2004 Indian Ocean earthquake

(source: http://en.wikipedia.org/wiki/2004_Indian_Ocean_earthquake#Tectonic_plates )

Quake characteristics

The earthquake was initially reported as 8.6 on the Richter scale. The Pacific Tsunami Warning Center (PTWC) also estimated it at 8.5 shortly after the earthquake. On the moment magnitude scale, which is more accurate for quakes of this size, the earthquake's magnitude was first reported as 8.1 by the U.S. Geological Survey. After further analysis, this was increased to 8.5, 8.9, and 9.0 [6]. In February 2005, some scientists revised the estimate of magnitude to 9.3. Although the Pacific Tsunami Warning Center has accepted this, the USGS has so far not changed its estimate of 9.0 [7]. The most definitive estimate so far has put the magnitude at 9.15 [8].

The hypocentre of the main earthquake was at 3.316°N, 95.854°E (3°19′N 95°51.24′E), some 160 km (100 mi) west of Sumatra, at a depth of 30 km (18.6 mi) below mean sea level (initially reported as 10 km). The earthquake itself (apart from the tsunami) was felt as far away as Bangladesh, India, Malaysia, Myanmar, Thailand, Singapore and the Maldives.

Indonesia lies between the Pacific Ring of Fire along the northeastern islands adjacent to and including New Guinea and the Alpide belt along the south and west from Sumatra, Java, Bali, Flores, and Timor. The December 2004 earthquake just off the coast of Sumatra was actually a part of the Alpide belt.

Since 1900 the only earthquakes recorded with a greater magnitude were the 1960 Great Chilean Earthquake (magnitude 9.5) and the 1964 Good Friday Earthquake in Prince William Sound (9.2). The March 9, 1957 earthquake [9] in the Andreanof Islands (9.1) was of comparable magnitude. The only other recorded earthquakes of magnitude 9.0 or greater were two off Kamchatka, Russia on October 16, 1737 and November 4, 1952 (magnitude 9.3 and 9.0 respectively) [10][11]. Each of these megathrust earthquakes also spawned tsunamis (in the Pacific Ocean), but the death toll from these was significantly lower; a few thousand for the worst one, probably because of the lower population density along the coasts near affected areas and the much greater distances to more populated coasts.

Other larger megathrust earthquakes occurred in 1868 (Peru, Nazca Plate and South American Plate); 1827 (Colombia, Nazca Plate and South American Plate); 1812 (Venezuela, Caribbean Plate and South American Plate) and 1700 (Cascadia Earthquake, western US and Canada, Juan de Fuca Plate and North American Plate). These are all believed to have been of greater than magnitude 9, but no accurate measurements were available in those days.

The US Embassy in China tried somewhat unsuccessfully to quell rumors that the tsunami-quake was caused by a nuclear explosion. The conspiracy theories were fed by the fact that a top-secret US-British weapons program -- which was quietly declassified in 1999 and published only in New Zealand newspapers [12] -- named Project Seal had the specific goal to develop and test tsunami-triggering weapons in 1944 and 1945. The project was allegedly cancelled.

Tectonic plates

Animation of the tsunami caused by the earthquake (see also the full-length version from NOAA/PMEL) or this more detailed image with time notations.The earthquake was unusually large in geographical extent. An estimated 1200 km (750 mi) of faultline slipped about 15 m (50 ft) along the subduction zone where the India Plate dives under the Burma Plate. The slip did not happen instantaneously but took place in two phases over a period of several minutes. Seismographic and acoustic data indicate that the first phase involved the formation of a rupture about 400 km (250 mi) long and 100 km (60 mi) wide, located 30 km (19 mi) beneath the sea bed - the longest rupture ever known to have been caused by an earthquake. The rupture proceeded at a speed of about 2.8 km/s (1.7 mi/s) or 10,000 km/h (6,300 mph), beginning off the coast of Aceh and proceeding north-westerly over a period of about 100 seconds. A pause of about another 100 seconds took place before the rupture continued northwards towards the Andaman and Nicobar Islands. However, the northern rupture occurred more slowly than in the south, at about 2.1 km/s (4,700 mph), continuing north for another five minutes to a plate boundary where the fault changes from subduction to strike-slip (the two plates push past one another in opposite directions) thus reducing the speed of the water displacement and so reducing the size of the tsunami that hit the northern part of the Indian Ocean [13].

