Huge underwater landslides dating back millions of years have enabled UK scientists to identify a potential new tsunami risk in Indonesia.

The research team, led by Heriot-Watt University in Edinburgh, said the findings suggest coastal communities in the country without tsunami warning or mitigation systems could be at risk from tsunamis generated by similar landslides.

Areas at risk include the proposed site for the new Indonesian capital on the island of Borneo.

More than 4,000 people across the country have died in the past two years alone from the Palu Bay and Anak Krakatau tsunamis.

The epicentre of the 2004 Boxing Day tsunami that killed around 230,000 people in several different countries was off the coast the Indonesian island of Sumatra, and the nation was the worst affected.

Now, scientists examining the seabed in the Makassar Strait between the islands of Borneo and Sulawesi have found evidence of 19 ancient submarine landslides they say can help predict the present day tsunami risk.

Most tsunamis are caused by large earthquakes but submarine landslides can also spark the devastating waves, as happened in 2018 on Sulawesi.

Researcher Rachel Brackenridge said: “We found evidence of submarine landslides happening over 2.5 million years.

“They happened every 160,000 years or so and ranged greatly in size.

“The largest of the landslides comprised 600 kilometres cubed of sediment, while the smallest we identified were five kilometres cubed.”

It’s like an underwater avalanche

The debris produced by the largest landslides identified would fill Sydney harbour 1,000 times over.

Ms Brackenridge said the team mapped underneath the seabed using seismic data.

She said: “We can see a layered and orderly seabed, then there are huge bodies of sediment that appear chaotic.

“We can tell from the internal characteristics that these sediments have spilled down a slope in a rapid, turbulent manner.

“It’s like an underwater avalanche.”

Uisdean Nicholson, who led the research at Heriot-Watt University, said the strong currents flowing through the strait could account for the prehistoric events and potential further underwater landslides.

He said: “The current acts as a conveyor belt, transporting sediment from the Mahakam Delta and dumping it on the upper continental slope to the south, making the seabed steeper, weaker and more likely to collapse.

“We estimate the largest, tsunamigenic events – those that displace 100 kilometres cubed – occurred every 500,000 years.

“Indonesia has mitigation and early warning measures in place in different parts of the country but not the area that would be affected by a tsunami wave generated from these landslides.

“This includes the cities of Balikpapan and Samarinda, which have a combined population of over 1.6 million people.”

He added: “Such an event could be concentrated and amplified by Balikpapan Bay, the site selected for the new capital city of Indonesia.

“Our next step is to quantify the risk in this area by building various numerical models of landslide events and tsunami generation.

“This could help us predict a threshold size that causes dangerous tsunamis and help inform any mitigation strategies.”

The British Geological Survey and University College London were also involved in the study, which was published in the Geological Society London publication Submarine Mass Movements and Their Consequences.