Column: 1,000-year-old tsunami gives glimpse of potential dangers ahead

On December 26, 2004, death came from the sea.

 A rupture along a fault on the boundary between two of Earth’s tectonic plates, the Burma Plate and the Indian Plate, produced a 9.2 magnitude earthquake, the third-largest in recorded history.

It was so large, it even triggered smaller quakes as far away as Alaska, but that wasn’t its deadliest effect. It was a tsunami, which reached shorelines 15 minutes to seven hours after the quake, and caused higher-than-normal tides as far away as Vancouver, Washington. 

The sudden movement of the sea floor had set in motion the water above. In the deep ocean, the wave was only about two feet high, but as it grew as it approached land, in some places to over 100 feet. And on those shores it killed an estimated 228,000 people in 14 countries, making it one of the deadliest natural disasters in recorded history. For a truly hair-raising experience, look up Sumatra tsunami videos on YouTube. 

Recent research suggests such tsunamis are not as rare as we might think. The research was published in the journal Geology by an international team of 13 scientists from the United States, New Zealand, Tanzania and Italy.   

Previous research had focused on the areas hardest hit by the 2004 event – the northern and eastern Indian Ocean coastlines. This research broadened the scope to include the western Indian Ocean, along the eastern coast of Africa. Tsunami risks there were thought to be low based on the little damage caused by the 2004 quake. This research shows otherwise. 

The study site was located in northern Tanzania, about 3 miles upstream from where the Pangani River enters the Indian Ocean. There, they found a sand layer holding the remains of an ancient coastal settlement. Radiocarbon dated to 1,008 to 802 years old, similar deposits of the same age have been found at other eastern Indian Ocean coastal sites including Thailand, India, Indonesia, Sri Lanka and the Maldives. 

The sand deposit is up to a foot thick, thins landward, and contains plant debris and pebbles similar those that can be found on the modern beach. It is interpreted as a tsunami deposit because it contains a mixture of organic remains from land, shore and ocean, including marine mollusks and the bones of fish, rodents, amphibians and birds. 

The clincher were the human remains, from isolated bones to full skeletons, of men, women and children. They occur in random locations, many with broken bones, and the lack of traditional funeral items indicates unexpected deaths. 

The researchers modelled tsunamis that could be produced by large quakes in the area and determined that the 1,000-year-old event was most-likely produced by a quake similar to the 2004 event, but along a different plate boundary – the one between the Sumatra and Andaman plates, one of many in that region of the world.   

Our Earth is restless – it is only a matter of time before another one of those faults lets go, sending waves crashing against shores near and far. 

gnidovec.1@osu.edu