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2004 Indian Ocean earthquake and tsunami
From Wikipedia, the free encyclopedia
2004 Indian Ocean earthquake and tsunami
The tsunami was initially documented as moment magnitude 8.8. In February 2005 scientists revised the estimate of the magnitude to 9.0.[16] Although the Pacific Tsunami Warning Center has accepted these new numbers, the United States Geological Survey has so far not changed its estimate of 9.1. The most recent studies in 2006 have obtained a magnitude of Mw 9.1–9.3. Dr. Hiroo Kanamori of the California Institute of Technology believes that Mw 9.2 is a good representative value for the size of this great earthquake.[17]
The hypocentre 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 (6.2 mi)). The northern section of the Sunda megathrust ruptured over a length of 1,300 km (810 mi).[12] The earthquake (followed by the tsunami) was felt simultaneously in Bangladesh, India, Malaysia, Myanmar, Thailand, Singapore and the Maldives.[18] Splay faults, or secondary "pop up faults", caused long, narrow parts of the sea floor to pop up in seconds. This quickly elevated the height and increased the speed of waves, causing the complete destruction of the nearby Indonesian town of Lhoknga.[19]
The epicenter of the 2004 Indian Ocean earthquake and associated aftershocks in French.
The epicentre of the earthquake, just north of Simeulue Island.
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, Flores to Timor.
Great earthquakes such as the Sumatra-Andaman event, which are invariably associated with megathrust events in subduction zones, have seismic moments that can account for a significant fraction of the global earthquake moment across century-scale time periods. Of all the seismic moment released by earthquakes in the 100 years from 1906 through 2005, roughly one-eighth was due to the Sumatra-Andaman event. This quake, together with the Good Friday earthquake (Alaska, 1964) and the Great Chilean earthquake (1960), account for almost half of the total moment. The much smaller but still catastrophic 1906 San Francisco earthquake is included in the diagram for perspective. Mw denotes the magnitude of an earthquake on the moment magnitude scale.
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 only other recorded earthquakes of magnitude 9.0 or greater were off Kamchatka, Russia, on 4 November 1952 (magnitude 9.0)[20] and Tōhoku, Japan (magnitude 9.0) in March 2011. Each of these megathrust earthquakes also spawned tsunamis in the Pacific Ocean. However, the death toll from these was significantly lower, primarily because of the lower population density along the coasts near affected areas and the much greater distances to more populated coasts and also due to the superior infrastructure and warning systems in MEDCs (More Economically Developed Countries) such as Japan.
Other very large 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 (western North America, Juan de Fuca Plate and North American Plate). All of them are believed to be greater than magnitude 9, but no accurate measurements were available at the time.
A pie chart comparing the seismic moment release for the largest earthquakes from 1906 to 2005 compared to all other earthquakes for the same period
The megathrust earthquake was unusually large in geographical and geological extent. An estimated 1,600 kilometres (1,000 mi) of fault surface slipped (or ruptured) about 15 metres (50 ft) along the subduction zone where the Indian Plate slides (or subducts) under the overriding 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 a rupture about 400 kilometres (250 mi) long and 100 kilometres (60 mi) wide, located 30 kilometres (19 mi) beneath the sea bed—the largest rupture ever known to have been caused by an earthquake. The rupture proceeded at a speed of about 2.8 kilometres per second (1.7 miles per second) (10,000 km/h or 6,200 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 (1.3 mi/s) (7,500 km/h or 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 great Indo-Australian Plate, which underlies the Indian Ocean and Bay of Bengal, and is drifting north-east at an average of 6 centimetres per year (2.4 inches per year). 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 sea floor is estimated to have risen by several metres, displacing an estimated 30 cubic kilometres (7.2 cu mi) of water and triggering devastating tsunami waves. The waves did not originate from a point source, as was inaccurately depicted in some illustrations of their paths of travel, but rather radiated outwards along the entire 1,600-kilometre (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 sea floor significantly reduced the capacity of the Indian Ocean, producing a permanent rise in the global sea level by an estimated 0.1 millimetres (0.004 in).[22]
Locations of initial earthquake and all aftershocks measuring greater than 4.0 from 26 December 2004 to 10 January 2005. The site of the original quake is marked by the large star in the lower right square of the grid.
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 8.7 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 epicenter, and was most likely triggered by stress changes associated with the 2004 event.[23] This earthquake was so large that it produced its own aftershocks (some registering a magnitude of as great as 6.1) and presently ranks as the 7th largest earthquake on record since 1900.
Other aftershocks of up to magnitude 6.6 continued to shake the region daily for up to three or four months.[24] 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 an uninhabited region west of New Zealand's subantarctic Auckland Islands, and north of Australia's Macquarie Island. This is unusual, since earthquakes of magnitude 8 or more occur only about once per year on average.[25] However, the U.S. Geological Survey sees no evidence of a causal relationship between these events.[26]
The December earthquake is thought to have triggered activity in both Leuser Mountain[27] and Mount Talang,[28] volcanoes in Aceh province along the same range of peaks, while the 2005 Nias–Simeulue earthquake had sparked activity in Lake Toba, an ancient crater in Sumatra.[29]
The tsunami strikes Ao Nang, Thailand.