The India Plate is part of the great Indo-Australian Plate, which underlies the Indian Ocean and Bay of Bengal, and is drifting north east at an average of 6 cm/year (2 inches per year). The India Plate meets the Australasian Plate (which is considered a portion of the great Eurasian Plate) at the Sunda Trench. At this point the India Plate subducts the Burma Plate, which carries the Nicobar Islands, the Andaman Islands and northern Sumatra. The India Plate slips 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 mantle above and trigger melt which exits the earth's mantle through volcanoes (see 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 sea bed is estimated to have risen by several metres, displacing an estimated 30 km³ (7 cu mi) of water and triggering devastating tsunami waves. The waves did not originate from a point source, as mistakenly depicted in some illustrations of their spread, but radiated outwards along the entire 1200 km (750 mi) length of the rupture. 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 sea bed significantly reduced the capacity of the Indian Ocean, producing a permanent rise in the global sea level by an estimated 0.1 mm. [14]

Aftershocks and other earthquakes

Locations of initial quake and aftershocks (Credit: USGS)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 largest aftershock of magnitude 8.7 was located off the Sumatran island of Nias [15]. A debate arose among seismologists over whether the 2005 Sumatra earthquake should be considered an aftershock of the December 2004 event, or a "triggered earthquake" as it was larger than typical aftershocks and on a different fault. Other aftershocks of up to magnitude 6.6 continue to shake the region on a daily basis [16][17].

The 2004 Indian Ocean earthquake came just three days after a magnitude 8.1 earthquake in an uninhabited region west of New Zealand's sub-Antarctic Auckland Islands, and north of Australia's Macquarie Island [18]. This is unusual, since earthquakes of magnitude 8 or more occur only about once per year on average [19]. Some seismologists have speculated about a connection between these two earthquakes, saying that the former one might have been a catalyst to the Indian Ocean earthquake, as the two quakes happened on opposite sides of the Indo-Australian Plate [20] (a 6.5 earthquake[21] occurred on 19 February 2005 off Sulawesi at the other end of the Indonesian island chain). However the US Geological Survey sees no evidence of a causal relationship [22].

Coincidentally the earthquake struck almost exactly one year (to the hour) after a magnitude 6.6 earthquake killed an estimated 30,000 people in the city of Bam in Iran[23].

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[24].

An earthquake of magnitude 8.7 was reported shortly at 16:09:37 UTC (23:09:37 local time) on March 28, 2005 approximately at the same location (see 2005 Sumatran earthquake). It is likely a very large aftershock of the original earthquake. This earthquake had strong aftershocks of its own, including magnitude 6.0 and 6.1 quakes. At 8.7, it ranks as the 7th largest earthquake since 1900.

An earthquake magnitude 6.7 struck on 10 April 2005 at 17:29 local time (10:29 GMT) about 120 km (75 mi) south-west of the city of Padang. [25][26]

Some scientists warn that geological stresses caused by the recent quakes may even have increased the possibility that the Lake Toba supervolcano could erupt. [27] According to the Toba catastrophe theory, this could threaten human life on Earth.

Some scientists confirm that the December quake had activated Leuser Mountain, a volcano in Aceh province along the same range of peaks as Talang, while the 2005 Sumatran earthquake had sparked activity in lake Toba, an ancient crater in Sumatra. [28]

Coincidentally, Mount Talang has since erupted [29] and is now on top alert.

Power of the earthquake

The total energy released by the 2004 Indian Ocean earthquake has been estimated as 3.35 exajoules (3.35×1018 joules) [30]. This is equivalent to 0.8 gigatons of TNT, or about as much energy as is used in the United States in 11 days. However, the most reliable seismic energy release estimate, as of Sept 30th 2005, is 1.1×1018 joules. This corresponds to about 0.25 gigatons of TNT. The earthquake is estimated to have resulted in an oscillation of the Earth's surface of about 20-30 cm (8 to 12 in), equivalent to the effect of the tidal forces caused by the Sun and Moon[31]. The shock waves of the earthquake were felt across the planet; as far away as Oklahoma, where vertical movements of 3 mm (0.12 in) were recorded [32]. The entire Earth's surface is estimated to have moved vertically by up to 1 cm.

