, causing stronger shaking than deeper earthquakes of a similar magnitude.

Immediately after the quake, the USGS warned that “high casualties and extensive damage are probable, and the disaster is likely widespread”.

Despite the strength of the earthquakes, tsunami warnings issued across the Caribbean were later lifted, partly because the epicentres were inland rather than beneath the sea.

What is the first response?

Within minutes of the earthquakes striking, authorities began activating emergency response plans across the country, with Rodriguez declaring a nationwide state of emergency as local authorities mobilised firefighters, police, medical teams and civil defence workers.

The Chacao municipality in the Caracas metropolitan area reported that it had activated its emergency mechanisms, while neighbouring regions also began assessing damage and dispatching first responders.

As often happens after natural disasters strike, neighbouring countries were among the first to offer assistance. El Salvador’s President Nayib Bukele said his government had offered support through its foreign ministry.

Brazil announced it would send a specialist search and rescue mission consisting of 36 firefighters, four telecommunications experts and four technicians from the National Civil Defence agency. Mexico also pledged assistance.

As the scale of the disaster became clearer, countries further afield joined the response, with France and Switzerland sending rescuers, while the United States pledged $150m in humanitarian assistance.

International organisations also began mobilising, with the International Federation of Red Cross and Red Crescent Societies activating its emergency network across Venezuela, saying its nationwide hospitals and polyclinics remained operational.

Meanwhile, United Nations-coordinated urban search and rescue teams also began travelling to the country.

When do search and rescue efforts begin?

Immediately, as there is no time to lose. While governments organise emergency responses, the race to save lives begins beneath collapsed buildings.

People who survive the initial collapse can remain alive for several days if they avoid catastrophic injuries, but their chances diminish rapidly with every passing hour.

“It is certainly possible for people who have not suffered major injuries in the initial collapse to survive for some days,” David Pyle, professor of earth sciences at the UK’s University of Oxford, told Al Jazeera.

“But without access to water or medical assistance, this requires rapid response.”

Water is often the deciding factor, Bill Murphy, professor of engineering geology at the University of Leeds in the UK, told Al Jazeera. “Three days without water is normally fatal,” he said.

“People who survive longer than this in rubble – it is normally because they have had some access to water.”

Murphy also said rescuers are most commonly dealing with impact and crush injuries in victims. “The longer people stay buried, the higher the likelihood of developing long-term problems from crush injury syndrome.”

While search and rescue teams continue scouring damaged neighbourhoods, authorities also begin confronting another immediate humanitarian challenge – attempting to house those displaced by the disaster via emergency shelters.

Venezuela’s Ministry of Education quickly announced that schools are being converted into emergency relief centres and shelters for families forced from their homes.

How do workers manage aftershocks?

Large earthquakes are almost always followed by aftershocks, some powerful enough to bring down buildings already weakened by the main quake, forcing rescuers to halt operations while they wait for the ground to stabilise again.

By 03:33 GMT – just over five hours after the earthquakes struck – Rodriguez said authorities had already recorded at least 20 aftershocks.

The following day, Cabello said more than 100 buildings had collapsed in the northern coastal region, while more than 70,000 families in La Guaira alone had been affected.

Vashan Wright, a geophysicist at the University of California San Diego, told Al Jazeera that statistics suggest Venezuela is almost certain to experience further tremors in the coming days.

“Based on historical knowledge and statistics of earthquakes, there is a 99 percent chance that you’ll have some magnitude 3 and magnitude 4 aftershocks in the next week,” he told Al Jazeera.

“A magnitude 5 has a high probability, over a 90 percent probability.”

Pyle added that aftershocks remain one of the greatest challenges facing rescue workers “but they become smaller and less frequent as time passes”.

“In the hours and days following an earthquake, the largest aftershocks will be smaller than the main event, but they may still cause significant shaking of buildings that have already been damaged or destabilised, and so may continue to hinder rescue and recovery efforts.”

Meanwhile, on the ground in Venezuela, authorities have repeatedly urged people not to return to damaged buildings, and to stay away from large trees and damaged infrastructure.

