December 26, 2024, marks the 20th year since the 2004 Indian Ocean earthquake and tsunami. The tsunami generated by the quake of magnitude 9.1 was sourced off the Sumatran coast and was the third largest (by magnitude) in the world since 1900. The source was 30 km below the ocean floor, in the Sunda trench, where part of the Indo-Australian plate subducts beneath the Burma microplate, which is a part of the Eurasian plate.

The 2004 earthquake ripped through 1,300 km of the plate boundary, the fault tearing from Sumatra in the south to Coco Islands in the north. The quake was felt in Indonesia, Bangladesh, India, Malaysia, the Maldives, Myanmar, Singapore, Sri Lanka, and Thailand. It caused severe damage and killed hundreds in Northern Sumatra and in the Andaman and Nicobar Islands. The tsunami was most impactful on distant shores, affecting 17 countries lining the Indian Ocean.

In all, with an astounding death toll of around 227,000 plus 1.7 million more displaced, the 2004 tsunami is the deadliest in recorded history.

Unprecedented magnitude

In less than six years, on March 11, 2011, a magnitude 9.1 earthquake hit the east coast of Japan, the largest ever recorded in that country. It generated a tsunami that reached as high as 39 metres and travelled up to 8 km inland. The twin disasters killed more than 18,000 people, displaced more than 500,000, and resulted in the Fukushima Daiichi nuclear power plant accident.

Although devastating tsunamis have occurred in the past — 1960 Chile and 1964 Alaska, for example — the two 21st century events taught us important lessons. Particularly, the 2004 tsunami highlighted how vulnerable the world was to natural hazards. It landed like a bolt from the sky, hitting the most unexpected locations, and placed a premium on the importance of tackling disaster risk through preparedness and resilience.

As Margareta Wahlström, head of the UN Office for Disaster Risk Reduction (UNISDR), observed in a panel discussion: “Ten years after the Indian Ocean tsunami, the world has taken significant measures to make the world a safer place against disasters.”

The 2004 tsunami surprised researchers and hazard managers alike with its transoceanic reach. With no recorded history of any event of such magnitude, the research community hadn’t anticipated it occurring along India’s eastern seaboard. The only previous tsunamis had 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 surges as recorded by tide gauges at different points on the east coast.

This view from the air shows coastal devastation on Katchal Island, part of the Andaman and Nicobar Islands, in 2005. The island lost some 90% of it’s population in the December 26, 2004, tragedy.
| Photo Credit:
AFP/Getty Images

However, in the two decades since 2004, researchers have made tremendous leaps in the scientific understanding of tsunami generation and the technical aspects of earthquake monitoring. The Indian Tsunami Early Warning Centre (ITEWC), established in 2007 by the Union Ministry of the Earth Sciences of the Government of India, is perhaps the most significant step in this direction.

Operating from the Indian National Centre for Ocean Information Services (INCOIS) at Hyderabad, ITEWC operates seismological stations as well as bottom pressure recorders and tidal stations across the Indian Ocean basin — all 24/7. These systems can transmit offshore and deep ocean tsunami observations that enable early warnings. Earthquake data from the stations operated by the India Meteorological Department (IMD) and 350 global stations are also available at INCOIS.

Ocean monitoring systems also pass data in real-time. In about 10 minutes, for example, the system can identify a potential tsunami-producing earthquake and issue tsunami alerts or warnings — depending on the expected severity — for countries bordering the Indian Ocean. India is the fifth country in the world, after the U.S., Japan, Chile, and Australia, to have an advanced tsunami warning system of this kind.

A new practice

The 2004 incident also spurred important new developments in research. The work of tsunami geology, pioneered by Brian Atwater of the U.S. Geological Survey, prompted researchers in Asian countries including India to search for evidence of tsunamis in history. Atwater’s work along the Washington coast of the western U.S. had revealed evidence of an earthquake and tsunami in 1700, plus their predecessors. One fascinating part of this work was the use of land elevation changes caused by the earthquake, which left trees stressed or just killed them. Atwater had used the imprints of these effects to determine when some piece of land had been deformed and thus when it was suffering the effects of a tsunamigenic earthquake.

