The archipelago, or string of islands, that makes up Indonesia is located on the so-called “Ring of Fire,” an arrangement of fault lines and volcanoes that surrounds the Pacific Basin. Before December 2004, the fault that triggered the massive earthquake (magnitude 9.1) had remained quiet for centuries, and the last big quake to hit near the region had occurred more than 500 years earlier. No oral history existed to make people aware of the dangerous effects such a temblor could have on the tides, and no warning system existed to detect tsunami activity in the Indian Ocean or warn people living nearby. As a result, even though there was a lag of several hours between the earthquake and when the tsunami hit the beaches, nearly all of the victims were caught completely unawares.
Since the deadly events of December 26, 2004, much research has been conducted to learn about the region’s past and prepare better for the future, including examination of sand deposits, uplifted coral and GPS data. Researchers discovered the natural limestone cave by chance, located within 200 yards of the coast near Banda Aceh. Set about three feet above knee-high tide, it is protected from storms and wind, and only huge waves that flood the entire coastal area are able to gush inside its walls.
In 2011, a team of researchers found seabed sand deposits inside the cave, swept there over thousands of years and layered among bat droppings. Through radiocarbon analysis of the materials, which included clamshells and remains of microscopic organisms, they have uncovered evidence of 11 tsunamis before 2004. According to lead researcher Charles Rubin from the Earth Observatory of Singapore, the disasters were not evenly spaced in time. The last one occurred around 2,800 years ago, and there were four others in 500 years before that. Additional evidence of other events might have been eroded, Rubin says; the historical record shows that there were massive earthquakes in the region around 1393 and 1450, and these could have triggered tsunamis as well.
Rubin and his colleagues are still working to determine the size of the waves that made an impact on the cave, and he says the record they have found so far does not provide any clear clues as to when the next tsunami might occur. He did caution that there might not be a 500-year gap between the 2004 earthquake-tsunami and the next similar event, saying that “I wouldn’t put out a warning that we’re going to have an earthquake, but [the record] shows that the timing is really variable.”