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Traces of a giant landslide deep in the southern Baltic Sea—researchers investigate a potential tsunami

Off the coast of Blekinge, in southern Sweden, at a depth of around 50–60 meters, the bottom sediment has scars that are tens of kilometers long and appear to have been caused by a huge submarine landslide. However, what caused this landslide and exactly when it may have occurred are unknown. The Geological Survey of Sweden investigated the bottom layers and documented the scar in 2014, and now Elinor Andrén, professor of environmental science at Södertörn University, and nine other researchers will try to discover what happened.

"When we studied sediment from the seabed off Blekinge we noticed anomalies. The sediment is like a history book, but the pages here are in completely the wrong order. Different types of layers are mixed up, and the order we were expecting to find them in simply doesn't exist. The question we want to answer is whether this is due to a tsunami," says Andrén.

"The hypothesis we will test is that around 8,000 years ago, a submarine landslide caused a tsunami that affected the coastal areas of the southern Baltic Sea. This theory is not really far-fetched, as a similar event occurred off the coast of Norway. The area's bedrock also has weaknesses that may cause earthquakes, which can lead to landslides. Something must have caused the landslide on the seabed," says Andrén.

The project covers three main areas, two of which focus on what may have happened and its consequences for coastal areas. The third will examine whether similar events could occur today and, if so, how this would affect the region's inhabitants and infrastructure.

"There are indications of landslides occurring in several other places on the bottom of the southern Baltic Sea, but we don't know whether they occurred over a long period of time or at around the same time. If it turns out this has happened on several different occasions, it could mean that landslides—and tsunamis—could even happen in our time," she says.

Rapid rises in sea level, for whatever reason, can affect infrastructure along our coasts and in the oceans. Landslides and earthquakes can also damage cables and pipelines on the seabed. The research team will therefore also assess the risks that could arise due to sudden and significant rises in sea level.

Elinor Andrén and her colleagues have been mulling over ideas about a tsunami in the southern Baltic Sea for almost 20 years. Time and again, the researchers have encountered sequences of sediment in areas that are significantly different to what would normally be found. Also, the geologists are not alone in perhaps finally being able to understand finds that have not yet had a proper explanation.

"Archaeologists studying archaeological remains on Blekinge's coast were enthusiastic about the idea of a sudden catastrophic event. One village had been more or less completely wiped out, entirely covered by sand and sediment that cannot be explained by natural coastal processes. A tsunami could be one reason for this," she says.