Physics explains crowd surge that killed 154 people

  • A Halloween party in Seoul ended in at least 154 deaths after the streets became very crowded.
  • The phenomenon, called crowd surge, is basic physics, an expert told Insider.
  • If a limit of around seven people per square meter is exceeded, things can quickly get deadly.

The surge in crowds – the deadly phenomenon that has claimed the lives of more than 150 people in South Korea – can be explained by simple physics, an expert told Insider.

At least 154 people died in Seoul on Saturday when a Halloween street party drew crowds so dense it crushed people to death.

The lightning strike took place in Seoul’s bustling Itaewon district, a popular nightlife spot that drew tens of thousands of people on Saturday, The Guardian reported.

No event was planned, according to Reuters. But crowds of bustling bars and nightclubs poured into a narrow, sloping lane connecting a subway station to a main street.

A map shows the location of the alley in Itaewon relative to the subway station.

A map of the Itaewon district of Seoul shows the location of the alley.

Google Maps/Insider

Sometime after 10 p.m., the street filled beyond capacity. Social media accounts from the night, compiled by Reuters, said people near the top of the aisle lost their footing and fell into the crowd below, triggering a fatal crush.

The tragedy prompted national mourning in South Korea and questions about whether more could have been done to prevent it.

Medhi Moussaïd, a researcher at the Max Plank Institute in Berlin who studies crowd dynamics, spoke to Insider about when crowds become deadly

“Most people don’t realize the danger,” he said, saying people should be better informed as cities get denser and large crowds grow.

Emergency services personnel are seen in a cordoned off alley in the Itaewon district where a Halloween stampede took place, October 30, 2022.

Emergency services are seen in the alley where the crowd crush took place, pictured here on October 30, 2022.


Crowds acting like waves

The influx of crowds is motivated by a simple principle. If a group of people becomes dense enough – more than six or seven people per square meter – a crowd begins to act like a fluid.

At this point, the people inside largely lose the power to control their own movement.

If someone is jostled, he will push his neighbour, who will fall on his neighbour, and so on.

“So this movement is transmitted,” Moussaïd said. It’s kind of like a ripple in water, as these movements spread they get bigger

The wave pressure may be too intense for people in the crowd to bear, especially if they are pushed into an obstacle. As seen in Seoul, this can be fatal.

“These waves are quite dangerous because people can be squeezed against the walls and also against each other. And whenever two waves cross, people can feel the pressure from both sides,” Moussaïd said.

What to do if you get caught up in a crush

In the overwhelming majority of cases, crowded events will be safe. But Moussaïd listed some things that could help if things ever got dangerous.

The key is awareness: if you’re feeling too cluttered, you’re probably right. Quickly move away to a less dense area. It can protect you and also relieve the pressure on others.

“If only a small portion of people start doing it, it reduces the density and solves the problem,” he said.

However, once the crowd reaches this critical threshold, the surge wave can develop very quickly. So it’s a matter of survival, says Moussaïd.

“If you feel the surging wave, don’t try to resist. Follow it and keep your balance.”

Do your best to stay upright. If a person falls, it will create a wave of falls. Those at the bottom of the pile then risk being crushed by the weight of the bodies above them.

Hold your arms against your rib cage like a boxer to make breathing easier. Wave pressure can cause fainting and falling.

Don’t struggle with the flow of the crowd. If you push back, the pressure in the system will increase, making the situation worse for the next few seconds or minutes, Moussaïd said.

An overhead view of a large outdoor stage with four square quadrants for attendees

Drone footage of the empty stage that hosted the 2021 Astroworld festival where people died due to crowds.

Nathan Frandino/Reuters

Information is essential

This is not the first time that a crowd has killed. Previous examples include the Love Parade in Germany in 2010 where 18 people died and Travis Scott’s Astroworld festival in Houston last year where eight people died.

With events like these, good planning can reduce the risk by ensuring that too many people don’t gather at once.

But Moussaïd said the event in Seoul was different because it was a spontaneous gathering in the streets. It would have been very difficult to prepare.

According to Reuters, authorities expected a crowd of around 100,000 but did not believe the area required more planning than a normal Halloween weekend.

“Many people gather every year for Halloween,” said an unnamed woman who identified to Reuters as living nearby.

“But there were so many last night, incomparably more than before COVID,” she said.

As the world’s population grows and more people crowd into urban areas, this could happen more often, Moussaïd said.

“An easy solution would be to let people know that crowds can be dangerous.”

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