Inspections of subsided mangrove swamps revealed how the 2004 earthquake had rendered changes in elevation of up to 3.5 metres at some places along the Andaman and Nicobar Islands. Scientists also wondered if there could have been past events that also caused the mangroves to subside. As it turned out the 2004 earthquake had reopened the coffins of the past and exposed their skeletons, in the form of dead roots sticking out from tidal platforms during a low tide. Such roots exposed near Port Blair were used to infer that the last earthquake had occurred about a thousand years ago.

Excavations at Mahabalipuram, a port of the Pallava dynasty, unearthed evidence of a tsunami of the same vintage. It was the first proof of a pre-2004 tsunami reported by an Indian team. Researchers also sifted through sedimentary deposits along the islands and coastal areas of 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 prompted the science of tsunami geology to become a new practice, leading to many new research papers and doctoral theses. The demand for more knowledge about tsunamis also facilitated quantum leaps in the use of GPS systems and earthquake instrumentation. With funding from the Ministry of Earth Sciences, research institutes established several new stations along the Andaman and Nicobar Islands, strengthening seismic observations and geodetic studies.

In another important step, the tsunami modelling using mathematical tools helped researchers determine inundation limits. In particular, the disaster provided a stark reminder that nuclear power plants established along Indian coasts could be vulnerable to a hitherto underestimated risk. While the Kalpakkam nuclear power plant withstood the giant waves, it also shut down automatically after the rising water levels tripped the detectors. There was no release of radioactive material and the reactor was restarted six days later.

The no. 3 nuclear reactor of the Fukushima Daiichi nuclear power plant is seen burning after a blast following an earthquake and tsunami in this handout satellite image taken March 14, 2011.
| Photo Credit:
DIGITALGLOBE

But the 2011 Tohoku earthquake reminded the world, and India, how quickly a nuclear disaster can happen in the absence of a failsafe. It was clear the radiation from the Fukushima facility had entered the human food chain. Researchers even found radioactive caesium in the breast milk of some women tested near Fukushima prefecture three months after the disaster. What if the waves in 2004 had been high enough to damage the reactors at Kalpakkam?

This question continues to resonate as the government has been pursuing large developmental projects in Great Car Nicobar, including the construction of an international transshipment terminal. Some experts have also argued that the last great earthquake that affected the region before 2004 was a millennium ago, so there is no imminent danger. But this question hinges on how much we still don’t know. What if an unbroken patch of the subduction zone between Myanmar and India gives way? A still-unexamined portion of the crust between Great Nicobar and Car Nicobar suddenly breaking into a powerful earthquake and a tsunami can’t be ruled out.

Experts and policymakers must also focus on other problem spots, like the Makran Coast in the northern Arabian Sea and the Myanmar coast adjoining the Northern Indian Ocean. Both of them have the potential to produce large tsunamis. The Makran Coast, cutting through Iran and Pakistan, could direct a tsunami’s energy towards India’s west coast, which also hosts nuclear reactors and the city of Mumbai.

A major milestone

Science tells us that stress builds between tectonic plates until it reaches a critical strain, at which point the accumulated potential energy is released as an earthquake. Subduction zones like the Andaman-Sumatra region are becoming significant as they provide clues to earthquake generation. The discovery of slow slips — tectonic faults that move many orders of magnitude slower and generally just a bit deeper — has also added a new dimension to this picture.

Of late, researchers have been studying seismic slips at plate boundaries to understand the processes that occur before and after major earthquakes. They have elucidated the occurrence of premonitory and post-seismic slip transients using laboratory experiments and numerical simulations. Some of these studies have implications for earthquake prediction: they indicate a creative process that initially involves stable, slow rupture growth within a confined zone on a fault just before unstable, high-speed rupture.

One paper published in 2015 (coauthored by one of the authors of this article) indicated a perceptible downward ground movement in South Andaman between 2003 and 2004, before the earthquake — a silent event with a moment magnitude of 6.3. This event could have been the precursor to the megathrust earthquake. Analyses of geodetic data on a wider set of global earthquakes published in Science also confirmed short-term precursory fault slips before large earthquakes.

After it happened, the 2004 Andaman-Sumatra earthquake became a major milestone in modern seismological research, providing science with a treasure trove of data to help glean new insights about earthquake generation and related hazards.

Kusala Rajendran is a former professor at the Centre for Earth Sciences, Indian Institute of Science, Bengaluru. CP Rajendran is an adjunct professor at the National Institute of Advanced Sciences, Bengaluru. They are authors of the book ‘The Rumbling Earth – The Story of Indian Earthquakes’.