MAJOR TSUNAMIS IN THE INDIAN OCEAN

MAJOR TSUNAMIS OF THE INDIAN OCEAN

Tsunami in the Indian Ocean between 1852 and 2004, from Researchgate

On the morning of December 26, 2004, an enormous 9.2-magnitude n earthquake occurred off the coast of Sumatra. The largest earthquake in four decades, it produced a powerful tsunami that struck Indonesia, Sri Lanka, India and Thailand particularly hard, killing about 227,000 people in 14 countries and leaving 2 million displaced and 1.8 million homeless. It was one of the worst natural catastrophes in the history of the human race. In the last hundred years the only disasters that were in the same league were some earthquakes and floods in China. The deadliest tsunami before this one was the one that killed 36,000 people after the eruption of Krakatoa in 1883. About 19,000 people were killed by the March 2011 earthquake and tsunami in Japan.

Tsunamis have been relatively rare in the Indian Ocean, at least in human memory. They are most prevalent in the Pacific. Earthquakes on Sunda Trench faults near Sumatra generated killer tsunamis in 1833 and 1861. The December 2004 earthquake and tsunami occurred along a fault with segments under the Batu and Mentawai islands. AP reported: “The Batu-Mentawai area produced a large earthquake in 1833, triggering a tsunami that devastated the adjacent coast of Sumatra. But because of the quake's location, most of the tsunami's energy flowed out into the open Indian Ocean. It passed south of Sri Lanka and India, which were hit hard by the December 2004 26 tsunami.

Tsunamis associated with the explosion of the Krakatau (Krakatoa) Volcano occurred with August 26-27, 1883 were generated by the paroxysmal eruptions, explosions, caldera collapses and slope failures of the Krakatau Volcano. The Krakatoa eruption produced huge tsunami that killed most the people associated with that event. The Krakatoa explosions and the tsunamis they generated killed 36,489 people, decimating 165 villages and nearly destroying 132 others. Most of those killed were carried away and drowned by tsunamis. Victims washed up months later as far away as the east coast of Africa. The tsunamis reached the height of four story buildings. They hit Java and Sumatra particularly hard and were felt as far away as California and England. These waves were generated by massive chunks of island that fell into the sea. At 10:32am a huge wave smashed into Tjaringin, 30 miles from Krakatoa, killing 10,000 people. The town of Penimbang, 10 miles inland, was submerged as were Tjeringr, Karang, Antoe, Telok Betong, Beneawany and Batavia

Other tsunamis in the Indian Ocean:
India in 2001 associated with- The Earthquake of January 25, 2001 in India
Papua New Guinea 1998 associated with the July 17, 1998 earthquake in Papua New Guinea.
Indonesia in 1977 associated with: an earthquake on August 19, 1977 in .
Philippines in 1976 associated with an earthquake on August 16, 1976 in the 13.
Vanuatu in 1999 associated with- an earthquake on November 26, 1999 in Vanuatu.

Tsunami and Earthquake Research at the USGS walrus.wr.usgs.gov/tsunami/ Here you will find general information on how local tsunamis are generated by earthquakes as well as animations, virtual reality models of tsunamis, and summaries of past research studies.

Tectonic Activity in the Indian Ocean Near Indonesia

Historical seismic events in the Indian Ocean: Lines represent the ridges (red), transform faults (orange) and trenches (purple), black dots are earthquakess between 1960 and 2013 and yellow stars represent tsunamigenic earthquakes; Source areas of simulated events are represented by the yellow ellipses Researchgate

Indonesia lies at one of the world's most complex tectonic junctions, where the Eurasian, Indo-Australian, Pacific, and Philippine Sea plates converge. Along the Indian Ocean margin, the Indo-Australian Plate is moving northward and subducting beneath the Eurasian Plate along the Sunda Trench, a major plate boundary that extends from the Andaman and Nicobar Islands past Sumatra and Java to the Lesser Sunda Islands. This subduction process generates intense seismic activity, volcanic eruptions, and crustal deformation throughout the region. The interaction of these plates has produced some of the largest earthquakes ever recorded, including the magnitude 9.1–9.3 Sumatra-Andaman earthquake of 26 December 2004. [Source: Dr. George Pararas-Carayannis, 2007]

The 2004 earthquake occurred when accumulated stress along the Sunda megathrust was suddenly released, causing a massive rupture that extended for approximately 600 kilometers off northern Sumatra and continued northward beneath the Nicobar and Andaman Islands. The seafloor was uplifted over a vast area, displacing enormous volumes of water and generating the devastating Indian Ocean tsunami.

