The Tragic Physics of the Deadly Explosion in Beirut

Shocks, because of their fascinating and complex physics, travel faster than the speed of sound, and cause far more damage than pressure waves. Thankfully, we know this blast did not produce a shock because the speed of the water vapor-filled white dome can be measured.

The speed of sound in air is 343 meters per second. Based on the viewing angle and distinctive red chairs pictured in some of the later frames, I traced one of the Beirut videos posted by The Guardian to its filming location on the rooftop terrace of La Mezcaleria Rooftop Bar, and measured it to be 885 meters from the center of the blast. From that vantage, the pressure wave can be seen neatly traveling from the center of the blast first to the point halfway between the end of the pier and the edge of the long, massive grey grain silo building, a distance of 151 meters, then to the end of the pier, 262 meters, then eventually to La Mezcaleria.

By measuring the times at which the pressure wave reaches these landmarks on the video, we know that, as it blazed down the pier, its rampage occurred at a speed of only 312 meters per second. That’s slow for a bomb. Then by the time the audible crash and mayhem reached the formerly peaceful and picturesque outdoor bar, it had slowed to at most 289 meters per second. The pressure wave, slower than the 343 meters per second of sound, caused destruction, horror, confusion, shattered glass, torn apart flat surfaces, and disorientation for onlookers as their ears were subjected to the rapid pressure fluctuations. But a shock wave could have caused them to drop dead from lung trauma as they watched.

In the six million pound Halifax explosion of 1917, the propagation of the shock wave through downtown left a swath of fatalities reaching 1.5 miles from the center of the blast, killing an estimated 1,950 and leaving another 8,000 with devastating injuries. (The ships that exploded in the harbor were known to be carrying high explosives, which by their nature always make shock waves.) In Beirut, thankfully, while building damage has been reported up to 5.6 miles away, because the low-explosive ammonium nitrate made a pressure wave rather than a shock wave, the fatality estimates so far are still in the hundreds, even though the charge size was likely larger than the bomb in Halifax.

Thanks to modern technology that charge size can be calculated scientifically too, even while waiting for more complete information to trickle out, using the size of the telltale crater. Analysis of the aerial photographs of the pier shows a crater in the range of 120 to 140 meters in diameter; blast physics mixed with history tell us that to carve a chunk that size from the side of the planet requires a charge equivalent to 1.7 to 5.4 million kilograms of TNT (that’s 3.8 to 11.8 million pounds for any Americans dragging their feet on converting to metric). For reference, the bombing of the Murrah Building in Oklahoma City in 1995 used the equivalent of 1.8 thousand kilograms of TNT. So, Beirut was at minimum a thousand times more boom than Oklahoma City.

As an aside, nuclear weaponry is set to detonate several hundred feet above ground level, and therefore doesn’t exert enough force directly on the soil to create a crater. The detonation of the first atomic weaponry above Hiroshima occurred almost exactly 75 years ago to the day, and despite its historically unprecedented trauma to the building and populace, it left behind no crater.

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Germany, too, knows the destructive power of improperly stored ammonium nitrate, and an accident in that country reinforces the calculation of the charge size. In 1921, a fertilizer explosion in Oppau, Germany, carved a remarkably similar crater. At 120 meters in diameter, following the explosion of 4.1 million kilos of ammonium nitrate, the size of the Oppau crater supports the idea that the Beirut pier, which early reports said held only 2,750 metric tons—2.75 million kilograms—may have held some number of millions more kilograms of charge. However even using only those 2,750 metric tons, Special Agent Wojtylak says his preliminary calculations indicate that those safe from all risk of carnage would have needed to be at least 15 kilometers from the Beirut pier.

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