The Megatsunami: Possible Modern Threat

SAN FRANCISCO -- Volcanic landslides that generate huge and devastating tsunamis tend to occur during historically warmer times on Earth, a new study suggests. Scientists don't know exactly why, but since the global climate is warming as you read this, the apparent connection was tossed out this week as a reason for scientists to be concerned about the threat now.

Tsunamis are waves that race across the ocean without much fanfare but grow to frightening proportions when they reach land. The waves are deep, and while they may appear just a few inches or feet tall on the open ocean, they can soar to the height of a multi-story building as they are forced upward near the shore.

A tsunami can be generated by the sudden uplift of the seafloor in an earthquake, or by the paddle-like effect of a landslide crashing into the sea from, say, an island volcano. Yet while quake-generated tsunamis have been observed from their genesis to the disastrous end, scientists have never witnessed a significant open-ocean tsunami generated by a landslide.

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Evidence exists at various locations around the world for megatsunamis, as scientists call the largest of these events. They seem to occur every 100,000 years or so, said Gary McMurtry of the University of Hawaii.

These monsters can be hundreds of feet tall and, depending on local topography, race miles inland.

One controversial event, about 110,000 years ago, appeared to create a 1,600-foot wave in Hawaii. Yes, you read that right: Nearly one-third of a mile, or about half a kilometer.

But the evidence -- marine fossils way up there where there's no sea -- is controversial. Perhaps the islands have been rising and carried the fossils up, critics suggest.

McMurtry's team looked at marine fossils at the Kohala volcano on the main island of Hawaii, which is known to be sinking about an inch per decade. The fossils simply could not have started at a lower elevation, McMurtry said Monday at a meeting of the American Geophysical Union held here. A submarine landslide from the giant Mauna Loa volcano has been dated to the same time and, the thinking goes, caused the tsunami.

McMurtry and his colleagues also re-examined evidence for a tsunami that may have struck Bermuda and other locations in the Atlantic 420,000 years ago.

Scientists agree that submarine landslides caused by the collapse of island volcanoes -- think of the destruction of Mount St. Helens -- could generate these megatsunamis. Evidence for such landslides can be found in topography scans of seafloors around various island volcanoes, McMurtry points out.

"These giant landslides seem to occur during periods of higher than normal sea level -- like we have now," he said.

High sea levels tend to correspond with wetter climates, he said. What this has to do with landslides is not known. But perhaps, McMurtry figures, excess rainfall can serve as a trigger for the cleaving of a volcano-in-waiting.

That might all sound like a lot of logic leaps, and McMurtry is the first to admit there isn't enough data to figure out whether global warming and tsunamis are correlated. But there is some independent thinking that supports the notion.

Peter Cervelli, of the Alaska Volcano Observatory, has studied the Hawaiian volcanoes and is not involved in McMurtry's work. Cervelli said it's possible that water during extended wet periods seeps down into natural faults on the flanks of a volcano -- volcanoes are known to be more porous than other land areas -- precipitating a collapse by "bringing it closer to failure."

And in other work, Emily Brodsky of the University of California, Los Angeles has modeled the friction involved in huge volcanic landslides. She agrees that it's possible that higher rainfall amounts could make a precarious situation more slippery.

So should we worry? "Maybe," says McMurtry. He thinks that a tsunami, which can race across an entire ocean in a matter of hours, is a real threat to urbanized coastlines. Other experts agree that a large tsunami would be bad news for, say, Los Angeles or New York City. And tsunamis are not parochial. One originating in Alaska in 1964 killed people in California and generated damaging surges clear down in Chile.

McMurtry believes the threat is greater than from an asteroid impact, but asteroid research has managed to lure more funding. More money should be spent to monitor the stability of oceanic volcanoes, McMurtry argues.

"Mauna Loa is as big as it's ever been, so the energy is there" for a giant submarine landslide, McMurtry said. He's even attached some odds to the threat: "The probability of a megatsunami in Hawaii in the next 10,000 years is about 50 percent."

How Tsunamis Work