Although much of the stored strain in the immediate northern Sumatra segment was released during this event, tectonic convergence continues, and stress is gradually rebuilding along the plate boundary. Furthermore, neighboring segments of the Sunda Trench that did not rupture completely during the 2004 event remain active and capable of producing large earthquakes in the future. South of the 2004 rupture zone, particularly off central and southern Sumatra, other sections of the megathrust have a long history of major earthquakes, including the powerful events of 1833 and 1861.

Likelihood of Earthquakes and Tsunamis in the Indian Ocean Near Indonesia

Because Indonesia sits astride several active plate boundaries, major earthquakes and tsunamis remain an ongoing hazard. Large earthquakes occurring in Indonesia's semi-enclosed seas, such as the Banda, Java, and Sulu Seas, can generate destructive local tsunamis with little warning time for nearby coastal communities. Along the Indian Ocean coast, earthquakes on the Sunda megathrust are capable of producing much larger tsunamis that can affect not only Indonesia but also distant coastlines throughout the Indian Ocean basin. Historical records show that major tsunami-generating earthquakes have repeatedly occurred along different segments of the trench, indicating that the hazard is persistent rather than confined to any single location. [Source: Dr. George Pararas-Carayannis, 2007]

Although the 2004 earthquake significantly reduced stress in the immediate northern Sumatra segment, it did not eliminate the possibility of future large earthquakes elsewhere along the Sunda Trench. Geological evidence suggests that major earthquakes similar to those that struck the Nicobar and Andaman Islands in 1881 and 1941 could recur in the northern section, while the central and southern Sumatra segments remain among the most likely sources of future destructive tsunamis.

Scientists have also noted the potential for significant tsunami-generating earthquakes farther south along the trench, including areas that could affect both Indonesia and northern Australia. Following major earthquakes, strong aftershocks may continue for months and can occasionally generate small local tsunamis. Coastal residents are therefore advised to treat strong ground shaking as a natural warning sign and move immediately to higher ground.

In the late 2000s, researchers at University College London estimated that the chance of a tsunami like the one in 2004 occurring again worldwide in the next 70 years was 2 to 1. The stimate was based in the fact there was one major earthquake in Chile in 1960 and one in Alaska in 1964 and if the one in 2004 — or three in 44 years. The estimate was made before the March 2010 earthquake and tsunami in Japan.

Huge Tsunami Hit Sumatra and Southeast Asia in 1394

Distribution of earthquakes in the northeast Indian Ocean and adjacent coast through time: a) By century, b) by decade between 1901 and 2012; Few earthquakes were reported before 1800. The number of earthquakes reported greatly increased after that Researchgate

The 2004 Indian Ocean tsunami unearthed evidence on the northwestern coast of Sumatrathat a previous tsunami had struck hundreds of years earlier. According to Archaeology magazine: A survey along 25 miles of coast near Banda Aceh revealed that at least 10 medieval communities once thrived there as part of the maritime Silk Road. A 1394 tsunami destroyed all but one of them. Thousands of recently recorded 15th-century Muslim gravestones suggest that Islamic traders eventually resettled the abandoned coast. [Source: Archaeology magazine, September-October 2019]

The mega-tsunami that struck Sumatra in 1394 affected parts of Southeast Asia and rivalling the deadly one in 2004, two teams of geologists, from Thailand and Indonesia, said in two articles published in in Nature magazine in 2008 after finding sedimentary evidence in coastal marshes. The tsunami hit around long before historical records of earthquakes in the region began. Some experts blame the massive loss of lives in 2004 on ignorance of the region's tsunami history.

"Tsunamis are something we never experienced before and after 2004, people thought it was something we would never experience again," Kruawun Jankaew of Thailand's Chulalongkorn University told Reuters. "But from this, we are able to identify that the place has been hit by a mega tsunami in the past. So even though it is infrequent for this part of the world, it still happens and there is a need to promote tsunami education for coastal peoples." [Source: Tan Ee Lyn, Reuters, October 29, 2008]

Reuters reported: Jankaew's team studied a grassy plain on Phra Thong, an island north of Phuket in Thailand, where the 2004 tsunami reached maximum wave heights of 20 meters above sea level. A separate team led by Katrin Monecke from the University of Pittsburgh looked at the sedimentary records on coastal marshes in Aceh, where the waves reached 35 meters. They explored low areas between beach ridges called "swales" — which are known to trap tsunami sand between layers of peat and other organic matter — and discovered a layer of sand beneath the most recent layer (2004), from 600 to 700 years ago. "Depending on where the depression is, it (the layer of the 1400 sand) can be 10 cm. But on higher ground, it can be two to five cm. Organic materials like bark and leaves, which contain carbon, were used for dating," Jankaew said.

