2004 Boxing Day Tsunami Part I

 

On 26 December 2004, at 07:58:53 local time (UTC+7), a major earthquake with a magnitude of 9.2–9.3 Mw struck with an epicenter off the west coast of northern Sumatra, Indonesia. The undersea megathrust earthquake, known by the scientific community as the Sumatra–Andaman earthquake, was caused by a rupture along the fault between the Burma Plate and the Indian Plate, and reached Mercalli intensity up to IX in some areas.

A massive tsunami with waves up to 30 m (100 ft) high, known as the Boxing Day Tsunami after the Boxing Day holiday, or as the Asian Tsunami, devastated communities along the surrounding coasts of the Indian Ocean, killing an estimated 227,898 people in 14 countries in one of the deadliest natural disasters in recorded history. The direct results caused major disruptions to living conditions and commerce in coastal provinces of surrounded countries, including Aceh (Indonesia), Sri Lanka, Tamil Nadu (India) and Khao Lak (Thailand). Banda Aceh reported the largest number of deaths. It is the deadliest natural disaster of the 21st century, and the worst tsunami disaster in history. It is also the worst natural disaster in the history of Indonesia, Sri Lanka and Thailand.

It was the most powerful earthquake ever recorded in Asia, the most powerful earthquake in the 21st century, and at least the third most powerful earthquake ever recorded in the world since modern seismography began in 1900. It had the longest fault rupture ever observed, between 1,200 km to 1,300 km (720 mi to 780 mi), and had the longest duration of faulting ever observed, at least ten minutes. It caused the planet to vibrate as much as 10 mm (0.4 in), and also remotely triggered earthquakes as far away as Alaska. Its epicenter was between Simeulue and mainland Sumatra. The plight of the affected people and countries prompted a worldwide humanitarian response, with donations totalling more than US$14 billion (equivalent to US$23 billion in 2023 currency).

Earthquake

2004 Indian Ocean

Humanitarian response

Military operations

The 2004 Indian Ocean earthquake was initially documented as having a moment magnitude of 8.8. The United States Geological Survey has its estimate of 9.1. Hiroo Kanamori of the California Institute of Technology estimates that Mw 9.2 is best representative of the earthquake's size. However, more recent studies estimate the magnitude to be Mw 9.3. A 2016 study estimated the magnitude to be Mw 9.25, while a 2021 study revised its 2007 estimate of Mw 9.1 to a new magnitude of Mw 9.2.

The hypocenter of the main earthquake was approximately 160 km (100 mi) off the western coast of northern Sumatra, in the Indian Ocean just north of Simeulue island at a depth of 30 km (19 mi) below mean sea level (initially reported as 10 km or 6.2 mi). The northern section of the Sunda megathrust ruptured over a length of 1,300 km (810 mi). The earthquake (followed by the tsunami) was felt in Bangladesh, India, Malaysia, Myanmar, Thailand, Sri Lanka and the Maldives. Splay faults, or secondary "pop up faults", caused long, narrow parts of the seafloor to pop up in seconds. This quickly elevated the height and increased the speed of waves, destroying the nearby Indonesian town of Lhoknga.

Indonesia lies between the Pacific Ring of Fire along the north-eastern islands adjacent to New Guinea, and the Alpide belt that runs along the south and west from Sumatra, Java, Bali, and Flores to Timor. The 2002 Sumatra earthquake is believed to have been a foreshock, preceding the main event by over two years.

Historical comparisons

Great earthquakes, such as the 2004 Indian Ocean earthquake, are associated with megathrust events in subduction zones. Their seismic moments can account for a significant fraction of the global seismic moment across century-scale periods. Of all the moment released by earthquakes in the 100 years from 1906 through 2005, roughly one eighth was due to the 2004 Indian Ocean earthquake. This quake, together with the Great Alaskan earthquake (1964) and the Great Chilean earthquake (1960), account for almost half of the total moment.

