The explosions that devastated Tianjin yesterday were so powerful, they registered as seismic activity by China's National Earthquake Network. And the "quakes" geophysicists saw don't even begin to capture the magnitude of the blasts.
Yesterday, two explosions originating at a warehouse owned by Tianjin Dongjiang Port Rui Hai International Logistics Co. ripped through the port city of Tianjin, smashing buildings and cars and leaving an apocalyptic scene in their wake. The first explosion was enormous, and the second was even bigger: the China Earthquake Networks Center is comparing it to 21 metric tons of TNT. A hundred miles northwest in Beijing, a seismometer station picked up the blasts as the equivalent of magnitude 2.3 and 2.9 earthquakes on the Ritcher scale.
Seismometers in Beijing detected the second Tianjin explosion as the equivalent of a 2.9 earthquake. Image via USGS
Now, this doesn't mean the explosions actually caused earthquakes: A seismometer simply measures motion in the ground due to propagating waves of energy. We typically use seismometers to detect tectonic activity, but they also register plenty of human technologies, including dynamite, airguns, and sonar. According to John Bellini, a geophysicist with the USGS, seismometers pick up man-made explosions in quarries all the time.
"We record dozens of them [explosions] every day in quarries and around world," Bellini told Gizmodo. "Usually they're in a known region, and they do look different from an earthquake on a seismometer."
Part of the reason quarry explosions look different from quakes, Bellini says, is that they're typically not point-source events. Instead miners will drill a bunch of boreholes over a couple acres and scatter explosives throughout them. This helps to minimise the overall impact of the explosion and prevent damage to the surrounding region.
What's interesting about the Tianjin explosions is that these were point source events, and what's more, they didn't take place underground. Usually, we can't detect surface explosions very well, because most of their energy dissipates into the air. The fact that the Tianjin explosion is registering on seismometers, and with as much energy as a small-sized earthquake, speaks to just how monstrously powerful these explosions were.
Smoke billows from the site of an explosion that reduced a parking lot filled with new cars to charred remains at a warehouse in northeastern China's Tianjin municipality, Thursday, Aug. 13, 2015. Image via Ng Han Guan / AP
"Imagine where the blast occurred: right at the surface," Bellini said. "Most of the force is going to be blown upward away from the ground. Whereas a quarry blast, for instance — generally the explosion is buried in the ground or in an underground mine. It takes a lot smaller explosion in a mine blast to be seen on a seismometer as the same size event."
"A quarry blast with this much explosive energy would probably register as an upper 3 to a low 4 quake," Bellini continued. "The larger quarry blasts I've seen are in the low 4 range, and that happens several times a year in quarry mines."
Fire fighters in protective gear watch partially pink smoke continue to billow after an explosion at a warehouse in northeastern China's Tianjin municipality, Thursday. Image via Ng Han Guan / AP
In other words, the modest-sized blip geophysicists detected at Tianjin doesn't capture the full force of the explosion. In fact, determining the total amount of energy released by explosions like these isn't a very easy thing to do. First, because our units of measure suck: We should be measuring explosions like this in joules of energy rather than tons of TNT. Second, because different types of explosions generate different types of waves, which affects how the energy is propagated and how much damage the blast ultimately inflicts.
The best way to appreciate the magnitude of the explosion might be the human toll: So far, there have been 50 confirmed deaths and more than 500 hospitalizations. Hollowed out building shells litter abandoned streets, an acrid chemical odor lingers in the air, and fires continue to rage.
Suffice it to say, this was a pretty nasty accident.
Top image: Charred remains of a warehouse and new cars are left burned after an explosion at a warehouse in northeastern China's Tianjin municipality, Thursday, Aug. 13, 2015, via Ng Han Guan / AP