The squeeze ‘n’ shove of Mecca pilgrimages are a-mighty frightenin. Thousands of people have been died in em. Now video analysis shows that a remarkable new phenomenon called crowdquakes are behind these stampedes. The analysis  also suggests how stampedes might be prevented.

Dizzy Dirk Helbing and his mates at the Swiss Federal Institute of Technology in Zurich have been a-studyin videos of crowd panic and stampedes at the Jamarat Bridge near Mecca, where 350 pilgrims were trampled to death in 2005 (250 also died in 2004).

Conventional models of crowd behaviour assume that the velocity of the flow is smooth and that it drops to zero as the density of people rises–in other words the crowd drifts to a halt.

That ain’t what happens at all though. Dizzy Dirk reckons that at densities of more than 7 people per square meter, an individual is no longer able to control his movement and the crowd begins to move as a whole.

The density within crowds varies from place to place. So where the density is above the threshold, whatcha get is regions that take on a life of their own, a-heavin and a-shovin and a- sendin pressure waves through less dense regions of the crowd.

Now here’s the interestin bit. Dizzy Dirk says that under these circumstances, chains of people can become so tightly compressed that they become momentarily locked, like sand jamming as it passes through an hour glass. When these force chains break, they release sudden, uncontrollable amounts of energy, like earthquakes.

It is these crowdquakes that knock people off their feet causing them to be trampled. Dizzy Dirk says he seen it happen in the Jamarat videos.

The solution? Prevent crowd densities exceeding 7 bodies per square meter. In the videos of the 2006 event in which a crowdquake killed more than 350 people, Dirk says the warning signs were clearly visible 11 minutes before it occurred. That should give the authorities some warning.

Ref: arxiv.org/abs/0708.3339: Crowd turbulence: the physics of crowd disasters

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