Since 1900, the only earthquakes recorded with a greater magnitude were the 1960 Valdivia earthquake (magnitude 9.5) and the 1964 Alaska earthquake in Prince William Sound (magnitude 9.2). The only other recorded earthquakes of magnitude 9.0 or greater were off Kamchatka, Russia, on 5 November 1952 (magnitude 9.0) and Tōhoku, Japan (magnitude 9.1) in March 2011. Each of these megathrust earthquakes also spawned tsunamis in the Pacific Ocean. In comparison to the 2004 Indian Ocean earthquake, the death toll from these earthquakes and tsunamis was significantly lower, primarily because of the lower population density along the coasts near affected areas.

Comparisons with earlier earthquakes are difficult, as earthquake strength was not measured systematically until the 1930s. However, historical earthquake strength can sometimes be estimated by examining historical descriptions of the damage caused, and the geological records of the areas where they occurred. Some examples of significant historical megathrust earthquakes are the 1868 Arica earthquake in Peru and the 1700 Cascadia earthquake in western North America.

Tectonic plates

Epicenter and associated aftershocks

The 2004 Indian Ocean earthquake was unusually large in geographical and geological extent. An estimated 1,600 km (1,000 mi) of fault surface slipped (or ruptured) about 15 m (50 ft) along the subduction zone where the Indian Plate slides under (or subducts) the overriding Burma Plate. The slip did not happen instantaneously but took place in two phases over several minutes: Seismographic and acoustic data indicate that the first phase involved a rupture about 400 km (250 mi) long and 100 km (60 mi) wide, 30 km (19 mi) beneath the sea bed—the largest rupture ever known to have been caused by an earthquake. The rupture proceeded at about 2.8 km/s (1.74 mi/s; 10,100 km/h; 6,260 mph), beginning off the coast of Aceh and proceeding north-westerly over about 100 seconds. After a pause of about another 100 seconds, the rupture continued northwards towards the Andaman and Nicobar Islands. The northern rupture occurred more slowly than in the south, at about 2.1 km/s (1.3 mi/s; 7,600 km/h; 4,700 mph), continuing north for another five minutes to a plate boundary where the fault type changes from subduction to strike-slip (the two plates slide past one another in opposite directions).

The Indian Plate is part of the Indo-Australian Plate, which underlies the Indian Ocean and Bay of Bengal, and is moving north-east at an average of 60 mm/a (0.075 in/Ms). The India Plate meets the Burma Plate (which is considered a portion of the great Eurasian Plate) at the Sunda Trench. At this point, the India Plate subducts beneath the Burma Plate, which carries the Nicobar Islands, the Andaman Islands, and northern Sumatra. The India Plate sinks deeper and deeper beneath the Burma Plate until the increasing temperature and pressure drive volatiles out of the subducting plate. These volatiles rise into the overlying plate, causing partial melting and the formation of magma. The rising magma intrudes into the crust above and exits the Earth's crust through volcanoes in the form of a volcanic arc. The volcanic activity that results as the Indo-Australian Plate subducts the Eurasian Plate has created the Sunda Arc.

As well as the sideways movement between the plates, the 2004 Indian Ocean earthquake resulted in a rise of the seafloor by several metres, displacing an estimated 30 km3 (7.2 cu mi) of water and triggering devastating tsunami waves. The waves radiated outwards along the entire 1,600 km (1,000 mi) length of the rupture (acting as a line source). This greatly increased the geographical area over which the waves were observed, reaching as far as Mexico, Chile, and the Arctic. The raising of the seafloor significantly reduced the capacity of the Indian Ocean, producing a permanent rise in the global sea level by an estimated 0.1 mm (0.004 in).

Aftershocks and other earthquakes

Aftershocks of 2004 Indian Ocean earthquake

Numerous aftershocks were reported off the Andaman Islands, the Nicobar Islands and the region of the original epicentre in the hours and days that followed. The magnitude 8.6 2005 Nias–Simeulue earthquake, which originated off the coast of the Sumatran island of Nias, is not considered an aftershock, despite its proximity to the epicentre, and was most likely triggered by stress changes associated with the 2004 event. The earthquake produced its own aftershocks (some registering a magnitude of as high as 6.9) and presently ranks as the third-largest earthquake ever recorded on the moment magnitude or Richter magnitude scale.

