Indonesia cave reveals history of ancient tsunamis
By Margie Mason, APJAKARTA--A cave discovered near the source of Indonesia's massive earthquake-spawned tsunami contains the footprints of past gigantic waves dating up to 7,500 years ago, a rare natural record that suggests the next disaster could be centuries away — or perhaps only decades.
December 25, 2013, 12:18 am TWN
The findings provide the longest and most detailed timeline for tsunamis that have occurred off the far western tip of Sumatra island in Aceh province. That's where 100-foot (30-meter) waves triggered by a magnitude-9.1 earthquake on Dec. 26, 2004, killed 230,000 people in several countries, more than half of them in Indonesia.
The limestone cave, located within a couple hundred yards (meters) of the coast near Banda Aceh, is about 3 feet (1 meter) above knee-high tide and protected from storms and wind. Only huge waves that inundate the coastal area are able to gush inside.
Researchers in 2011 uncovered seabed sand deposits that were swept into the cave over thousands of years and neatly layered between bat droppings like a geological cake. Radiocarbon analysis of materials, including clamshells and the remains of microscopic organisms, provided evidence of 11 tsunamis before 2004.
The disasters were by no means evenly spaced, said lead researcher Charles Rubin from the Earth Observatory of Singapore. The last one occurred about 2,800 years ago, but there were four others in the preceding 500 years.
And it's possible there were others. Researchers know, for instance, that there were two mammoth earthquakes in the region around 1393 and 1450. Rubin said a big tsunami could have carried away evidence of other events through erosion.
The scientists are still working to determine the size of the waves that entered the cave.