A thousand years ago, a prosperous early Swahili village was booming along the banks of the Pangani River in Tanzania, a few miles inland from the Indian Ocean. Residents built their houses from wooden lattices daubed with mud. They filled their nets with fish and improvised beads from shells. The ceramics were simple and functional.
And then, one day, a tsunami barreled in, caused by an earthquake on the Indian Ocean’s opposite side. Recent work, sponsored by the National Geographic Society and published in Geology Today, describes a geological record of macabre rarity. Clearly the villagers had little chance to escape from the torrent that overtook them.
Many drowned and were buried in the wreckage in their razed houses. The Tanzanian site is, as far as the authors of the study know, the first –and oldest –tsunami deposit bearing human remains discovered in East Africa. The world’s oldest such deposit of human remains, located across the Indian Ocean in Papua New Guinea, is 7,000 years old.
The Tanzanian site provides a critical data point in the study of tsunamis in the Indian Ocean that can prove devastating. While large tsunamis occur relatively infrequently in the region, they do happen about once every 300 to 1,000 years or so— and the stakes are high in East Africa. Tanzania’s Dar es Salaam economic center, situated on the coast, is one of the fastest-growing cities in the world. Projections by the United Nations forecast Dar es Salaam to become a “megacity” of more than 10 million residents by 2030 and by the end of the century may reach 70 million.
East Africa got extremely lucky in 2004 when more than 227,000 people were killed by a tsunami triggered by a huge earthquake off the Indonesian coast. The bulk of that tsunami ‘s damage and fatalities occurred in South and Southeast Asia. The waves even made their way to East Africa’s shores, but during an intense low tide, the first wave brought landfall on the continent, minimizing its effects.
Nonetheless, the tsunami that occurred a thousand years ago was a different story. “It doesn’t seem to have been that big of a tsunami, but if people are living on low ground, and they have no idea what’s coming, it’s probably the worst situation,” says Jody Bourgeois, a sedimentologist and tsunami specialist at the University of Washington who reviewed the new study before publication. “There’s no earthquake to warn you, because you’re on the other side of the Indian Ocean.”
Uncovering an ancient disaster
Despite the possibility for Indian Ocean tsunamis to reach the shores of East Africa, not much has been researched about tsunami risk in the area. The Tanzanian site is one of only a handful of known tsunami deposits in East Africa from the past 12,000 years. “This kind of information needs to be known by governments and by the population,” says lead study author and National Geographic Explorer Vittorio Maselli, a geologist at Canada’s Dalhousie University.
In the spring of 2017, Maselli started researching the thousand-year-old tsunami, while working in the department of geology at Dar es Salaam University. He then found out about the work of archeologist Elinaza Mjema, also at Dar es Salaam University, who worked at a 95-mile northwest site near Pangani City.
The area once abounded in early Swahili fishing village with beads and ceramics was used by the university to teach proper archaeological field techniques. But when Mjema took the students there in 2010, test pit showed human bone. “Every student was going: Teacher, there is a skeleton,” he says. “It was a surprise.”
In 2012, 2016, and 2017, Mjema returned to the location and his further investigations found corpses randomly oriented in the dirt— including one with intact iron bracelets around its ankles. War and disease apparently did not account for the sudden disappearance of the village. There were no cut marks on any of the bones or signs of disease.
As it looks, the men, women, and children of the village had drowned and were buried in the shattered ruins of their homes. A team of researchers including Maselli and the study co-author Andrew Moore, a sedimentologist at Richmond, Indiana’s Earlham College, visited the place in 2017 to take further sediment samples.
Dar es Salaam University had begun digging holes on the property to create fish ponds for teaching aquaculture, damaging some of the archaeological sites in the process. The researchers dug up their trenches along the edges of the ponds and gathered everything they could. “Within about three months’ time, maybe less, they were going to fill those ponds with water,” Moore says. “Some of this was very much salvage geology.”
The sand that covered the village held the remains of fish, mice, birds, amphibians, and even small marine mollusk shells— a sign that water washed in from the Indian Ocean many miles downriver. And wherever the researchers worked, they have managed to find more human bones. “Sometimes, it was kind of emotional—we needed to think about the science, but in the meantime, we were working with the people who died there,” Maselli says.
Radiocarbon dating in the deposits of charcoal and bone indicated that the flooding event occurred around a thousand years ago. Tsunami deposits from across the Indian Ocean also date back to that period, indicating a similar occurrence in scale and intensity to the tsunami of 2004 occurred a millennium ago.
Computer simulations showed that an earthquake along the Sunda megathrust— the fault off Indonesia’s coast that caused the tsunami of 2004—could have created large enough waves to explain the deposits in Pangani. The funnel-shaped bay of the Pangani River would have intensified the tsunami waves as they raced upriver, making the site flooding much more devastating.
“If we go to other places around Africa that saw 2004-like events … would we find this ancient event writ in the rock history?” Moore wonders.
Revisiting risk
The team of researchers sincerely hope their study will encourage work to assess East Africa’s tsunami risks. Additionally, more detailed mapping of the ocean floor of the region is necessary, says Maselli.
Just like mountain ranges channel airflow, the topography of the ocean floor influences the movement of waves and currents. And tsunamis don’t just come from earthquakes; underwater landslides can cause them too.
“The United States has a huge program for mapping the entire shelf and slope all along the Atlantic coast for understanding landslides,” Maselli says. “We don’t know for East Africa.”
As for the ancient civilization, archeological research by Mjema is showing signs of post-tsunami renewal. Within 50 to 100 years, people were creating deposits atop the floods. Construction in the region continues even today. While local officials have started to push construction out of the lowlands, Mjema says recent buildings were built along the Pangani River on old tsunami deposits.
“We can learn a lot from the past, if we really want to understand what we are facing,” Mjema says.
