At the weekend, people in the east of the Danish Baltic Sea island of Bornholm were startled by tremors. What was first thought to be a small earthquake was believed to be acoustic pressure waves, experts say. But what is their source? There are many theories.
On Saturday afternoon, the east of the Danish holiday island of Bornholm trembles. Witnesses report rattling windowsone pressure drop, strange noises, vibrations. Bottles fell from supermarket shelves. The police report that they have received several reports of the tremors and are investigating the background.
Was it an earthquake? The Geological Institute for Denmark and Greenland (Geus) already shares a few hours after the earthquake with: The tremors probably came from an earthquake in Poland. Bornholm is about 100 kilometers north of the Polish coast.
The exact origin of the quake, which had a magnitude of 2.3 on the Richter scale was given, but was unknown. The police, together with the Danish military, checked whether a military exercise in Poland was behind it. The exercise takes place there together with NATO allies “Anaconda 23” instead, which runs until May 26th.
An explosion in Poland as the cause?
The next day Geus reports: It wasn’t an earthquake, but probably sound waves from a blast. “We don’t know exactly why people on Bornholm experienced these tremors in their homes, but we can rule out that they were caused by an earthquake,” said Trine Dahl-Jensen, senior researcher at Geus, broadcaster DR Bornholm.
Their data showed that there was a major explosion in Poland shortly before 3 p.m., i.e. shortly before the Bornholm earthquake. “But that’s not something we’ve gotten confirmed,” said Dahl-Jensen.
They think the vibrations are due to an acoustic phenomenon. It is unlikely that the people on Bornholm felt any shock from a possible explosion.
“We think it’s sound that has spread. (…) But we haven’t figured out what caused the sound yet,” she told the TV station TV2.
“Acoustic pressure waves” as the cause of tremors on Bornholm?
Geus then corrected himself again on Monday. Yes, it was “acoustic pressure waves” that caused the shaking on Bornholm. But the seismologists now consider it unlikely that a blast in Poland is the source. According to Geus, the pressure waves came from one or more events in the atmosphere. But what exactly is behind it is unclear.
That started the great guesswork. Experts throw out their theories as to who or what could be responsible for Saturday’s earthquake.
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Henning Haack, geophysicist and meteorite expert, spoke in the newspaper “Berlingske” of two possibilities: either it was a meteorite or multiple sonic booms from military aircraft.
“I doubt it was a meteorite though as the area appeared to be clear on Saturday afternoon and someone would have seen a fireball – especially if it was violent enough to shake the ground.”
Experts believe military aircraft are the most likely cause
So it was military aircraft?
Flemming Christensen, lecturer in acoustics at Aalborg University, thinks this is the most likely. “My closest guess would be that it was a sonic boom from aircraft, the common feature of which is breaking the sound barrier,” he told the newspaper “politics”.
In neighboring Sweden – where the tremors were also felt in the south – fighter jets are also pointed to as the cause. At least that’s what Björn Lund, a seismologist at Uppsala University, says.
“When we get reports of rumbles and tremors, it is almost always that the Luftwaffe broke the sound barrier at sea and the atmospheric conditions were such that the sonic boom came over land.”
But which country is behind the possible sonic boom is still unclear. In any case, the Danish Air Force informed “Berlingske” that they were not active in the area at that time.
However, the weather could also have played a role.
Inversion layer in the atmosphere may have played a role
René Fleron, civil engineer and project manager at DTU Space, told the newspaper “BT”: “The atmosphere is structured in such a way that we have different layers. The higher we go, the lower the temperature. But under the right weather conditions, an inversion layer can form where pressure and temperatures change.” This could then have the effect that sound is reflected.
“You can think of it like a sound hitting a wall and going around. In other words, a sound can come from the floor and go back again. And such a sound can be heard a long way away.”
Knud-Jacob Simonsen from the Danish Meteorological Institute told the newspaper “Jylland’s Post”: “It seems that there was an inversion layer over Poland between 2,800 and 3,600 meters on Saturday. This means that if there was a loud noise from an explosion in Poland, it is very conceivable that there would have been a reflection at this altitude or something like that.”
However, that would contradict Geus, who believes the blast waves came from the atmosphere – and not from a blast in Poland that could possibly be traced back to the “Anaconda” exercise.
Whatever the cause of the shaking, it is clear that the vibrations, the rattling of windows and the drop in pressure were not imagined. “The people of Bornholm have clearly experienced something. There is no doubt about that,” says Geus expert Trine Dahl-Jensen.
Sources: geus, Denmark Radio, TV2, “Berlingske”, “Jylland’s Post”, TT news agency, “BT”, “Express”, “politics”