The shift of mass and the massive release of energy very slightly altered the Earth's rotation. The exact amount is yet undetermined, but theoretical models suggest the earthquake shortened the length of a day by 2.68 microseconds (2.68 µs) (or about one billionth of the length of a day) [33] due to a decrease in the oblateness of the Earth. It also caused the Earth to minutely "wobble" on its axis by up to 2.5 cm (1 in) in the direction of 145°east longitude [34], [35] or perhaps by up to 5 or 6 cm (2.0 to 2.4 in) [36]. However, due to tidal effects of the Moon, the length of a day increases at an average of 15 µs per year, so any rotational change due to the earthquake will be lost quickly. Similarly, the natural Chandler wobble of the Earth can be up to 15 m (50 ft).

More spectacularly, there was 10 m (33 ft) movement laterally and 4 to 5 m (13 to 16 ft) vertically along the fault line. Early speculation was that some of the smaller islands southwest of Sumatra may have moved southwest by up to 20 m (66 ft). There were also calculations that the northern tip of Sumatra, which is on the Burma Plate (the southern regions are on the Sunda Plate), may have moved up to 36 m (118 ft) southwest. Since movement was vertical as well as lateral, some coastal areas may now be below sea level. Measurements using GPS and satellite imagery are being used to determine the extent and nature of actual geophysical change [37]. The Andaman and Nicobar Islands appear to have shifted southwest [38] by around 4 m (13 ft), according to GPS data.

In February 2005, the Royal Navy vessel HMS Scott surveyed the sea bed around the earthquake zone, which varies in depth between 1,000 m (3,300 ft) and 5,000 m (16,500 ft) west of Sumatra. The survey, conducted using a high-resolution multi-beam sonar system, revealed that the earthquake had had a huge impact on the topography of the sea bed. It had created large thrust ridges, about 1,500 m high, which have collapsed in places to produce large landslides several kilometres across. One landslide consisted of a single block of material some 100 m (300 ft) high and 2 km (1.25 mi) long. The force of the displaced water was such that individual blocks of rock, massing millions of tons apiece, were dragged as much as 10 km (7 mi) across the sea bed. A newly-formed oceanic trench several kilometres wide was also found in the earthquake zone [39].

By a beneficial and remarkable coincidence, satellites TOPEX/Poseidon and Jason 1 happened to pass over the tsunami as it was crossing the ocean [40]. These satellites carry radars that measure precisely the height of the water surface; anomalies of the order of 50 cm (20 in) were measured. Measurements from these satellites may prove invaluable for the understanding of the earthquake and tsunami [41]. 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 complex effects close to the coast. Inversion of this height data may help adjust the parameters for the source earthquake.

Tsunami characteristics

Radar imaging of the tsunami two hours after the earthquake

Ripples were felt around the world. See also animation: MPG / AVI / MOV.

Maximum Computed Tsunami Heights from around the GlobeThe 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 which causes damage far away from its source is sometimes called a "teletsunami", and is much more likely to be produced by vertical motion of the seabed than by horizontal motion (Earthquakes and tsunamis, Lorca et al.).

See a full-length animation of how the waves travelled (large file, about 1 MiB) to see exactly how and why some countries were more affected than others

The tsunami, like all others, behaved very differently in deep water than in shallow water. In deep ocean water, tsunami waves form only a small hump, barely noticeable and harmless, which generally travels at a very 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 an hour but in doing so forms large destructive waves [42]. 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.[43]

Radar satellites recorded the heights of tsunami waves in deep water: at two hours after the earthquake, the maximum height was 60 cm (2 ft). These are the first such observations ever made. However, these observations could not have been used to provide a warning, because the satellites were not intended for that purpose and the data took hours to analyze [44][45].

