Cave holds 7,500 years of data on tsunamis

SINGAPORE- It was the end of a long, humid summer of fieldwork, and the Indonesian and American geologists were on their way to one of their last sites of the year on the far western tip of Sumatra.

The cave was not top of the research team's list that afternoon in May 2011 but National University of Singapore anthropologist Patrick Daly, 38, an expert on the human impact of disasters, suggested they stop off at the site.

When the team of 10 cut and sliced into the cave floor, what they found amazed them.

Sandwiched between layers of bat droppings and soil were strata of sand and other deposits swept in from tsunamis that occurred over the past 7,500 years.

It was to be the longest and most detailed record yet of the natural disasters that had pummelled the Aceh coast.

Within an hour, the team recognised that the cave would merit much more work.

Over the following year or so, they mapped, surveyed and collected data, said lead researcher Charles Rubin in a phone interview with The Sunday Times.

Associate Professor Rubin, 62, is from the Earth Observatory of Singapore (EOS) at Nanyang Technological University.

Earlier this month, EOS visiting research fellow Jessica Pilarczyk presented the findings of their research at the annual American Geophysical Union meeting in San Francisco, in the United States.

The cave discovery is part of an overall effort to understand how often tsunamis and earthquakes recur at the Sunda megathrust. Aceh is located along this megathrust.

This is knowledge that could be applied to help coastal communities prepare for future disasters, said Prof Rubin.

The Sunda megathrust off Sumatra is a fault zone where one tectonic plate grinds under another, producing earthquakes and tsunamis as the seabed is lifted up.

The most recent of such disasters occurred nine years ago on Dec26, 2004, when waves 30m high devastated towns in the Indonesian province of Aceh and other countries across the Indian Ocean.

The tsunami followed a 9.1 magnitude earthquake, which had its epicentre just off the Aceh coast.

Since then, scientists have been examining sand deposits and corals to work out when the last big quakes occurred.

They knew there had been two large earthquakes around 1393 and 1450.

But by studying debris like charcoal from brush fires in the layers of the cave, they now know that tsunamis may cluster in time.

"You might have 1,500 years of quiescence and you might have a cluster of five within 500 years," said Prof Rubin. Why the events might cluster is not known but the team is working to better understand those patterns, he added.

The knowledge could help the local government plan its development and response to future disasters, said team member Nazli Ismail, 42, who is head of the physics and geophysics department at Syiah Kuala University in Banda Aceh.

He said via e-mail: "Aceh is a great natural laboratory for tsunamis and other disasters. Here, not only the people of Aceh can learn but also the world."

After the 2004 tsunami, for instance, a coastal road was moved to the inland side of the cave that the team explored. As a result, it is now further away from the coastline and less vulnerable to another tsunami.

But such caves are rare, said Prof Rubin. "For any of these sites to be good, you need... I don't want to say 'the stars to align' but a lot of things have to line up."

The cave south of Banda Aceh is about 100m inland from the coast and its mouth is high enough that waves do not wash in sand all the time - which would bury evidence of tsunamis.

The most important point is that caves must contain material that can be dated.

For example, researchers are working out how to tell the age of the bat guano using the insect remains within the cave.

This will help to further pin down the dates of tsunamis.

But if sites are buried under roads and other development, their secrets cannot be unlocked.

Still, finding the Aceh cave was not just dumb luck.

Before they ventured out, researchers used remote sensing and satellite images to pinpoint potential sites, and visited two to three of these sites a day.

Prof Rubin said: "We understood what we were doing and knew where we wanted to go. Sometimes, you put yourself in the right place at the right time."

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