COPENHAGEN, Denmark (AP) – A series of minor tremors recorded on the Danish Baltic Sea island of Bornholm on Saturday have puzzled scientists who now say they were caused by “acoustic pressure waves from an unknown source”.
At first it was assumed that the tremors were caused by earthquakes. Then seismologists theorized that they came from controlled explosions in Poland, more than 140 kilometers (nearly 90 miles) south.
On Monday, the Geological Survey of Denmark and Greenland, an official body that monitors the subsoil, said the tremors were caused “not by earthquakes but by pressure waves from an event in the atmosphere.” However, they came from an “unknown source”.
“The seismologists can report that the tremors are unlikely to have come from a controlled explosion in Poland that took place shortly before the first reports of tremors on Bornholm,” the organization known as GEUS said in a statement.
On Saturday, GEUS said it had received “more than 60” tips from people on Bornholm that “earthquake-like tremors” – described as deep rumbling, shaking and rattling, alternating pressure in the ear – were reported on Bornholm in the afternoon.
Nobody was hurt. Police said they too have been contacted by the public about the quake in the eastern part of the island. Danish media reported that the tremors caused a crack in the wall of a house.
According to GEUS, seismic tremors were measured with a magnitude of 2.3.
Polish authorities said that during exercise Anakonda23 in Ustka, northern Poland, there was intense activity with jet fighters and live artillery shelling.
GEUS, an independent research and advisory body at the Danish Ministry of Climate, Energy and Utilities, said it has two seismographs on Bornholm that collect data 24/7.
Bornholm, home to almost 40,000 people, is a rocky island in the Baltic Sea, south of Sweden, north-east of Germany and north of Poland.