Scenes of the deadly crowd surge in Seoul on Oct. 29 (Provided by Yukkuri JKR Channel)

The deadly human “avalanche” in Seoul was likely inevitable after the crowd reached a certain size on the narrow site of the tragedy, a professor specializing in crowd disasters said.

Toshihiro Kawaguchi at Kansai University said a “crowd avalanche” typically occurs after an overcrowded area reaches a density of 10 or more people per 1 square meter.

At that threshold, people who fall down have no chance to escape from the bodies of others piling up on top, Kawaguchi said. The unfortunate ones at the bottom will likely die under the great pressure.

He also noted that the Oct. 29 disaster in Seoul occurred on a narrow alley on a sloping path.

He compared that alley with a bridge in Akashi, Hyogo Prefecture, on which 11 people died in a crowd crush during a fireworks festival in 2001.

“The accident in Akashi occurred at a crossover bridge with a width of 6 meters,” Kawaguchi said.

“It is very dangerous when people crowd together in an enclosed space because there is no way to escape the force.”

The Itaewon district of Seoul shortly before the deadly crowd crush on Oct. 29 (Provided by a witness)

He said the alley in Itaewon, a popular nightlife district in Seoul, was more dangerous because it was only 3 meters wide and sloped. The people there were likely unable to see their feet, which further heightened their risk of falling down, he said.

The alley is near a subway station located along an arterial road and runs north-south. The alley is also connected to World Food Street, a wider area with many shops.

The slope of the alley is about 10 degrees.

The huge crowd was formed when people heading from the subway station toward the World Food Street area ran onto others moving in the opposite direction.

Kawaguchi said a video shot immediately before the accident showed people so tightly packed together that they were hardly able to move while swaying right and left.

“It is a typical warning sign for a crowd surge like an avalanche,” he said. “There is nothing that people involved can do” once such a surge occurs.

Rescue workers treat injured people on the street in Seoul on Oct. 30. (AP Photo)

In Indonesia, more than 130 people were killed in a crowd surge after police fired tear gas in a soccer stadium on Oct. 1.

In 2015, more than 2,000 people died in a stampede during the Muslim pilgrimage to Mecca in Saudi Arabia.

Toshiro Yonemura, a former superintendent general who was in charge of security details for the 2021 Tokyo Olympics, also said people trapped in such crowds can do nothing once it starts moving.

“You can take preparations by identifying narrow spaces (to later avoid) and collecting information on potential factors that can make crowds move,” Yonemura said.

He said the Itaewon area has many narrow streets, and it was likely difficult for police to keep watch on the crowds.

“I think the South Korean police did not obtain information in advance that the crowds could suddenly change,” he said.

(This article was written by Susumu Sakamoto and Yoshihiro Makino.)