
December 26, 2024 marks the 20th anniversary of the 2004 Indian Ocean earthquake and tsunami. The source of the tsunami was the 9.1 magnitude earthquake off the coast of Sumatra and was the third largest tsunami (in terms of strength) in the world since 1900. The source was 30 kilometers below The ocean floor, in the Sunda Trench, where part of the Indus River lies. – The Australian plate is subducted under the Burma microplate, which is part of the Eurasian plate.
The 2004 earthquake swept 1,300 kilometers from the plate boundary, and the fault extended from Sumatra in the south to the Coco Islands in the north. The earthquake was felt in Indonesia, Bangladesh, India, Malaysia, the Maldives, Myanmar, Singapore, Sri Lanka and Thailand. It caused massive damage and killed hundreds in North Sumatra and in the Andaman and Nicobar Islands. The tsunami’s impact was greater on distant shores, affecting 17 countries bordering the Indian Ocean.
Overall, with a staggering death toll of some 227,000 plus 1.7 million displaced people, the 2004 tsunami was the deadliest in recorded history.
Unprecedented size
Less than six years later, on March 11, 2011, a 9.1-magnitude earthquake struck the east coast of Japan, the largest earthquake ever recorded in that country. It generated a tsunami that reached 39 meters high and traveled up to 8 kilometers into the country. The twin disasters resulted in the deaths of more than 18,000 people, the displacement of more than 500,000, and resulted in the Fukushima Daiichi nuclear power plant accident.
Although devastating tsunamis have occurred in the past – in Chile in 1960 and Alaska in 1964, for example – these two twenty-first century events have taught us important lessons. In particular, the 2004 tsunami highlighted the world’s vulnerability to natural hazards. It came down like a bolt from the sky, striking in unexpected locations, and underscoring the importance of addressing disaster risks through preparedness and resilience.
As Margareta Wallström, head of the United Nations Office for Disaster Risk Reduction, noted in a panel discussion: “Ten years after the Indian Ocean tsunami, the world has taken important measures to make the world a safer place against disasters.”
The 2004 tsunami surprised researchers and risk managers alike with its transoceanic reach. With no recorded history of any event of this magnitude, the research community did not expect it to occur along the eastern coast of India. The only previous tsunamis occurred in 1881, caused by a large earthquake (magnitude 8) off Car Nicobar Island, and another in 1883 due to the explosion of Krakatoa. These events produced only small sea waves as recorded by tide gauges at various points on the East Coast.

This aerial view shows the coastal devastation that struck Kachal Island, part of the Andaman and Nicobar Islands, in 2005. The island lost about 90% of its population in the December 26, 2004 tragedy. | Image source: AFP/Getty Images
However, over the past two decades since 2004, researchers have made huge leaps in scientific understanding of tsunami generation and the technical aspects of earthquake monitoring. The Indian Tsunami Early Warning Center (ITEWC), set up by the Union Ministry of Earth Sciences of the Government of India in 2007, is perhaps the most important step in this direction.
Operating from the Indian National Center for Ocean Information Services (INCOIS) in Hyderabad, ITEWC operates seismic stations as well as bottom pressure recorders and tidal stations across the Indian Ocean Basin – all 24/7. These systems can transmit deep-sea and deep-ocean tsunami observations, allowing early warnings. Seismic data from stations operated by the Indian Meteorological Department (IMD) and 350 global stations are also available in INCOIS.
Ocean observing systems also pass data in real time. Within about 10 minutes, for example, the system can identify a potential earthquake that could trigger a tsunami and issue tsunami alerts or warnings – depending on its expected severity – to countries bordering the Indian Ocean. India is the fifth country in the world, after the United States, Japan, Chile and Australia, to have an advanced tsunami warning system of this type.
New practice
The 2004 incident also stimulated important new developments in research. Work in the field of tsunami geology, pioneered by Brian Atwater of the US Geological Survey, has led researchers in Asian countries, including India, to search for evidence of tsunamis in history. Atwater’s work along the Washington coast in the western United States has uncovered evidence of the 1700 earthquake and tsunami, as well as their predecessors. One of the cool parts of this work was using ground elevation changes caused by the earthquake, which left trees stressed or just killed them. Atwater used the fingerprints of these impacts to determine when a piece of land was deformed and thus when it was suffering the effects of a tsunami earthquake.