According to Tad Murty, vice-president of the Tsunami Society, the total energy of the tsunami waves was about five megatons of TNT (20 petajoules). This 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 [46]. In many places the waves reached as far as 2 km (1.24 mi) inland[47].

Because the 1,200 km of faultline affected by the quake 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 very few casualties despite being a low-lying country relatively near the epicenter. It also benefitted 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. [48]

Coasts that have a land mass between them and the tsunami's location of origin are usually safe; however, tsunami waves can sometimes diffract around such land masses. Thus, the Indian state of Kerala was hit by the tsunami despite being on the western coast of India, and the western coast of Sri Lanka also suffered substantial impacts. Also distance alone is no guarantee of safety; 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 (for Somalia) to reach the various coastlines[49][50]. The northern regions of the Indonesian island of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly 90 minutes to two hours later. Thailand was also 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, some 8,500 km (5,300 mi) away, where a 1.5 m (5 ft) high ‘tide’ surged onshore about 16 hours after the quake. It took a relatively long time to reach this spot 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 [51].

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 20 to 40 cm (7.9 to 15.7 in) [52]. At Manzanillo, Mexico, a 2.6 m (8.5 ft) crest-to-trough tsunami was measured. This 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. [53]

Signs and warnings

Despite a lag of up to several hours between the earthquake and the impact of the tsunami, nearly all of the victims were taken completely by surprise; there were no tsunami warning systems in the Indian Ocean to detect tsunamis, or equally importantly, to warn the general populace living around the ocean. Tsunami detection is not easy because while a tsunami is in deep water it has a very low height and a network of sensors is needed to detect it. Setting up the communications infrastructure to issue timely warnings is an even bigger problem, particularly in a relatively poor part of the world.

Scientists were also hampered by the incorrect initial estimates for the magnitude of the earthquake, which was originally put at 8.1. The determination that the earthquake had actually been much stronger (and the resulting tsunami much larger) was not made until after the tsunami had already struck.

Tsunamis are much more frequent in the Pacific Ocean due to earthquakes in the "Ring of Fire", and an effective tsunami warning system has long been in place there. Although the extreme western edge of the "Ring of Fire" extends into the Indian Ocean (the point where this 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 Krakatoa eruption of 1883. It should be noted that not every earthquake produces large tsunamis; on March 28, 2005 a magnitude 8.7 quake hit roughly the same area of the Indian Ocean but did not result in a major tsunami.

In the aftermath of the disaster there is now an awareness of the need for a tsunami warning system for the Indian Ocean. The UN has started working on an Indian Ocean Tsunami Warning System and aims to have initial steps in place by end 2005 Some have even proposed creating a unified global tsunami warning system, to include the Atlantic Ocean and Caribbean.

Unfamiliarity with warning signs

Maximum recession of tsunami waters at Kata Noi Beach, Thailand, before the 3rd, and strongest, tsunami wave (sea visible in the right corner), 10:25 AM local time.The first warning sign of a possible tsunami is the earthquake itself. However, tsunamis can strike thousands of miles away, where the earthquake is only felt weakly or not at all. Also, in the minutes preceding a tsunami strike, the sea often recedes temporarily from the coast. People in Pacific regions are more familiar with tsunamis and often recognize this phenomenon as a sign to head for higher ground. However, around the Indian Ocean, 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 [54].

One of the few coastal areas to evacuate ahead of the tsunami was on the Indonesian island of Simeulue, very close to the epicentre. Island folklore recounted an earthquake and tsunami in 1907 and the islanders fled to inland hills after the initial shaking — before the tsunami struck [55]. On Maikhao beach in northern Phuket, Thailand, a 10 year old British girl named Tilly Smith had studied tsunamis in geography class 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 [56]. John Chroston, a biology teacher from Scotland, also recognised the signs at Kamala Bay, north of Phuket, taking a busload of holidaymakers and locals to safety on higher ground.

Retreat and rise cycle

The tsunami was a succession of several waves, occurring in retreat and rise cycles with a period of over 30 minutes between each peak. The third wave was the most powerful, and reached highest, occurring about an hour and a half after the first wave.