Other aftershocks of up to magnitude 7.2 continued to shake the region daily for three or four months. [42] As well as continuing aftershocks, the energy released by the original earthquake continued to make its presence felt well after the event. A week after the earthquake, its reverberations could still be measured, providing valuable scientific data about the Earth's interior.

The 2004 Indian Ocean earthquake came just three days after a magnitude 8.1 earthquake in the sub-Antarctic Auckland Islands, an uninhabited region west of New Zealand, and Macquarie Island to Australia's north. This is unusual since earthquakes of magnitude eight or more occur only about once per year on average. The U.S. Geological Survey sees no evidence of a causal relationship between these events.

The 2004 Indian Ocean earthquake is thought to have triggered activity in both Leuser Mountain and Mount Talang, volcanoes in Aceh along the same range of peaks, while the 2005 Nias–Simeulue earthquake sparked activity in Lake Toba, a massive caldera in Sumatra.

Energy released

The energy released on the Earth's surface (Me, the energy magnitude, which is the seismic potential for damage) by the 2004 Indian Ocean earthquake was estimated at 1.1×1017 joules (110 PJ; 26 Mt). This energy is equivalent to over 1,500 times that of the Hiroshima atomic bomb, but less than that of Tsar Bomba, the largest nuclear weapon ever detonated.

The earthquake generated a seismic oscillation of the Earth's surface of up to 200–300 mm (8–12 in), equivalent to the effect of the tidal forces caused by the Sun and Moon. The seismic waves of the earthquake were felt across the planet, as far away as the U.S. state of Oklahoma, where vertical movements of 3 mm (0.12 in) were recorded. By February 2005, the earthquake's effects were still detectable as a 20 μm (0.02 mm; 0.0008 in) complex harmonic oscillation of the Earth's surface, which gradually diminished and merged with the incessant free oscillation of the Earth more than four months after the earthquake.

Because of its enormous energy release and shallow rupture depth, the earthquake generated remarkable seismic ground motions around the globe, particularly due to huge Rayleigh (surface) elastic waves that exceeded 10 mm (0.4 in) in vertical amplitude everywhere on Earth. The record section plot displays vertical displacements of the Earth's surface recorded by seismometers from the IRIS/USGS Global Seismographic Network plotted with respect to time (since the earthquake initiation) on the horizontal axis, and vertical displacements of the Earth on the vertical axis (note the 1 cm scale bar at the bottom for scale). The seismograms are arranged vertically by distance from the epicenter in degrees. The earliest, lower amplitude signal is that of the compressional (P) wave, which takes about 22 minutes to reach the other side of the planet (the antipode; in this case near Ecuador). The largest amplitude signals are seismic surface waves that reach the antipode after about 100 minutes. The surface waves can be clearly seen to reinforce near the antipode (with the closest seismic stations in Ecuador), and to subsequently encircle the planet to return to the epicentral region after about 200 minutes. A major aftershock (magnitude 7.1) can be seen at the closest stations starting just after the 200-minute mark. The aftershock would be considered a major earthquake under ordinary circumstances but is dwarfed by the mainshock.

The shift of mass and the massive release of energy slightly altered the Earth's rotation. Weeks after the earthquake, theoretical models suggested the earthquake shortened the length of a day by 2.68 microseconds, due to a decrease in the oblateness of the Earth.  It also caused the Earth to minutely "wobble" on its axis by up to 25 mm (1 in) in the direction of 145° east longitude, or perhaps by up to 50 or 60 mm (2.0 or 2.4 in). Because of tidal effects of the Moon, the length of a day increases at an average of 15 microseconds per year, so any rotational change due to the earthquake will be lost quickly. Similarly, the natural Chandler wobble of the Earth, which in some cases can be up to 15 m (50 ft), eventually offset the minor wobble produced by the earthquake.

