Confusion over cause of tremors on Danish island off Poland’s coast

There is ongoing confusion over the cause of “earthquake-like” tremors that struck a Danish island off Poland’s Baltic coast over the weekend. While some have claimed that military exercises taking place in northern Poland were to blame, Danish authorities are still investigating the cause.

According to Danish state broadcaster TV 2, on Saturday 13 May at around 3:15 p.m., tremors and a rumbling sound were reported by residents of Bornholm, an island in the Baltic Sea about 95 kilometres (59 miles) from the Polish port city of Kołobrzeg.

The tremors caused windows to vibrate and cracks to appear in the walls of some homes. No injuries were reported in relation to the incident.

Some residents initially feared that another explosion around the Nord Stream gas pipelines, which run along the seabed near the island, had occurred.

But on Saturday evening, TV 2 reported that, according to the National Geological Survey of Denmark and Greenland (GEUS), the tremors were probably caused by an earthquake in northern Poland, the epicentre of which was estimated to be 150 kilometres (93 miles) south of Bornholm.

TV 2 also interviewed Bornholm deputy police inspector Hans-Henrik Dam on Saturday evening, who revealed that the Danish police and armed forces were “investigating whether the tremors could be related to military exercises in Poland.”

Meanwhile, the Polish Geological Institute (PIG) also confirmed that a seismic anomaly of roughly 2.3 magnitude was recorded at two seismic stations in northern Poland at around 3 p.m. on Saturday.

This led to initial speculation that large-scale military drills, titled Anakonda 23, taking place in northern Poland during the time of the reported tremors may have been to blame. The training ground being used for the military exercise was roughly 125 km (77 miles) from Bornholm.

Speculation heightened on Monday when PIG announced that, according to information received from the Polish defence ministry, “on 13 May 2023, at the Central Air Force training ground in Ustka (between 2 and 4 p.m.), as part of the Anakonda 23 exercise, the air force, ground troops and navy carried out intensive firing.”

This led PIG to suggest that, as the tremors recorded in Poland were uncharacteristic of seismic shocks “in the geological sense” and could have been caused by human activity, “explosions associated with military exercises may have been the source” of Saturday’s minor seismic event in northern Poland.

More than 12,000 Polish soldiers and several hundred soldiers from NATO and partner countries have been taking part in the military exercise, which is organised by the Polish armed forces. The exercise seeks to test capabilities in response to threats in the Baltic Sea region and will run until 26 May.

The exercise involves the use of F-16 fighter jets, K9 howitzers, and WR-40 Langusta rocket launchers, among other equipment.

However, later on Monday, GEUS released a statement in which it retracted the previous theory and hypothesised that the tremors on Bornholm had not been caused by earthquakes, but instead by “acoustic pressure waves from one or more events somewhere in the atmosphere”.

They revealed that, while their seismologists had located a tremor in Poland – which appeared to be a minor blast – about 20 minutes earlier on Saturday, it had been “ruled out” as having triggered the tremors in Bornholm.

The statement also announced that it is “not possible for GEUS seismologists to speculate on what produced the powerful pressure waves”, as acoustic waves in the atmosphere are outside the scope of their work.

While GEUS has seemingly ruled out the training exercise in Poland as the explanation for the tremors, the head of emergency management at the Danish Meteorological Institute has speculated that they could have been caused by acoustic sound waves from the Polish military drills.

Knud-Jacob Simonsen told Danish newspaper Jyllands-Posten that a particular weather phenomenon, known as a temperature inversion layer, occurred over northern Poland on Saturday afternoon.

“Normally, the temperature drops the higher up in the atmosphere you go, but occasionally a layer of air forms where the temperature is constant or rising at a certain altitude. This layer is called an ‘inversion layer’ and can act as a ‘lid’ that reflects sound waves and sends them back down towards the ground,” he was quoted as saying.

“It seems that there was such an inversion layer over Poland between 2,800 and 3,600 metres on Saturday. This means that it is very conceivable that there would have been a reflection at that height if there had been a loud sound from an explosion or the like in Poland,” continued Simonsen.

The Danish authorities continue to investigate the exact cause of the tremors.

Main image credit: Lilly M/Wikimedia Commons (under CC BY-SA 3.0)

Anna Hackett

Anna Hackett is an assistant editor at Notes from Poland. She is a recent graduate of European Studies from Trinity College Dublin and has had previous journalistic experience with the Irish Independent News & Media group.