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Ancient submarine megaslides warn of tsunami risk

Three giant underwater landslides, which took place up to half a million years ago, have been discovered off the coast of Southern Chile.

17 October 2011

The largest of the three landslides moved nearly 500 cubic kilometres of rock material and sediment, making them some of the biggest ever found at active continental plate boundaries. They were discovered by researchers at the Leibniz Institute of Marine Sciences (IFM-GEOMAR) in Kiel and the Federal Institute for Geosciences and Natural Resources in Hanover.

‘The main hazard of such an event is the tsunami that would be generated’ says Jacob Geersen of the Leibniz Institute. ‘But there are others: slides can destroy offshore installations like cables and oil platforms, or affect populated shorelines.’

The giant landslides – which moved 253, 388 and 472 km3 of sediment – occurred at the edge of the South American plate, where the Nazca plate subducts beneath it. The slope gradient of the seafloor from the shelf to the deep sea can be as high as 30 degrees, which researchers believe was the main cause of the landslides. While slope failure is highly likely to recur in future, it is difficult to predict.

‘Giant submarine slides of this magnitude are very rare on human timescales’ says Geersen. ‘Only a few are known that occurred within the last 100,000 years. The likelihood of another in the near future is low – but we cannot rule it out.’

The landslides took place during the Pleistocene period – a time when humans were evolving into their present form. Woolly mammoths, sabre toothed tigers and cave bears also emerged during the period, which saw repeated glaciations.

‘The slides may have been triggered by one of the strong earthquakes that characterise this region’ says Geersen, ‘but we can’t be sure of that. Strong earthquakes with magnitudes close to 9 occur in the area every 100-200 years, and the time interval between these slides is 200,000 years, so earthquakes obviously do not always cause giant landslides.’

Landslides are responsible for about 15% of all tsunamis worldwide. With the data suggesting that each landslide was one event, rather than multiple failures, the Pleistocene slides would have caused very large tsunamis. Tsunami amplitudes are hard to determine after the event - but a comparatively small slide which took place in 1998 off Papua New Guinea, displacing around 5 – 10 km3, caused a local tsunami-wave of up to 15 m height which killed around 2,200 people.

The research, published in the Journal of the Geological Society on 17 October, used high frequency acoustic signals to map the seafloor, and low frequency signals to investigate subsurface structures, which enabled them to identify the landslide debris. The Leibniz Institute of Marine Sciences has carried out several research trips over the last 20 years to identify giant landslides, and hope to carry out more surveys.

‘To better understand and evaluate the hazard of submarine slides, it is important to investigate more case studies’ says Geersen. ‘We also need to perform more laboratory studies to understand conditions needed for failure, and run analog and numeric models to test different scenarios for failure.’

  • J. Geersen, D. Volker, J. H. Behrmann, C. Reichert & S. Krastel., 'Pleistocene giant slope failures offshore Arauco Peninsula, Southern Chile', Journal of the Geological Society, London, Vol. 168, 2011, pp. 1 - 12