There was 10 m (33 ft) movement laterally and 4–5 m (13–16 ft) vertically along the fault line. Early speculation was that some of the smaller islands south-west of Sumatra, which is on the Burma Plate (the southern regions are on the Sunda Plate), might have moved south-west by up to 36 m (120 ft), but more accurate data released more than a month after the earthquake found the movement to be about 0.2 m (8 in). Since movement was vertical as well as lateral, some coastal areas may have been moved to below sea level. The Andaman and Nicobar Islands appear to have shifted south-west by around 1.25 m (4 ft 1 in) and to have sunk by 1 m (3 ft 3 in).

In February 2005, the Royal Navy vessel HMS Scott surveyed the seabed around the earthquake zone, which varies in depth between 1,000 and 5,000 m (550 and 2,730 fathoms; 3,300 and 16,400 ft). The survey, conducted using a high-resolution, multi-beam sonar system, revealed that the earthquake had made a considerable impact on the topography of the seabed. 1,500-metre-high (5,000 ft) thrust ridges created by previous geologic activity along the fault had collapsed, generating landslides several kilometres wide. One such landslide consisted of a single block of rock some 100 m (330 ft) high and 2 km (1.2 mi) long. The momentum of the water displaced by tectonic uplift had also dragged massive slabs of rock, each weighing millions of tonnes, as far as 10 km (6 mi) across the seabed. An oceanic trench several kilometres wide was exposed in the earthquake zone.

The TOPEX/Poseidon and Jason-1 satellites happened to pass over the tsunami as it was crossing the ocean. These satellites carry radars that measure precisely the height of the water surface; anomalies in the order of 500 mm (20 in) were measured. Measurements from these satellites may prove invaluable for the understanding of the earthquake and tsunami. Unlike data from tide gauges installed on shores, measurements obtained in the middle of the ocean can be used for computing the parameters of the source earthquake without having to compensate for the complex ways in which proximity to the coast changes the size and shape of a wave.

Assessment of potential earthquakes in the future

Before the 2004 quake there were three arguments against a large earthquake occurring the Sumatra region. After the quake it was considered that earthquake hazard risk would need to be reassessed for regions previously thought to have low risk based on these criteria:

The subducting plate at the location of the 2004 quake is older and denser. Before the 2004 earthquake it was thought that only the subduction of young and buoyant crust could produce giant earthquakes.

Slow plate motion. Previously it was thought that the convergence rate had to be fast.

Before the 2004 quake it was thought that giant earthquakes only occurred in regions without back-arc basins.

Tsunami

The sudden vertical rise of the seabed by several metres during the earthquake displaced massive volumes of water, resulting in a tsunami that struck the coasts of the Indian Ocean. A tsunami that causes damage far away from its source is sometimes called a teletsunami and is much more likely to be produced by the vertical motion of the seabed than by horizontal motion.

The tsunami, like all the others, behaved differently in deep water than in shallow water. In deep ocean water, tsunami waves form only a low, broad hump, barely noticeable and harmless, which generally travels at the high speed of 500 to 1,000 km/h (310 to 620 mph); in shallow water near coastlines, a tsunami slows down to only tens of kilometres per hour but, in doing so, forms large destructive waves. Scientists investigating the damage in Aceh found evidence that the wave reached a height of 24 m (80 ft) when coming ashore along large stretches of the coastline, rising to 30 m (100 ft) in some areas when travelling inland. Radar satellites recorded the heights of tsunami waves in deep water: the maximum height was at 600 mm (2 ft) two hours after the earthquake, the first such observations ever made.

According to Tad Murty, vice-president of the Tsunami Society, the total energy of the tsunami waves was equivalent to about 5 megatons of TNT (21 PJ), which is more than twice the total explosive energy used during all of World War II (including the two atomic bombs) but still a couple of orders of magnitude less than the energy released in the earthquake itself. In many places, the waves reached as far as 2 km (1.2 mi) inland.