The scientists next went to work on trying to figure out the scale of that catastrophe in 1394. "We will look at the thickness and grain size of the sediment and we can calculate how fast the tsumani was, how far inland it went, and the floor depth," she said. Jankaew said there are two more layers of sand under the 1400 layer but more studies would need to be done to date these.

Great Indian Ocean Tsunami of 1833

India plate

In 1833 a massive earthquake off the coast of Central Sumatra generated a destructive tsunami that must have affected Sumatra and the entire Indian Ocean basin and western coast of Australia. George Pararas-Carayannis wrote: There is not much data on the effects of the 1833 tsunami in Sumatra, Australia, Sri Lanka, India, the Maldives or on any of the islands or countries bordering the Indian Ocean. Undoubtedly the tsunami must have been devastating on the adjacent coastline of Sumatra. However, because of the orientation of the generating area, most of the tsunami energy radiated primarily towards unpopulated coastal areas of the Southwest Indian Ocean. [Source: George Pararas-Carayannis, 2007]

The tsunami must have had an impact on Sri Lanka, the Maldives, Madagascar and other islands and coastlines of Africa. Not much of its energy radiated towards the Bay of Bengal, so India - with the exception perhaps of the Tamil Nadu State - was not greatly affected nor was the coastlines of Bangladesh, Burma or Thailand. Also because of the tsunami's source area orientation, western Australia was not impacted in any significant way.

The rupture of the 1833 earthquake is estimated to have been between 300 to 350 kilometers. It was considerably less than the 1,200 kilometers rupture of the December 26, 2004. Although the magnitude of the 1833 earthquake was great, the tsunami generating area was much smaller than that of 2004 - the latter extending along two segments of the great Sunda tectonic boundary in northern Sumatra and the Andaman Sea.

Massive Earthquake off Central Sumatra That Caused the 1833 Tsunami

The earthquake that generated the 1833 tsunami was one of the largest earthquakes ever recorded along the coast of Sumatra, Indonesia. Modern studies estimate its magnitude at between magnitude 8.7 and 8.8, although some research based on coral records suggests it may have reached magnitude 9.2. If this higher estimate is correct, the earthquake was nearly as powerful as the devastating Indian Ocean earthquake of December 26, 2004. The event occurred along the Sunda megathrust, a major fault where oceanic crust is forced beneath the Indonesian island arc. [Source: George Pararas-Carayannis, 2007]

Scientists have learned much about the 1833 earthquake by studying ancient coral formations on the Mentawai Islands, particularly Sipora, North Pagai, and South Pagai. Corals grow close to sea level and preserve evidence of sudden changes in land elevation. When the earthquake struck, parts of these islands were lifted upward by as much as 1 to 2.3 meters. The uplift was greatest on the side facing away from the Sunda Trench, indicating that the islands tilted as the fault moved.

Location of 562 earthquakes between 810 BC and 2012, showing four clusters (shown by black dashed lines) of earthquake: west coast of Sumatra, Andaman and Nicobar Island, Bangladesh and adjoining regions and northwest coast of India; Main features include the AT = Andaman Trench and SSZ = Sunda Subduction Zone Researchgate

Research shows that the earthquake involved an enormous rupture beneath the Mentawai Islands and extended well beyond South Pagai Island. The amount of movement along the fault, known as slip, is estimated to have reached about 12 meters. This was much greater than the estimated 3 meters of slip during the earlier 1797 earthquake, which occurred on much of the same fault segment. The larger displacement in 1833 explains why it generated a much more powerful earthquake and tsunami.

The 1833 event demonstrated that earthquakes along the Sunda megathrust can vary greatly in size and impact. Although the 1797 and 1833 earthquakes occurred on the same section of the plate boundary, the later event released much more energy and caused greater deformation of the seafloor. Geological evidence suggests that similar giant earthquakes have occurred repeatedly over the last thousand years, often in pairs separated by a few decades. These findings indicate that the Mentawai region remains one of the most significant earthquake and tsunami sources in the Indian Ocean.