Across Caracas and other cities, many residents spent the night on the streets, fearful that further aftershocks could bring down buildings already weakened by the twin earthquakes.

When will we know how many people have died?

One of the defining features of major earthquakes is that the true scale of the disaster often remains unclear for days.

In the first hours, casualty reports usually emerge piecemeal from local officials, hospitals and emergency services before a national picture begins to form.

Immediately following Wednesday’s quake, Gustavo Duque, mayor of Caracas’s Chacao municipality, said four buildings had completely collapsed, killing one person. Another 22 people were reported to have been rescued from damaged buildings, while another 30 structures had been left unsafe. Victor Clark, governor of Falcon state, said at least 15 people remained trapped after a residential building collapsed in the coastal region.

Just over six hours later, at 05:02 GMT, Rodriguez announced the first nationwide death toll – 32 people had been killed and about 700 injured.

Within 24 hours, Health Minister Carlos Alvarado said the number of people killed had risen to at least 235, with more than 4,300 injured.

As the hours turn into days, officials expect the death toll to rise as more missing people are confirmed dead.

In all, the USGS has estimated there was a potential for between 10,000 and 100,000 deaths using its Prompt Assessment of Global Earthquake Response (PAGER) system.

How is that estimate arrived at?

Before rescuers can reach every collapsed building, scientists have already estimated how severe a disaster could become.

The PAGER system estimated between 10,000 and 100,000 deaths for the Venezuela disaster as a whole.

“Essentially, what it does is take all known knowledge about the magnitude, depth, location, population density, historical earthquakes and building inventory… and makes a prediction for what might be expected,” Wright said.

Local geology can also make a dramatic difference. Some of Caracas’s hardest-hit neighbourhoods are built on soft sediments that amplify seismic waves, making buildings more vulnerable to collapse than those constructed on solid bedrock.

Informal housing, common across parts of Venezuela and often built on hillsides, is also generally less able to withstand powerful earthquakes. About half of Caracas’s five million population is thought to live in informal housing.

The slums in Caracas are known as barrios and are densely populated, lacking proper infrastructure. They comprise self-constructed housing or structures built with unreinforced cinderblocks or bricks, often without formal foundations or steel reinforcement, mostly on the mountainous hillsides surrounding the capital.

Beyond the physical destruction, humanitarian agencies say broader economic conditions can also influence how quickly countries will recover. The US-based Center for Economic and Policy Research (CEPR) has called on the United States and other governments to lift sanctions on Venezuela to allow aid to reach those affected more quickly.

“The Venezuelan government must be free to receive and allocate earthquake relief and to send humanitarian support to those who need it,” CEPR said.

“Current US and other sanctions threaten to hobble the overall earthquake response.”

Why do earthquakes seem to kill fewer people today than historically?

On Friday, some experts were saying that the overall death toll in Venezuela may not rise as high as first expected.

James Jackson, head of earth sciences at the UK’s University of Cambridge, said there are already signs that some buildings had fared better than many experts initially expected.

“It appears that the construction may have been better quality than many feared, though there were still many collapses,” he told Al Jazeera.

He believes the unusual sequence of the two earthquakes may also have saved lives.

“Undoubtedly, people were helped by the first, smaller shock getting them out of buildings before the bigger one 40 seconds later, which will have caused some already-weakened buildings to collapse.”

The location of the earthquakes may also have limited the scale of the disaster. “The earthquakes were a significant distance from Caracas and in this context location is important,” Murphy said.

Powerful earthquakes are no less common today than they were hundreds of years ago.

For instance, in 1556, the deadliest earthquake in recorded history in China’s Shaanxi killed about 830,000 people. In 2023, a similar earthquake hit northwestern China near the Shaanxi province, killing just 127 people.

Experts say what has changed is how societies prepare for them.

Countries regularly affected by major earthquakes, including Japan, Chile, New Zealand and parts of the United States, have steadily strengthened building regulations, invested in earthquake-resistant construction and improved emergency planning.

“Those that are subjected to frequent large earthquakes … have become more resilient and serious about building codes,” Jackson said.

By contrast, regions that experience major earthquakes less frequently may have older or more vulnerable buildings because seismic risk has historically received less attention.