How the 2004 Indian Ocean earthquake changed tsunami science

December 26, 2024, marks the 20th year since the 2004 Indian Ocean earthquake and tsunami. The tsunami generated by the 9.1 magnitude earthquake came off the Sumatran coast and was the world’s third largest (by magnitude) since 1900. The source was 30 km below the sea floor in the Sunda Trench, where part of India was. – The Australian plate subducts beneath the Burma microplate, which is part of the Eurasian. plate.

The 2004 earthquake ruptured a 1,300 km plate boundary, the fault extending from Sumatra in the south to the Cocoa Islands in the north. The earthquake was felt in Indonesia, Bangladesh, India, Malaysia, Maldives, Myanmar, Singapore, Sri Lanka and Thailand. It caused severe damage to northern Sumatra and the Andaman and Nicobar Islands, killing hundreds. The tsunami had the greatest impact on the far coast, affecting 17 countries in the Indian Ocean.

Overall, with a staggering death toll of nearly 227,000 and 1.7 million more displaced, the 2004 tsunami is the deadliest in recorded history.

unprecedented magnitude

Less than six years later, 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 up to 39 meters and reached up to 8 kilometers 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 – for example in 1960 in Chile and 1964 in Alaska – two events in the 21st century teach us important lessons. In particular, the 2004 tsunami highlighted how vulnerable the world is to natural hazards. It descended like a bolt from the sky, hitting the most unexpected places, and placed a premium on the importance of coping with disaster risk through preparedness and resilience.

As Margaretta 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 important steps 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 on the eastern seaboard of India. Previous tsunamis were caused by a large earthquake (~8) in Car Nicobar Island in 1881 and another in 1883 by the eruption of Krakatoa. These events produced only small tidal surges as recorded by tide gauges at various points along the East Coast.

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

Operated from the Indian National Center for Ocean Information Services (INCOIS) in Hyderabad, ITEWC operates seismological stations as well as low 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. Seismic data from stations operated by India Meteorological Department (IMD) and 350 global stations are also available in INCOIS.

Ocean monitoring systems pass data in real time. In about 10 minutes, for example, the system can identify a potential tsunami-producing earthquake and issue a tsunami alert or warning—based on expected severity—for countries bordering the Indian Ocean. India is the fifth country in the world to have such an advanced tsunami warning system after the US, Japan, Chile and Australia.

A new practice

The 2004 event also spurred important new developments in research. The work on tsunami geology started by Brian Atwater of the US Geological Survey inspired researchers in Asian countries including India to look for evidence of tsunamis in history. Atwater’s work on the Washington coast in the western US revealed evidence of earthquakes and tsunamis in the 1700s, as well as their predecessors. A fascinating part of this work was the use of land elevation changes caused by earthquakes, which stressed the trees or killed them. Atwater used the imprints of these impacts to determine when a piece of land had deformed and thus was experiencing the effects of a tsunamigenic earthquake.

Observations of dormant mangrove swamps revealed how the 2004 earthquake caused elevation changes of up to 3.5 meters in some parts of the Andaman and Nicobar Islands. Scientists also speculated that there may have been past events that may have reduced mangroves as well. The 2004 earthquake reopened the coffins of the past and exposed their skeletons in the form of dead roots sticking out of the tidal platforms during low tide. Such roots, uncovered near Port Blair, were used to date the last earthquake about 1,000 years ago.

Excavations at Mahabalipuram, a port city of the Pallava dynasty, have found evidence of the same ancient tsunami. This was the first evidence of a pre-2004 tsunami reported by an Indian team. The researchers also searched sediment deposits on mainland islands and coastal areas 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 inspired 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 seismic instruments. With funding from the Ministry of Earth Sciences, research institutes set up several new stations in the Andaman and Nicobar Islands, strengthening seismic observations and geodetic studies.

In another important step, tsunami modeling using mathematical tools helped researchers determine the extent of inundation. In particular, the disaster provided a stark reminder that nuclear power plants installed off the Indian coast can be vulnerable to hitherto underestimated risks. When the Kalpakkam nuclear power plant faced huge waves, it shut down automatically after the rising water level tripped the detectors. There was no release of radioactive material and the reactor restarted six days later.

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 that radiation from the Fukushima facility had entered the human food chain. Researchers also found radioactive cesium 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?

The question continues to resonate as the government embarks on major development projects, including the construction of an international transshipment terminal at Great Car Nicobar. Some experts have also argued that the last major earthquake to affect the region was a millennium ago, before 2004, so there is no imminent danger. But this question depends on how much we still don’t know. What if the unbroken patch of subduction zone between Myanmar and India gives way? It cannot be ruled out that the as yet unexamined portion of the crust between Great Nicobar and Car Nicobar is subject to sudden violent earthquakes and tsunamis.

Experts and policy makers should also focus on other trouble spots, such as the Makran coast in the northern Arabian Sea and the coast of Myanmar along the northern Indian Ocean. Both of them have the potential to produce large tsunamis. The Makran coast, which cuts between Iran and Pakistan, could direct the energy of a tsunami to India’s west coast, which also hosts nuclear reactors and the city of Mumbai.

A major milestone

Science tells us that stress builds up between tectonic plates until it reaches a critical stress, at which point the stored potential energy is released as an earthquake. Subduction zones such as the Andaman-Sumatra region are becoming important because they provide clues to earthquake generation. The discovery of slow slips—tectonic faults that move several orders of magnitude slowly and usually at little depth—has added a new dimension to this picture.

Recently, researchers have been studying seismic slip at plate boundaries to understand the processes that occur before and after large 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 formative process that involves steady, slow rupture growth within a confined region on a fault just before an initially unstable, high-speed rupture.

A paper published in 2015 (co-authored by one of the authors of this article) indicated a perceptible ground movement between 2003 and 2004, before the South Andaman earthquake, a silent event with a moment magnitude of 6.3. This event may be a precursor to a megathrust earthquake. Analysis of geodetic data on a broad set of global earthquakes published in Science Short-term antecedent fault slips were also confirmed before major earthquakes.

After it happened, the 2004 Andaman-Sumatra earthquake became a major milestone in modern seismological research, providing a wealth of data to help science glean new insights into earthquake generation and associated hazards.

Kusala Rajendran is a former professor at the Center for Economics, Indian Institute of Science, Bengaluru. CP Rajendran is an Assistant Professor at the National Institute of Advanced Sciences, Bengaluru. He is the author of the book ‘The Rumbling Earth – The Story of Indian Earthquakes’.