Tectonic Activity That Generated the Great Earthquake and Tsunami in 1833

The 1833 earthquake was caused by movement along the Sunda Subduction Zone, one of the most active tectonic boundaries on Earth. Here, the Indian and Australian oceanic plates move northward and dive beneath the Sunda Plate, which forms part of the larger Eurasian Plate system. This process, known as subduction, has been shaping the Indonesian archipelago for about 40 million years. It has also created the Sunda Trench, a deep ocean trench that stretches for thousands of kilometers along the western coast of Sumatra and Java. [Source: George Pararas-Carayannis, 2007]

As the oceanic plate slowly sinks beneath Sumatra, friction causes the two plates to become locked together. Even though the plates continue moving at a rate of about 60 millimeters per year in the Mentawai region, the locked fault prevents smooth motion. Over many decades, enormous amounts of stress accumulate within the Earth's crust. Eventually the fault can no longer withstand the pressure and ruptures suddenly, releasing centuries of stored energy in the form of a major earthquake.

The 1833 earthquake occurred on the Central and Southern Sumatra segment of the Sunda megathrust, often referred to as the Mentawai segment because it lies beneath the Mentawai Islands. Geological studies indicate that the rupture extended over a vast area of the fault and reached deeper beneath the islands than the rupture during the 1797 earthquake. The movement caused widespread uplift of the islands and displacement of the seafloor, which in turn generated a large tsunami across the eastern Indian Ocean. Such vertical movements are a hallmark of great subduction-zone earthquakes.

The Mentawai segment has experienced repeated major earthquakes throughout recorded history, including events in 1797, 1833, and 2007. The giant earthquakes of 2004, 2005, and 2007 along neighboring segments of the Sunda megathrust demonstrated that tectonic activity remains extremely high throughout the region. Studies of coral records and other geological evidence suggest that stress is not always released in a single earthquake. Instead, it may be released through a series of large events occurring over several decades, making the Sunda megathrust one of the world's most important regions for earthquake and tsunami monitoring.

Ancient Corals Give Insights Into Indian Ocean Tsunamis of the Past and Future

Research on ancient corals in the areas affected by the 2004 Indian Ocean Tsunami have provided evidence of large tsunamis in the past which also gives insights into when they may occur in the future. According to The Times: It has long been said that earthquakes occur in cycles, yet until recently evidence has been hard to come by. A pioneering study of the corals off the west coast of Sumatra — published in 2008 by an international team led by Kerry Sieh, a professor at Nanyang Technological University and the California Institute of Technology’s Tectonics Observatory — revealed that the region’s earthquakes during the past 700 years occurred in series of shocks that spanned decades. [Source: Frank Pope, The Times, December 12 2008]

Corals lay down growth rings every year, just like trees, and these record environmental changes including upheavals of the seabed. They show that the region’s earthquakes usually come in a sequence. “If previous cycles are a reliable guide we can expect one or more very large west Sumatran earthquakes ... within the next two decades,” Sieh, told reporters in Singapore. [Source: Reuters, December 12, 2008]

Scientists said that the earthquake could be similar to the one that sparked the 2004 Indian Ocean Tsunami. the devastating 2004 tsunami and left 230,000 people either dead or missing across Asia Sieh said while Thailand and Sri Lanka were unlikely to be affected, people in Sumatra should be prepared. “The tsunami could be at five meters in Padang (in Sumatra). This is a worse case scenario,” he said.

Sieh, whose team’s research was published reported in Science journal, said corals off Sumatra’s Mentawai Islands showed a major earthquake had occurred every 200 years since 1300. “When earthquakes push the seafloor upward, lowering local sea level, the corals can’t grow upward and grow outward instead,” the researchers wrote in Science. Earlier in 2008, month, Sieh and his colleagues reported in the journal Nature that an area off Sumatra that has been the source of disastrous earthquakes, still carried a lot of pent-up pressure that could result in another strong quake.

Sumatran Cave Contains Evidence of Tsunamis Dating Back 7,500 Years Ago

A cave discovered in Sumatra near the source of the massive earthquake that spawned the 2004 Indian Ocean Tsunami contains evidence of past tsunamis dating up to 7,500 years ago. The findings provide the longest and most detailed timeline for tsunamis that have occurred off western Sumatra in Aceh province. near Banda Aceh. The limestone cave is about one meter above knee-high tide and protected from storms and wind. Only huge waves that inundate the coastal area are able to penetrate inside the cave.

