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Scientists uncover evidence of prehistoric 'megatsunami'

Researchers have found evidence of an 800-foot wave, caused by the collapse of a volcano in the Cape Verde Islands some 73,000 years ago.

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    A tsunami generated by a volcanic flank collapse is believed to have swept huge boulders like this one into the highlands of Santiago island. Here, a researcher chisels out a sample for dating.
    Ricardo Ramalho
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Scientists have found evidence of an unprecedented “megatsunami” off the Cape Verde Islands that occurred some 70,000 years ago. 

Caused by sudden volcanic collapse, the 800-foot wave would have engulfed what is now Santiago Island, some thirty miles away. That estimate, which was published today in Science Advances, could prompt scientific community to re-evaluate the threat of catastrophic collapses near coastal communities.

When volcanoes collapse, the resulting landslides can cause tsunamis of varying severity. Previous research proposed a gradual model for volcanic breakdown, which would result in multiple smaller waves.

But a series of van-sized boulders, found nearly 2,000 feet inland, suggest otherwise. Lead author Ricardo Ramalho noted that the boulders were composed of marine rock, while the surrounding terrain was made of young volcanic rock.

These shoreline boulders were most likely deposited by a massive wave, Dr. Ramalho and colleagues argue. They calculated the height of the wave based on the weight of the rocks, many of which weigh several hundred tons. 

“We were all very surprised by the findings,” Ramalho says, “especially because we found them by chance. When we realized the potential implications of these findings, we were naturally excited (academically speaking). But also respectful of what this represents in terms of hazard.”

Ramalho stresses that, while megatsunamis are devastating, they are also incredibly rare.

“These are what we scientists call ‘very low frequency, very high impact events,’” Ramalho says. “Due to their very low frequency, we estimate that the probability for them to happen again is very small. But they may and will happen nevertheless, at some point in time. And since their impact can be absolutely devastating, we need to be vigilant and improve our society’s resilience to their possible occurrence.”

Like most natural disasters, tsunamis cannot be prevented by technology. But there are ways to improve our resilience to them, Ramalho says. Further research, particularly on what triggers flank collapse, could help society to better understand the hazards associated with these events.

Meanwhile, improved monitoring networks could provide additional warning time before a collapse. While some of these networks are able to detect volcanic unrest, many aren’t designed to pick up on the ground deformation that occurs prior to flank collapse.

But perhaps most importantly, a thorough response plan is essential to minimizing damage in the wake of natural disaster.

“We need to start thinking, coolly and rationally, what can be done in terms of disaster risk reduction,” Ramalho says. “How may we respond to such a crisis, and what measures can be taken at short, medium, and long term to increase our society’s resilience to their threat? For example, better territorial and urban planning help in mitigating the effects of natural catastrophes in general, and this would not be an exception – and that can be implemented at any time.”

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