Bright meteor sets off earthquake sensors in Michigan
Friday, January 19, 2018, 12:24 PM - Residents of the US Midwest and southwestern Ontario were treated to a spectacular event, and a bit of a shake-up on Tuesday night, as a bright meteor fireball exploded north of Detroit, registering as a magnitude 2.0 earthquake.
Just after 8 p.m. EST on Tuesday, a bright fireball streaked across the night sky in southeast Michigan, turning the darkness to near daylight for just a few moments. As of Wednesday afternoon, nearly 400 reports of the event had shown up on AMSMeteors.org - the website of the American Meteor Society, who routinely catalogue fireball sightings by witnesses around the world.
The observers map for the January 16, 2018 Michigan fireball. Credit: AMSmeteors.org
"This was a very slow moving meteor," Vincent Perlerin of the AMS wrote on their website, saying that it was likely travelling at a speed of around 45,000 kilometres per hour (or about 28,000 miles per hour). "This fact, combined with the brightness of the meteor (which suggests a fairly big space rock), shows that the object penetrated deep into the atmosphere before it broke apart."
On their Facebook page, NASA Meteor Watch said that they agreed with the AMS's conclusions, and estimated that the space rock was probably a little over a meter across.
Bill Cooke, of NASA's Meteoroid Environment Office, told Detroit News the same, and that it likely tipped the scales at about one tonne.
Remarkably, both Perlerin and NASA Meteor Watch reported that several witnesses reported hearing sounds from the asteroid as it exploded, and the shockwave produced by the explosion actually registered on seismic sensors in Michigan, as a magnitude 2.0 earthquake.
United States Geologic Survey earthquake map shows the M2.0 'quake' set off by this bolide. Credit: USGS
So, this was not an actual earthquake, generated by the asteroid hitting the ground. The seismic instruments simply picked up the pressure wave that travelled through the air from the point of the explosion to the ground.
Professor Peter Brown, from the University of Western Ontario's Western Meteor Physics Group, posted to Twitter, showing the infrasonic signal that was picked up by the university's detectors at the time of the meteor explosion.
While the explosion produced an audible boom, which was heard in the direct vicinity of the event a few minutes after the flash went by, the bolide also generated waves of infrasound. This very low frequency sound is beyond the limit of human hearing, but it can carry for much farther distances than higher frequency sound waves. These signals are very useful for detecting meteors (and nuclear detonations), and meteor scientists set up special instruments to detect them.
Not rare, but rare that we saw it
It's estimated that several tonnes of rock and ice fall from space every day, as Earth sweeps up these remnants of the the solar system's formation on its travels around the Sun.
Most of this debris is made up of very tiny grains, which are often missed, even though they can produce meteor streaks across the sky. Meteoroids the size of pebbles or gravel can easily produce bright fireballs as they travel at tens of thousands of kilometres per hour through the atmosphere. Larger pieces, like the one over Michigan on Tuesday night, are more rare, with maybe one falling somewhere on Earth every day. With these rocks falling randomly across Earth's surface, most are missed, as they fall into the oceans, or into very remote regions of the planet. Thus, it is somewhat rare that we saw this one.
Even larger chunks of space rock only make it so far into the atmosphere before they explode, raining smaller debris down over the area. The main concern with these is the "airburst" - the shockwave set off by the asteroid exploding - which can shatter windows and cause injuries. It takes a very large asteroid, or one made of the toughest materials (nickel-iron), to penetrate all the way to the ground for a direct impact.
Even the 2013 Chelyabinsk meteor, which was likely around 20 metres across and weighed around 13,000 tonnes, did most of its damage from the airburst as it exploded in the air. It produced many meteorites, the largest of which was about 60 centimetres across and weighed around 300 kilograms. That chunk was recovered from nearby Lake Chebarkul, and it would have likely produced a small hole in the ground had it struck land, instead.
Did this produce meteorites?
With an asteroid of this size exploding in the atmosphere, the experts are saying that it's very likely that pieces of it survived to hit the ground. That means it's possible that meteorites could be found somewhere to the northwest of Detroit.
According to NASA Meteor Watch, "one of our colleagues at [Johnson Space Center] has found a Doppler weather radar signature characteristic of meteoritic material falling to earth."
Doppler radar, normally used to track precipitation and storm motion, has become a valuable tool for meteorite hunters in recent years. For pieces of space rock to show up on a radar scan requires excellent timing, on the parts of the space rock and the radar station, but this has already been used several times to track the location of meteorite falls.
Meteorite hunters will likely be tracking down the fragments of this space rock, if they are not already on the scene now. For anyone who happens to find meteorites from this fall, or any other, these rocks are perfectly safe to touch, but be aware of the laws of the area that you are hunting. For example, in the United States, meteorites belong to the owner of the land where they are found.