Inspections of low-lying mangrove swamps revealed how the 2004 earthquake had caused elevation changes of up to 3.5 meters in some places along the Andaman and Nicobar Islands. Scientists also wondered whether there were previous events that also caused the mangroves to decline. As it turns out, the 2004 earthquake reopened the coffins of the past and revealed their skeletons, in the form of dead roots emerging from tidal platforms during low tide. Such roots uncovered near Port Blair have been used to infer that the last earthquake occurred about a thousand years ago.
Excavations at Mahabalipuram, a Pallava port, have uncovered evidence of a tsunami of the same type. This was the first evidence of a tsunami before 2004 reported by an Indian team. The researchers also examined sedimentary deposits along islands and coastal areas on the mainland to find evidence of other ancient tsunamis, while learning to distinguish between tsunami and storm deposits.
This effort is a good example of how the 2004 tsunami stimulated tsunami geology to become a new practice, leading to many new research papers and doctoral dissertations. The demand for more knowledge about tsunamis has also facilitated quantum leaps in the use of GPS systems and seismographs. With funding from the Ministry of Earth Sciences, research institutes have established several new stations along the Andaman and Nicobar Islands, to enhance seismic observations and geodetic studies.
In another important step, tsunami modeling using mathematical tools helped researchers determine flood boundaries. In particular, the disaster was a stark reminder that nuclear power plants located along India’s coast may be vulnerable to hitherto unappreciated risks. While the Kalpakkam Nuclear Power Plant withstood the giant waves, it also shut down automatically after rising water levels tripped detectors. There was no leak of radioactive material and the reactor was restarted six days later.
no. Reactor No. 3 at the Fukushima Daiichi Nuclear Power Plant burns after an explosion following an earthquake and tsunami in this satellite image taken on March 14, 2011. | Image credit: Digital Globe
But the 2011 Tohoku earthquake reminded the world and India how quickly a nuclear disaster can occur in the absence of a safety system. It was clear that radiation from the Fukushima facility had entered the human food chain. Researchers even found radioactive cesium in the breast milk of some women tested near Fukushima Prefecture three months after the disaster. What if in 2004 the waves were high enough to destroy the reactors at Kalpakkam?
This question continues to resonate as the government pursues major development projects in Great Nicobar, including the construction of an international transshipment terminal. Some experts also argue that the last major earthquake to hit the region before 2004 was a thousand years ago, so there is no imminent danger. But this question depends on how much we don’t know yet. What if an unbroken portion of the subduction zone between Myanmar and India collapses? It cannot be ruled out that an as-yet unexamined part of the Earth’s crust between Great Nicobar and Car Nicobar suddenly collapsed and caused a strong earthquake and tsunami.
Experts and policymakers should also focus on other problem areas, such as the Makran coast in the northern Arabian Sea and the Myanmar coast adjacent to the northern Indian Ocean. Both have the potential to produce large tsunamis. The Makran Coast, which runs through Iran and Pakistan, could direct the tsunami’s energy towards India’s western coast, which also hosts nuclear reactors and the city of Mumbai.
A major milestone
Science tells us that pressure builds between tectonic plates until it reaches a critical pressure, at which point the accumulated potential energy is released in the form of an earthquake. Subduction zones such as the Andaman-Sumatra region have become important because they provide evidence of earthquake generation. The discovery of slowslips – tectonic faults that move much more slowly and generally a little deeper – has also added a new dimension to this picture.

Recently, researchers have been studying seismic slips at plate boundaries to understand the processes that occur before and after major earthquakes. They demonstrated the occurrence of pre- and post-seismic transitions using laboratory experiments and numerical simulations. Some of these studies have implications for earthquake prediction: they point to a degenerative process that initially involves stable, slow rupture growth within a fault-confined zone before unstable, high-velocity rupture occurs.
One sheet Published in 2015 (co-authored by one of the authors of this article) noted notable subsidence ground motion in South Andaman between 2003 and 2004, before the earthquake — a silent 6.3 magnitude event. This event may have been a precursor to the massive earthquake. Geodetic data analyzes on a wide range of global earthquakes Published in sciences Short-term precursor fault slips before large earthquakes have also been confirmed.
After its occurrence, the 2004 Andaman-Sumatra earthquake became a major milestone in modern earthquake research, providing science with a trove of data to help derive new insights into earthquake generation and associated hazards.
Kosala Rajendran is a former professor at the Center for Earth Sciences, Indian Institute of Science, Bengaluru. CB Rajendran is Assistant Professor at National Institute of Advanced Sciences, Bengaluru. They are the authors of the book ‘The Roaring Earth – The Story of Indian Earthquakes’.
Published – 26 December 2024 at 5:30 AM IST