Because the 1,600 km (1,000 mi) fault affected by the earthquake was in a nearly north–south orientation, the greatest strength of the tsunami waves was in an east–west direction. Bangladesh, which lies at the northern end of the Bay of Bengal, had few casualties despite being a low-lying country relatively near the epicentre. It also benefited from the fact that the earthquake proceeded more slowly in the northern rupture zone, greatly reducing the energy of the water displacements in that region.

Average height of the waves

Coasts that have a landmass between them and the tsunami's location of origin are usually safe; however, tsunami waves can sometimes diffract around such landmasses. Thus, the state of Kerala was hit by the tsunami despite being on the western coast of India, and the western coast of Sri Lanka suffered substantial impacts. Distance alone was no guarantee of safety, as Somalia was hit harder than Bangladesh despite being much farther away.

Because of the distances involved, the tsunami took anywhere from fifteen minutes to seven hours to reach the coastlines. The northern regions of the Indonesian island of Sumatra were hit quickly, while Sri Lanka and the east coast of India were hit roughly 90 minutes to two hours later. Thailand was struck about two hours later despite being closer to the epicentre because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast.

The tsunami was noticed as far as Struisbaai in South Africa, about 8,500 km (5,300 mi) away, where a 1.5-metre-high (5 ft) tide surged on shore about 16 hours after the earthquake. It took a relatively long time to reach Struisbaai at the southernmost point of Africa, probably because of the broad continental shelf off South Africa and because the tsunami would have followed the South African coast from east to west. The tsunami also reached Antarctica, where tidal gauges at Japan's Showa Base recorded oscillations of up to a metre (3 ft 3 in), with disturbances lasting a couple of days.

Some of the tsunami's energy escaped into the Pacific Ocean, where it produced small but measurable tsunamis along the western coasts of North and South America, typically around 200 to 400 mm (7.9 to 15.7 in). At Manzanillo, Mexico, a 2.6 m (8.5 ft) crest-to-trough tsunami was measured. As well, the tsunami was large enough to be detected in Vancouver, which puzzled many scientists, as the tsunamis measured in some parts of South America were larger than those measured in some parts of the Indian Ocean. It has been theorized that the tsunamis were focused and directed at long ranges by the mid-ocean ridges which run along the margins of the continental plates.

Early signs and warnings

Despite a delay of up to several hours between the earthquake and the impact of the tsunami, nearly all of the victims were taken by surprise. There were no tsunami warning systems in the Indian Ocean to detect tsunamis or to warn the general population living around the ocean. Tsunami detection is difficult because while a tsunami is in deep water, it has little height and a network of sensors is needed to detect it.

Tsunamis are more frequent in the Pacific Ocean than in other oceans because of earthquakes in the "Ring of Fire". Although the extreme western edge of the Ring of Fire extends into the Indian Ocean (the point where the earthquake struck), no warning system exists in that ocean. Tsunamis there are relatively rare despite earthquakes being relatively frequent in Indonesia. The last major tsunami was caused by the 1883 eruption of Krakatoa. Not every earthquake produces large tsunamis: on 28 March 2005, a magnitude 8.7 earthquake hit roughly the same area of the Indian Ocean but did not result in a major tsunami.

The first warning sign of a possible tsunami is the earthquake itself. However, tsunamis can strike thousands of kilometres away where the earthquake is felt only weakly or not at all. Also, in the minutes preceding a tsunami strike, the sea sometimes recedes temporarily from the coast, which was observed on the eastern earthquake rupture zone such as the coastlines of Aceh, Phuket Island, and Khao Lak area in Thailand, Penang island of Malaysia, and the Andaman and Nicobar islands. This rare sight reportedly induced people, especially children, to visit the coast to investigate and collect stranded fish on as much as 2.5 km (1.6 mi) of exposed beach, with fatal results. However, not all tsunamis cause this "disappearing sea" effect. In some cases, there are no warning signs at all: the sea will suddenly swell without retreating, surprising many people and giving them little time to flee.

