Boxing Day tsunami: here’s what we have learned in the 20 years since the deadliest natural disaster in modern history

On Boxing Day 2004, an earthquake in the Indian Ocean near Indonesia set off a tsunami which killed almost 250,000 people. It was the deadliest natural disaster this century, and was probably the deadliest tsunami in human history.

As coastal engineers who specialise in tsunamis and how to prepare for them, we have seen how the events of 2004 reshaped our global disaster management systems. Among the lessons learned since that day, three themes stand out.

First, the importance of early warning systems, providing time to escape impact zones. Second, the importance of local preparations and educating people about the risks. Finally, the ongoing need for – but not overreliance on – coastal defences.

The evolution of early warning systems

The absence of a comprehensive early warning system contributed to the devastating loss of life in 2004. About 35,000 people died in Sri Lanka, for instance, which wasn’t hit until two hours after the earthquake.

Significant investment has been made in the years since, including the Indian Ocean tsunami warning system which operates across 27 member states. This system was able to issue warnings within eight minutes when another earthquake struck the same part of Indonesia in 2012. Similarly, when an earthquake hit Noto, Japan, in January 2024, swiftly issued tsunami warnings and evacuation orders undoubtedly saved lives.

However, these systems are not in use globally and weren’t able to detect the tsunamis that swept the Tongan islands in 2022 following the eruption of an undersea volcano in the South Pacific. In this instance, better monitoring of the volcano would have helped detect the early signs of a tsunami.

Championing community resilience

But early warning systems alone are not enough. We still need education and awareness campaigns, evacuation drills, and disaster response plans.

This sort of planning proved effective in the village of Jike, Japan, which was hit by the Noto tsunami in January 2024. Having learned from a major tsunami in 2011 (the one that hit Fukushima nuclear power plant), engineers constructed new evacuation routes to tsunami shelters. Though the village was destroyed, residents evacuated up a steep stairway and no casualties were reported in Jike.

The role of engineering defences

In the years since the Boxing Day tsunami, countries at risk have invested in “hard” engineering defences including seawalls, offshore breakwaters and flood levees. While these structures offer a measure of protection, their effectiveness is limited.

In Japan, the idea that hard measures can protect against the loss of life has been discarded, with the view that large-scale tsunamis can overwhelm even the most robust defences. For instance, in 2011, even a rubble breakwater followed by a five-metre-high wall could not protect the city of Watari. The tsunami covered half the city and hundreds of people died.

Tsunamis in the past decade or two have exposed vulnerabilities in existing protection strategies, with our field surveys showing breakwaters and other structures having suffered severe damage. While complete failure is expected in the face of extreme events, it’s crucial that certain critical infrastructure, such as power plants, are designed to withstand the biggest tsunamis. This requires further research into resilient engineering designs that may be able to partially fail but remain functional.

After the 2011 tsunami, Japanese engineers created two tsunami measurement levels. Level one tsunamis are more frequent, occurring perhaps once every century, but less dangerous.

Level two tsunamis are the big ones that any given bit of coastline might expect only once every thousand or so years: Indian Ocean 2004, Japan 2011. It is these tsunamis that critical infrastructure like power plants must prepare for. Nothing will entirely hold back a 2004-sized tsunami, but the goal is for structures to overflow without being destroyed. They should still be able to assist the evacuation process by reducing tsunami height and delaying the time it takes.

Despite evolving views on hard defences, there remains value in building and planning coastal urban areas in more sustainable and responsible ways. In particular, critical infrastructure and densely populated areas in tsunami-threatened regions should be built on higher ground where possible.

Engineering advancements must also account for environmental consequences, including damage to ecosystems and disruption of natural coastal processes, with consideration given to nature-based solutions. Strengthening coral reefs with rock armour or heavy sandbags, and planting coastal forests as buffer zones may be a cheaper and more ecologically sensitive option than building high walls.

Climate change and the road ahead

The progress is undeniable. However, tsunami and earthquake data still isn’t shared widely around the world, and local authorities and experts often don’t communicate the risk to residents of flood-prone communities. The passage of time can erode the memory of best practice when it comes to people’s disaster preparedness.

Added to that, rapid climate change is making sea levels rise and extreme weather, such as storms, more frequent. This doesn’t cause more tsunamis, but it can make them worse, and it does make “hard” defences less sustainable in the long term.

While significant and urgent challenges remain, they are not insurmountable. By continuing to learn more about tsunamis and to prepare for the worst, we can minimise their impact and protect millions of lives.

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Ravindra Jayaratne receives funding from the Great Britain Sasakawa Foundation, the Daiwa Anglo-Japanese Foundation, the Global Challenges Research Fund, the Royal Society, University of East London, UK and Waseda University, Japan.

Tomoya Shibayama receives funding from the KAKENHI grant, the Japan Society for the Promotion of Science, and Waseda University, Japan.