13 Dec 2013

Tsunami layers found in cave in Sumatra

4:52 am on 13 December 2013

Scientists say a coastal cave on the Indonesian island of Sumatra holds a remarkable record of big tsunamis in the Indian Ocean.

The limestone cave, close to Banda Aceh, retains the sandy deposits washed ashore by huge waves induced by earthquakes over thousands of years.

The scientists are using the site to help determine the frequency of catastrophes like the event of 26 December 2004.

The BBC reports 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)," said Dr Jessica Pilarczyk.

''From a geologist's point of view, this cave has the most amazing stratigraphy," she told BBC News in San Francisco at a meeting of the American Geophysical Union.

Dr Pilarczyk is part of a team of researchers from the Earth Observatory of Singapore, an institute that is investigating the coastal history of the 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.

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

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

The team thinks it can see deposits from perhaps seven - 10 tsunamis. The geometry of the cave means these events would likely have been generated by earthquakes of magnitude 8.0, or more. By way of comparison, the devastation wrought on 26 December, 2004, stemmed from a magnitude 9.2 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.