Deep inside a limestone cave on the Sumatran coast, a team of researchers have unearthed a natural timeline of the biggest tsunamis to hit the region over the last 7,500 years. They hope that the discovery will shed light on the frequency of major disasters like the 2004 Indian Ocean earthquake and tsunami that left more than 200,000 dead in 14 countries.
The team is led by Charles Rubin, who heads the Division of Earth Sciences at the Earth Observatory of Singapore.
Work in the cave, located near Banda Aceh, is far from glamorous. The tsunami record was created by a combination of sediment being washed into the cave by large waves and creating layers between existing guano deposits – thousands of years’ worth of bat droppings.Enjoying this article? Click here to subscribe for full access. Just $5 a month.
“The tsunami sands just jump right out at you because they’re separated by guano layers. There’s no confusing the stratigraphy,” Jessica Pilarczyk, a member of Rubin’s team, told BBC. “It makes for interesting field work; I’m not going to lie to you. The bats get very excited when people are disrupting their space. But from a geologist’s point of view, this cave has the most amazing stratigraphy.”
The stratigraphy, or layering, was uncovered by digging trenches in the base of the cave. Its raised entrance, which is situated approximately 100 meters from the high tide area, ensures that only the largest waves contribute a new layer of sand. The researchers estimate that between 7 and 10 tsunamis contributed to the stratigraphy between 7,500 and 3,000 years ago.
The date of each layer is estimated using radiocarbon analysis of organic debris caught in the sandy layers – including sea creatures and bits of charcoal from human-lit fires.
The cave’s geometry means that only tsunamis generated by magnitude 8 or higher earthquakes would be large enough to enter – the 2004 tsunami was sparked by a magnitude 9.2 “megathrust” quake that was estimated to have released as much energy as 23,000 atomic bombs. The researchers added that such a severe event likely hadn’t occurred in more than 500 years.
“2004 caught everybody by surprise. And why was that? Because nobody had been looking back to see how often they happen, if they’d ever happened,” said co-investigator Kerry Sieh. “In fact, because people thought they had no history of such things, they thought it was impossible. Nobody was prepared; nobody had even given it a second thought. So the reason we look back in time is so we can learn how the Earth works and how it might work during our watch.”