According to Associated Press: Researchers in 2011 uncovered seabed sand deposits that were swept into the cave over thousands of years and neatly layered between bat droppings like a geological cake. Radiocarbon analysis of materials, including clamshells and the remains of microscopic organisms, provided evidence of 11 tsunamis before 2004. The disasters were by no means evenly spaced, said lead researcher Charles Rubin from the Earth Observatory of Singapore. The last one occurred about 2,800 years ago, but there were four others in the preceding 500 years. [Source: Margie Mason, The Associated Press, December 24, 2013]

And it's possible there were others. Researchers know, for instance, that there were two mammoth earthquakes in the region around 1393 and 1450. Rubin said a big tsunami could have carried away evidence of other events through erosion. In 2013 the scientists are still working to determine the size of the waves that entered the cave. "The take-home message is perhaps that the 2004 event doesn't mean it won't happen for another 500 years," said Rubin, who added that the cave was discovered by chance and not part of planned field work. "We did see them clustered together closer in time. I wouldn't put out a warning that we're going to have an earthquake, but it shows that the timing is really variable."

The quake that triggered the 2004 tsunami surprised scientists because the fault that unleashed the megathrust temblor had been quiet for hundreds of years. And since the last big earthquake had struck more than 500 years earlier, there was no surviving oral history that could have helped people understand the risk. Since 2004, much research has been done to try to learn about the area's past by examining sand deposits, uplifted coral and GPS data.

"The findings are very significant," Katrin Monecke, a geosciences professor at Wellesley College in Massachusetts wrote. She worked on tsunami sand deposits discovered in marshes in the area, but was not involved with the cave research, which was presented this month at an American Geophysical Union conference in San Francisco. "The sand sheets in the cave cover a very long time span and give an excellent idea about earthquake frequency."

The most puzzling and perhaps alarming part of the data, according to Andy Moore, a geologist at Earlham College, is the streak of four tsunamis in a single century. “The clustered tsunamis is to me the biggest thing that needs to be clarified because that has real world impacts on how we handle the tsunami hazards on the coast of the Indian Ocean,” he told the Atlantic. How much should countries invest in infrastructure protecting against a potential tsunami? How important is a warning system? “The answer if the tsunami could happen once every few decades to a century, that’s a vastly different answer than [if it happens] every 500 years,” he says. [Source: Sarah Zhang, The Atlantic July 19, 2017]

How Bat Feces Provided Key Data to Understanding Past Tsunami

Benjamin Horton of Rutgers University and Nanyang Technological University was involved in studying the cave near Banda Aceh that gave insights into past tsunamis in the Indian Ocean described above. Sarah Zhang wrote in The Atlantic: Horton knew the cave was special the moment he set foot inside in 2011. His colleague, Patrick Daly, an archeologist at Nanyang, had heard about it from locals. The first thing they noticed is that the opening of the cave did not directly face the ocean—a good sign because that positioning slows the movement of water, allowing sand brought by the tsunami to settle in the cave.[Source: Sarah Zhang, The Atlantic July 19, 2017]

Then they stepped in the dark, second chamber. “The next thing you know we were faced with thousands of bats. We were just drenched in bat pee,” says Horton. These bats turned out to be key. Tsunamis had been inundating this cave for thousands of years, during which time bats were also pooping on the cave floor. A tsunami came. Bats pooped. Tsunami, bat poop, tsunami, bat poop, and so on. So when Horton and Daly dug into the sand in the cave, they saw these perfect layers of sand, separated by dark bands of bat poop. “It was a holy grail moment,” says Horton. “We knew we had found something very, very unique.”

Over several years, Horton and his colleagues dug six major trenches up to six-and-a-half feet deep. They carbon dated the animal shells and charcoal in the sand layers as well as the bat poop itself. There was one odd hiccup in data. The cave was missing the last 2,900 years of records because, Horton and his colleagues think, the 2004 tsunami actually washed away those layers. The bottom of the sand layer from 2004 was all irregular, as if something (perhaps the 2004 tsunami) had ripped out uneven chunks of the sand bed. This made the team worry that other parts of the tsunami record might be missing. But they did not see the irregular border in any other time period.

Horton and his colleagues are continuing to work in the cave. They hope that by looking at the thickness of the sand layers and the grain size of the sand, they might determine how big each of those tsunamis were. They’re also driving down the coastal highway, in search of more caves that can corroborate the records from this one. This highway is relatively new; it was built after the 2004 tsunami.

Image Sources: Wikimedia Commons

Text Sources: National Geographic, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Encyclopedia.com, Library of Congress, Indonesia Tourism website (indonesia.travel), Indonesia government websites, Live Science, The Conversation, The New Yorker, Time, BBC, CNN, Reuters, Associated Press, AFP, Lonely Planet Guides, Google AI, Wikipedia, The Guardian and various websites, books and other publications.

Last updated June 2026