Those lucky enough to look skyward over parts of Michigan yesterday at around 8.10pm local time would have been fortunate enough to see a meteor streak through the sky. It quickly encountered a layer of the atmosphere about 80 kilometers (50 miles) up, the mesosphere. The air in front of it became highly compressed, heated up, and caused its outer layers to ignite, which lit up the night sky.
Some thought it might have been a weather-based phenomenon, but no: the National Weather Service out of Detroit explained that “the flash and boom was NOT thunder or lightning, but instead a likely meteor,” something that was later confirmed.
As reported by the United States Geological Survey (USGS), and as picked up by meteorologist Eric Holthaus, the meteorite also generated quite the shockwave when it broke up in the atmosphere. Consequently, this registered as a 2.0M tremor on seismometers – a type of pseudo-earthquake, but definitely not a real one.
Bonafide earthquakes require tectonic movement. When two segments of the crust suddenly move past each other across a fault plane, that’s an earthquake.
Seismographs pick up any kind of shaking caused by anything large enough, though: from nuclear weapon tests over in North Korea to powerful hurricanes drifting over the Atlantic Ocean, these all rock the crust to some degree to cause a tremor. Even jumping football fans triggered a recent alert over in earthquake-prone Peru.
They’re not really earthquakes though, and their seismic signals are all distinct. Geophysicists know the difference between an earthquake’s signal and a nuclear blast’s.
Meteors that burn up particularly violently in the atmosphere, then, can also be added to this fake earthquake list. The Detroit firework wasn’t the first of its kind, not by a long shot.
Remember that far larger meteor – and eventually meteorite, when its remnants hit the ground – over in Chelyabinsk back in 2013? This object, 20 meters (66 feet) across and moving at 19 kilometers (12 miles) per second, burst apart in the rarefied air with the energy of about 500,000 tonnes of TNT. The subject shockwave registered as a 2.7M event on the USGS’s seismographs.
Incidentally, some are referring to the Michigan meteor as a bolide – meaning “missile” – which, according to astronomers, means a meteor that’s burning so brightly that its flash can be seen for miles around. Weirdly, geologists sometimes refer to a bolide as a particularly large meteorite or asteroid whose collision with Earth’s surface creates a fairly sizeable impact crater, like the one that finished off the landlocked dinosaurs.
So just in case you’re wondering: Detroit stands. It has not been squashed by a spaceborne horror.
By the way, that meteor now has its own Twitter account. What a time to be alive, eh?
Update: It seems an angry Twitter user complained about the use of the word "earthquake" to NWS Detroit, who responded thusly.