M 1.8 Meteorite - 0 km N of Walled Lake, Michigan

Meteor seen and heard in Detroit area. Location is approximate. The experienced sound and shaking from this event was created in the atmosphere by the meteor and not the impact of the meteorite. The shaking detected by seismic and infrasound instruments originated in the atmosphere and not from the impact. Therefore the magnitude reported for this meteor cannot be directly used to compare its size to an earthquake because the source of the seismic signals are different.

On January 18th at 8:08 PM local time hundreds of people in Michigan, and about six surrounding U.S. states and southernmost Canada, witnessed a spectacular sight in the sky. A meteor travelling at an estimated speed of 58,000 km/hr (from NASA) broke up across southeastern Michigan, creating a light show that dazzled all who were lucky enough to see it. Any large object travelling that fast creates a shockwave as it moves through the atmosphere, and the shockwave from this meteor just happened to be recorded by an array of sensitive instruments on the ground in the area.

Sensitive infrasound instruments were co-deployed with seismometers across the central and eastern U.S. as part of a National Science Foundation funded U.S. Array Experiment, and 158 of the instruments were left permanently when the rest of them moved on to another region. The normal seismometers recorded the shockwave energy that was transferred into the ground, and the measurement was the equivalent of a magnitude 1.8 earthquake. Infrasound instruments, however, measure acoustic (sound) waves in the air by sensing a change in the air pressure.

The infrasound sensors in the Central and Eastern U.S. recorded the shockwave above the ground as the meteor raced across the sky overhead. Data from this type of instrument are used to study events in the atmosphere such as severe storms, and they can be used to estimate the size and trajectory of meteors. The sensors are most sensitive in a frequency range (.02 Hz to 15 Hz) that is below the lower limit of what humans can typically hear (20Hz) and perfect for shockwaves.

The top image shows the recordings of the shockwave on sensors as it radiated through the sky and along the ground; the red line shows the estimated arrival time of the wave at each instrument. The bottom image shows location of the infrasound instruments that recorded the shockwave.