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Sumatra coastal cave records stunning tsunami history

_71685558_photo_1By Jonathan Amos

A cave on the northwestern coast of Sumatra holds a remarkable record of big tsunamis in the Indian Ocean. The limestone opening, close to Banda Aceh, retains the sandy deposits washed ashore by huge, earthquake-induced waves over thousands of years.

Scientists are using the site to help determine the frequency of catastrophes like the event of 26 December 2004. This is being done by dating the cave’s tsunami-borne sediments, which are easy to see between layers of bat droppings.

“The tsunami sands just jump right out at you because they’re separated by guano layers. There’s no confusing the stratigraphy (layering),” explains Dr Jessica Pilarczyk.

“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,” she told BBC News.

Dr Pilarczyk was speaking here in San Francisco at the American Geophysical Union (AGU) Fall Meeting, the world’s largest annual gathering of Earth scientists.

She is part of a team of researchers – led by Prof Charles Rubin – from the Earth Observatory of Singapore, an institute of Nanyang Technological University that is investigating the coastal history of Indonesia’s largest island.

Sumatra’s proximity to the Indo-Australia and Sunda tectonic plate boundary, and the giant earthquakes that occur there, means its shores are at risk of major inundations.

Understanding how often these occur is important for policy and planning in the region.

The Acehnese cave lies about 100m back from the swash zone at current high-tide. Its entrance is also raised somewhat, and this prevents all waters from getting into the opening – apart from tsunamis and severe storm surges.

Dr Pilarczyk and colleagues have dug trenches through the alternating bands of bat guano and sand to piece together the cave’s history.

The scientists know they are looking at tsunami deposits because they can find debris in the sediments of seafloor organisms such as microscopic foraminifera. Only the most energetic waves could have lifted and carried this material into the cave.

The investigations are ongoing but the team thinks it can see deposition from perhaps 7-10 tsunamis. The geometry of the cave means these events would likely have been generated by earthquakes of Magnitude 8, or more. By way of comparison, the devastation wrought by 26 December 2004 stemmed from a M9.2 tremor.

Dating the old deposits is obtained by radiocarbon analysis of organic debris caught up in the bands, such as molluscs and pieces of charcoal from old human-lit fires.

Work is under way to date even the insect remains eaten by the bats and now immersed in the guano layers.

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