One of the few coastal areas to evacuate ahead of the tsunami was on the Indonesian island of Simeulue, close to the epicentre. Island folklore recounted an earthquake and tsunami in 1907, and the islanders fled to inland hills after the initial shaking and before the tsunami struck. These tales and oral folklore from previous generations may have helped the survival of the inhabitants. On Maikhao Beach in north Phuket City, Thailand, a 10-year-old British tourist named Tilly Smith had studied tsunamis in geography at school and recognised the warning signs of the receding ocean and frothing bubbles. She and her parents warned others on the beach, which was evacuated safely. John Chroston, a biology teacher from Scotland, also recognized the signs at Kamala Bay north of Phuket, taking a busload of vacationers and locals to safety on higher ground.

Anthropologists had initially expected the aboriginal population of the Andaman Islands to be badly affected by the tsunami and even feared the already depopulated Onge tribe could have been wiped out. Many of the aboriginal tribes evacuated and suffered fewer casualties, however. Oral traditions developed from previous earthquakes helped the aboriginal tribes escape the tsunami. For example, the folklore of the Onges talks of "huge shaking of ground followed by high wall of water". Almost all of the Onge people seemed to have survived the tsunami.

Indonesia

Aceh

The tsunami devastated the coastline of Aceh province, about 20 minutes after the earthquake. Banda Aceh, the closest major city, suffered severe casualties. The sea receded and exposed the seabed, prompting locals to collect stranded fish and explore the area. Local eyewitnesses described three large waves, with the first wave rising gently to the foundation of the buildings, followed minutes later by a sudden withdrawal of the sea near the port of Ulèë Lheue. This was succeeded by the appearance of two large black-colored steep waves which then travelled inland into the capital city as a large turbulent bore. Eyewitnesses described the tsunami as a "black giant", "mountain" and a "wall of water". Video footage revealed torrents of black water, surging by windows of a two-story residential area situated about 3.2 km (2.0 mi) inland. Additionally, amateur footage recorded in the middle of the city captured an approaching black surge flowing down the city streets, full of debris, inundating them.

Apung 1, a 2,600-ton vessel, was flung some 2 km (1.2 mi) to 3 km (1.9 mi) inland. In the years following the disaster, it became a local tourist attraction and has remained where it came to rest.

The level of destruction was extreme on the northwestern areas of the city, immediately inland of the aquaculture ponds, and directly facing the Indian Ocean. The tsunami height was reduced from 12 m (39 ft) at Ulee Lheue to 6 m (20 ft) a further 8 km (5.0 mi) to the north-east. The inundation was observed to extend 3–4 km (1.9–2.5 mi) inland throughout the city. Within 2–3 km (1.2–1.9 mi) of the shoreline, houses, except for strongly-built reinforced concrete ones with brick walls, which seemed to have been partially damaged by the earthquake before the tsunami attack, were swept away or destroyed by the tsunami. The area toward the sea was wiped clean of nearly every structure, while closer to the river, dense construction in a commercial district showed the effects of severe flooding. The flow depth at the city was just at the level of the second floor, and there were large amounts of debris piled along the streets and in the ground-floor storefronts. In the seaside section of Ulee Lheue, the flow depths were over 9 m (30 ft). Footage showed evidence of back-flowing of the Aceh River, carrying debris and people from destroyed villages at the coast and transporting them up to 40 km (25 mi) inland.

A group of small islands: Weh, Breueh, Nasi, Teunom, Bunta, Lumpat, and Batee lie just north of the capital city. The tsunami reached a run-up of 10–20 m (33–66 ft) on the western shorelines of Breueh Island and Nasi Island. Coastal villages were destroyed by the waves. On the island of Pulau Weh, strong surges were experienced in the port of Sabang, yet there was little damage with reported run-up values of 3–5 m (9.8–16.4 ft), most likely due to the island being sheltered from the direct attack by the islands to the south-west.