Ten-Story Tsunami Thrashed Mediterranean 8,000 Years Ago

Thursday, November 30, 2006

By Ker Than


A volcano avalanche in Sicily 8,000 years ago triggered a devastating tsunami taller than a 10-story building that spread across the entire Mediterranean Sea, slamming into the shores of three continents in only a few hours.

A new computer simulation of the ancient event reveals for the first time the enormity of the catastrophe and its far-reaching effects.

The Mt. Etna avalanche sent 6 cubic miles of rock and sediment tumbling into the water — enough material to cover the entire island of Manhattan in a layer of debris thicker than the Empire State Building is tall.

The mountain of rubble crashed into the water at more than 200 mph. It pummeled the sea bed, transformed thick layers of soft marine sediment into jelly and triggered an underwater mudslide that flowed for hundreds of miles.

Recreating an ancient disaster

To create their computer simulation, researchers at the National Institute of Geology and Volcanology in Italy used sonar-equipped boats to survey seafloor sediment displaced by the Mt. Etna avalanche.

Their recreation suggests the tsunami's waves reached heights of up to 130 feet and maximum speeds of up to 450 mph, making it more powerful than the Indonesian tsunami that killed more than 180,000 people in 2004.

The researchers have also linked the ancient tsunami with the mysterious abandonment of Atlit-Yam, a Neolithic village located along the coast of present-day Israel.

When archeologists discovered the village about 20 years ago, they found evidence of a sudden evacuation, including a pile of fish that had been gutted and sorted but then left to rot.

"A tsunami was not suspected before," lead researcher Maria Pareschi told LiveScience.

Could happen today

According to Pareschi, if the same tsunami struck today, southern Italy would be inundated within the first 15 minutes.

In one hour, the waves would reach Greece's western coasts.

After an hour and a half, the city of Benghazi in Northern Africa would be hit.

At the three-and-a-half-hour mark, the waves would have traversed the entire Mediterranean to reach the coasts of Israel, Lebanon and Syria.

Avalanches and minor eruptions still occur on Mt. Etna today, but so far, nothing approaching the magnitude of the ancient event.

"Should the Neolithic Etna tsunami have occurred today, the impact is tremendous because the Eastern Mediterranean coasts are very inhabited ones," Pereschi said.

Once the tsunami reaches shore, how long does it remain 130' high and how far inland does one go?

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That would depend on the terrain and how fast it hit the land. If it was moving 300 mph and hit, say, Puget Sound, it would travel inland about 20 miles, since there are very few hills and not much high ground in the area. The same tsunami, hitting San Francisco, wouldn't travel as far inland, but it would be enough to totally wipe out the city. If it hit where I'm moving, it would travel about 15 miles inland, but after about the first 3 or 4 miles, it would channel through and follow the valleys, since the cascade range would be acting as a natural water break. It WOULD, however, remove all the trees, bushes, dirt, homes and so forth in its path, just as if a giant reached down and scooped it up.

Interestingly, this is not the largest tsunami on record. That honor is held by a tsunami that occurred in a Fjord type bay. The cause was the same as in this story...an earthquake dislodged a great deal of rocks and dirt which fell into the bay. The resulting tsunami swashed against the other side of the bay, then rebounded and splashed back against the original wall. Measures taken a few years ago (since the damage can still be seen even though that happened so long ago) showed that the wave was a little over 1,100 feet high.

How could a 130' wave in the ocean make the turn into Puget Sound. I could see how it could stir up action in Puget Sound, but I don't think it would have the same height and velocity.

It looks like the Olympic Peninsula from the ocean to Puget Sound ranges from 50 to 80 miles across.

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The front of a tsunami will range from 300 to 500 miles long, occasionally longer, so there are few harbors that are 'safe'. Also, it isn't the height of the wave that is the killer. There is an enormous mass of water behind and under it, and that is where it's power comes from. When such a wave approaches relatively shallow water, like all the way down the Pacific coast, that power is seen visually by the wave crest. The area around Italy is very deep, and in that deep water, the wave crest was still 130 feet. On the Pacific coast, the same wave would most likely end up being about 250-300 feet high, with the majority of the power pushing it in, so it would continue until either it hit an obstacle, or the power gradually dissipated. To simply dissipate will take a LONG distance. So if it hit the Columbia, as a for instance, Portland would suffer enormous damage, though they aren't really all that close to the coast.

It is exactly for the same reason that the tsunami in 1964 that was generated in Alaska totally destroyed Crescent City California. And that tsunami was nowhere near as big as the one that this article is talking about.

Incidentally, the tsunami of 1964 generated wave heights along the Oregon coast of around 44 inches. The wave height that destroyed Cresent City was only about 38". That may not seem like much, but as it came in, pushed by a huge amount of force, the wave actually raised substantially when it reached shore. So that 3 foot wave of water may have easily topped 20 feet by the time it actually was on shore. Tsunami wave heights are generally measured at the shore line.

I want to correct myself. The wave at Cresent City was 41.77 inches in height, not 38 inches.

Thank you for the interesting post!

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