The energy released on the Earth's surface only (ME, which is the seismic potential for damage) by the 2004 Indian Ocean earthquake and tsunami was estimated at 1.1×1017 joules,[30] or 26 megatons of TNT. 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. However, the total work done MW (and thus energy) by this quake was 4.0×1022 joules (4.0×1029 ergs),[31] the vast majority underground. This is over 360,000 times more than its ME, equivalent to 9,600 gigatons of TNT equivalent (550 million times that of Hiroshima) or about 370 years of energy use in the United States at 2005 levels of 1.08×1020 J.
The only recorded earthquakes with a larger MW were the 1960 Chilean and 1964 Alaskan quakes, with 2.5×1023 joules (250 ZJ) and 7.5×1022 joules (75 ZJ) respectively.[32]
The earthquake generated a seismic oscillation of the Earth's surface of up to 20–30 cm (8–12 in), equivalent to the effect of the tidal forces caused by the Sun and Moon. The shock 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 4 months after the earthquake.[33]
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 1 cm (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. This aftershock would be considered a major earthquake under ordinary circumstances, but is dwarfed by the mainshock.
Vertical-component ground motions recorded by the IRIS/USGS Global Seismographic Network.
The shift of mass and the massive release of energy very slightly altered the Earth's rotation. The exact amount is not yet known, but theoretical models suggest the earthquake shortened the length of a day by 2.68 microseconds, due to a decrease in the oblateness of the Earth.[34] 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,[35] or perhaps by up to 5 or 6 cm (2.0 or 2.4 in).[36] However, because of 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, which in some cases can be up to 15 m (50 ft), will eventually offset the minor wobble produced by the earthquake.
More spectacularly, there was 10 m (33 ft) movement laterally and 4–5 m (13–16 ft) vertically along the jade 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 20 cm (8 in).[37] 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).[38]
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 huge 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 high and 2 km long (300 ft by 1.25 mi). The momentum of the water displaced by tectonic uplift had also dragged massive slabs of rock, each weighing millions of tons, as far as 10 km (6 mi) across the seabed. An oceanic trench several kilometres wide was exposed in the earthquake zone.[39]
The TOPEX/Poseidon and Jason-1 satellites 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 the complex ways in which close proximity to the coast changes the size and shape of a wave.
From Wikipedia, the free encyclopedia
2004 Indian Ocean earthquake and tsunami
The tsunami was initially documented as moment magnitude 8.8. In February 2005 scientists revised the estimate of the magnitude to 9.0.[16] Although the Pacific Tsunami Warning Center has accepted these new numbers, the United States Geological Survey has so far not changed its estimate of 9.1. The most recent studies in 2006 have obtained a magnitude of Mw 9.1–9.3. Dr. Hiroo Kanamori of the California Institute of Technology believes that Mw 9.2 is a good representative value for the size of this great earthquake.[17]
The hypocentre 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 (6.2 mi)). The northern section of the Sunda megathrust ruptured over a length of 1,300 km (810 mi).[12] The earthquake (followed by the tsunami) was felt simultaneously in Bangladesh, India, Malaysia, Myanmar, Thailand, Singapore and the Maldives.[18] Splay faults, or secondary "pop up faults", caused long, narrow parts of the sea floor to pop up in seconds. This quickly elevated the height and increased the speed of waves, causing the complete destruction of the nearby Indonesian town of Lhoknga.[19]
The epicenter of the 2004 Indian Ocean earthquake and associated aftershocks in French.
The epicentre of the earthquake, just north of Simeulue Island.
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, Flores to Timor.
Great earthquakes such as the Sumatra-Andaman event, which are invariably associated with megathrust events in subduction zones, have seismic moments that can account for a significant fraction of the global earthquake moment across century-scale time periods. Of all the seismic moment released by earthquakes in the 100 years from 1906 through 2005, roughly one-eighth was due to the Sumatra-Andaman event. This quake, together with the Good Friday earthquake (Alaska, 1964) and the Great Chilean earthquake (1960), account for almost half of the total moment. The much smaller but still catastrophic 1906 San Francisco earthquake is included in the diagram for perspective. Mw denotes the magnitude of an earthquake on the moment magnitude scale.
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 only other recorded earthquakes of magnitude 9.0 or greater were off Kamchatka, Russia, on 4 November 1952 (magnitude 9.0)[20] and Tōhoku, Japan (magnitude 9.0) in March 2011. Each of these megathrust earthquakes also spawned tsunamis in the Pacific Ocean. However, the death toll from these was significantly lower, primarily because of the lower population density along the coasts near affected areas and the much greater distances to more populated coasts and also due to the superior infrastructure and warning systems in MEDCs (More Economically Developed Countries) such as Japan.
