17 November 2022–Data collected by a large-N seismic array and distributed acoustic sensing (DAS) in Iceland offer one of the most detailed acoustic fingerprints of a meteoroid entering and disintegrating in the atmosphere.
The dense record, described in Seismological Research Letters, allowed the researchers to distinguish acoustic phases that are not often identified with more sparse networks, said Ismael Vera Rodriguez, formerly of NORSAR, the Norwegian Seismic Array, and now at Silixa LLC.
The acoustic data allowed Vera Rodriguez and his colleagues to develop a model that they used to place the July 2021 Iceland meteoroid within a class of slow meteoroids that end their fireball flight at relatively low atmospheric heights of about 23 kilometers above the Earth’s surface. The researchers calculate the Icelandic meteoroid was on the order of several centimeters in size—several hundred of which fall through Earth’s atmosphere each year.
Meteoroids can generate acoustic waves in several different ways as they disintegrate in the atmosphere, including the shock waves generated by the “fireball” as it deposits its ablated material in the atmosphere, and the fragmentation of the meteoroid into small pieces as it is crushed by increasing air pressure in its descent.
The researchers observed ground-coupled acoustic waves from the 2 July 2021 meteoroid using a large temporary seismic array, 50-kilometer-long DAS “dark fiber,” and several local and regional seismic stations. All stations were located within 300 kilometers of the meteoroid’s trajectory.
The network and DAS arrays were already being monitored as part of the DEEPEN geothermal project in Iceland, with the dark fibers being used to look for microseismic activity. “We were first informed by colleagues from Iceland that there was possibly a meteoroid in South Iceland, Vera Rodriguez explained. “We looked to our DAS recordings and indeed found suspicious signals which matched the suspected time of the meteoroid.”
The analysis turned up three main sets of acoustic waves distinguished by their arrival times, the researchers found. But the DAS array added even more detail to the record, detecting other sets of weaker, spatially shorter arrivals in between the main sets. These data could help explain local “earwitness” accounts such as “a loud rumble” or “motorcycle revving its engine,” and inconsistencies between reported observations such as “single blast” versus “fireworks,” depending on the geographical location of the observers, the researchers write.
The late arrival phase, one of the main three sets observed, “does not seem to have been identified before without a dense network of recording channels such as DAS,” said Vera Rodriguez. The researchers constructed a model that incorporated this late arrival phase, “based on concepts often used to describe directivity of earthquake rupture but adapted to the hypersonic speed of a meteoroid,” he added.
With the new model, Vera Rodriguez and colleagues were able to better map the trajectory of the Icelandic meteoroid and determine that the orbit range of its origin was consistent with the solar system’s main asteroid belt.