Ripple effect.
An Etna collapse 8000 years ago spawned a huge tsunami in the Mediterranean.

Pareschi et al., Geophysical Research Letters (2006)

Tracking a Killer Tsunami

Geophysicists may have finally fingered the source of one of the largest tsunamis to strike the Mediterranean in the past several millennia. The culprit, they say, was a massive eruption of Italy's Mount Etna, which hurdled the equivalent of 10,000 Cheops Pyramids into the Mediterranean Sea 8000 years ago. Although the find is controversial, experts say the new study strengthens the link between volcanism and megatsunamis.

The Mediterranean basin is a crucible of killer waves. More than 300 tsunamis have been recorded in the last 3300 years, with volcanic activity known to have triggered a dozen in the last 2 millennia. The most recent occurred in December 2002, when a colossal chunk of the Stromboli volcano slid into the Aeolian Sea, creating a 10-meter-high tsunami that snapped moorings of oil tankers in Milazzo harbor, 100 kilometers away, but did little other damage.

That was a kiddy wave compared to the mystery tsunami, which left a trail of sediment between Sicily and North Africa. The leading suspect has been a collapse of the Santorini volcano in the Aegean Sea some 3600 years ago. However, computer simulations made by Maria Pareschi of the National Institute of Geology and Volcanology (INGV) and colleagues suggest the Santorini event was largely confined to the Aegean. "Outside that basin, the impact was negligible," says Pareschi.

Instead, the INGV researchers turned their attention to Etna, a highly active volcano on Sicily. They carried out seismic surveys in the Mediterranean and found telltale debris from a landslide spreading 20 kilometers off the island. The team carbon-dated the debris to approximately 8000 years ago. Next they mapped similarly aged mudslides that flowed hundreds of kilometers, from the Ionian Sea all the way to the Sidra Gulf off Libya. Intriguing corroborating evidence comes from an excavation at Atlit-Yam, a coastal village in present-day Israel, which appears to have been abandoned suddenly 8 millennia ago. The team reports its findings today at a press briefing in Rome and in the current issue of Geophysical Research Letters.

"This is a very careful and reasonable work," says Stephan Grilli, an ocean engineer at the University of Rhode Island, Narragansett. Not everyone agrees. Costas Synolakis, a top tsunami modeler at the University of Southern California in Los Angeles, says INGV's model uses "unrealistic" initial conditions, including an impossibly fast underwater velocity of the Etna collapse.

Not in dispute is the notion that volcanism could spawn future megatsunamis. Last year, scientists at the Benfield Hazard Research Center in London warned that a massive collapse of Cumbre Vieja, a volcano in the Canary Islands, would trigger a towering tsunami that could pummel coasts on both sides of the Atlantic, deluging cities such as New York, Miami, and Lisbon.

For more in-depth coverage of this story, stay tuned for the 8 December issue of Science.

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