Other very large 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 (western North America, Juan de Fuca Plate and North American Plate). All of them are believed to be greater than magnitude 9, but no accurate measurements were available at the time.
A pie chart comparing the seismic moment release for the largest earthquakes from 1906 to 2005 compared to all other earthquakes for the same period
The megathrust earthquake was unusually large in geographical and geological extent. An estimated 1,600 kilometres (1,000 mi) of fault surface slipped (or ruptured) about 15 metres (50 ft) along the subduction zone where the Indian Plate slides (or subducts) under the overriding 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 a rupture about 400 kilometres (250 mi) long and 100 kilometres (60 mi) wide, located 30 kilometres (19 mi) beneath the sea bed—the largest rupture ever known to have been caused by an earthquake. The rupture proceeded at a speed of about 2.8 kilometres per second (1.7 miles per second) (10,000 km/h or 6,200 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 (1.3 mi/s) (7,500 km/h or 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 great Indo-Australian Plate, which underlies the Indian Ocean and Bay of Bengal, and is drifting north-east at an average of 6 centimetres per year (2.4 inches per year). 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 sea floor is estimated to have risen by several metres, displacing an estimated 30 cubic kilometres (7.2 cu mi) of water and triggering devastating tsunami waves. The waves did not originate from a point source, as was inaccurately depicted in some illustrations of their paths of travel, but rather radiated outwards along the entire 1,600-kilometre (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 sea floor significantly reduced the capacity of the Indian Ocean, producing a permanent rise in the global sea level by an estimated 0.1 millimetres (0.004 in).[22]
Locations of initial earthquake and all aftershocks measuring greater than 4.0 from 26 December 2004 to 10 January 2005. The site of the original quake is marked by the large star in the lower right square of the grid.
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 8.7 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 epicenter, and was most likely triggered by stress changes associated with the 2004 event.[23] This earthquake was so large that it produced its own aftershocks (some registering a magnitude of as great as 6.1) and presently ranks as the 7th largest earthquake on record since 1900.
Other aftershocks of up to magnitude 6.6 continued to shake the region daily for up to three or four months.[24] 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 an uninhabited region west of New Zealand's subantarctic Auckland Islands, and north of Australia's Macquarie Island. This is unusual, since earthquakes of magnitude 8 or more occur only about once per year on average.[25] However, the U.S. Geological Survey sees no evidence of a causal relationship between these events.[26]
The December earthquake is thought to have triggered activity in both Leuser Mountain[27] and Mount Talang,[28] volcanoes in Aceh province along the same range of peaks, while the 2005 Nias–Simeulue earthquake had sparked activity in Lake Toba, an ancient crater in Sumatra.[29]
The tsunami strikes Ao Nang, Thailand.
The energy released on the Earth's surface only (ME, which is the seismic potential for damage) by the 2004 Indian Ocean earthquake and tsunami was estimated at 1.1×1017 joules,[30] or 26 megatons of TNT. 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. However, the total work done MW (and thus energy) by this quake was 4.0×1022 joules (4.0×1029 ergs),[31] the vast majority underground. This is over 360,000 times more than its ME, equivalent to 9,600 gigatons of TNT equivalent (550 million times that of Hiroshima) or about 370 years of energy use in the United States at 2005 levels of 1.08×1020 J.
The only recorded earthquakes with a larger MW were the 1960 Chilean and 1964 Alaskan quakes, with 2.5×1023 joules (250 ZJ) and 7.5×1022 joules (75 ZJ) respectively.[32]
The earthquake generated a seismic oscillation of the Earth's surface of up to 20–30 cm (8–12 in), equivalent to the effect of the tidal forces caused by the Sun and Moon. The shock 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 4 months after the earthquake.[33]
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 1 cm (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. This aftershock would be considered a major earthquake under ordinary circumstances, but is dwarfed by the mainshock.
Vertical-component ground motions recorded by the IRIS/USGS Global Seismographic Network.
The shift of mass and the massive release of energy very slightly altered the Earth's rotation. The exact amount is not yet known, but theoretical models suggest the earthquake shortened the length of a day by 2.68 microseconds, due to a decrease in the oblateness of the Earth.[34] 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,[35] or perhaps by up to 5 or 6 cm (2.0 or 2.4 in).[36] However, because of 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, which in some cases can be up to 15 m (50 ft), will eventually offset the minor wobble produced by the earthquake.
More spectacularly, there was 10 m (33 ft) movement laterally and 4–5 m (13–16 ft) vertically along the jade 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 20 cm (8 in).[37] 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).[38]
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 huge 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 high and 2 km long (300 ft by 1.25 mi). The momentum of the water displaced by tectonic uplift had also dragged massive slabs of rock, each weighing millions of tons, as far as 10 km (6 mi) across the seabed. An oceanic trench several kilometres wide was exposed in the earthquake zone.[39]
The TOPEX/Poseidon and Jason-1 satellites 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 the complex ways in which close proximity to the coast changes the size and shape of a wave.
