A cave discovered near the source of the deadly earthquake-triggered tsunami that hit Indonesia on Dec. 26, 2004, contains the footprints of past ancient tsunamis dating back 7,500 years. This rare natural record will help scientists better understand the nature and frequency of these massively destructive waves.
The findings presented this month at an American Geophysical Union conference in San Francisco, Calif., provide the longest and most detailed timeline for tsunamis that have occurred off the far western tip of the island of Sumatra. Finding the cave was pure luck, according to lead researcher Charles Rubin from the Earth Observatory of Singapore. It was discovered by chance and not the result of planned fieldwork, he said.
Sumatra’s Aceh province is where, the day after Christmas in 2004, 100-foot waves triggered by magnitude 9.1 earthquake smashed onto the shore, killing some 230,000 people in several countries, mostly in Indonesia.
The limestone cave, within a couple of hundred yards (meters) of the coast near Banda Aceh, is about three feet (one meter) above knee-high tide and protected from storms and wind. Only huge waves coming onto the coastal area can reach the cave.
In 2011, researchers uncovered layers of sand deposits brought up from the sea that were swept into the cave over thousands of years. They say the seabed sand layers were neatly stratified between layers of bat droppings, like a geological cake.
Using radiocarbon analysis to date the materials, which included clam shells and the remains of microscopic organisms, the team found evidence of 11 tsunamis prior to 2004. The last one happened about 2,800 years ago, but there were four others in the 500 years before that.
And it’s possible there were others. Rubin said a really big tsunami could have carried away evidence of other events through erosion. The researchers are still trying to determine the size of the waves that gushed into the cave.
The tsunami events were not evenly spaced, Rubin noted.
“The take-home message is perhaps that the 2004 event doesn’t mean it won’t happen for another 500 years,” he said. “We did see them clustered together in time. I wouldn’t put out a warning that we’re going to have an earthquake, but it shows that the timing is really variable.”
Scientists were surprised by the quake that set off the 2004 tsunami disaster because the fault that triggered the megathrust temblor had been quiet for hundreds of years. And as the last big earthquake happened 500 years earlier, there is no surviving oral history to help people understand the risk.
Geologist Kerry Sieh, director of the Singapore group has predicted another massive quake could hit the region in the next few decades. They tend to come in cycles, he said, and the 2004 temblor put more pressure on the fault. But the history of quakes is so variable, it’s impossible to make predictions.
“By learning about the type of tsunamis that happened in the past, maybe we can do planning for mitigation for the next tsunami,” said team member Nazli Ismail, head of the physics and geophysics department at Syiah Kuala University in Banda Aceh.