Aftershocks and other earthquakes

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 was 8.7 epicentred off the Sumatran island of Nias [12]. Other aftershocks of up to magnitude 6.6 continue to shake the region on a daily basis [13] [14].

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 [15]. This is unusual, since earthquakes of magnitude 8 or more occur only about once per year on average [16]. 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 [17] (a 6.5 earthquake 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 [18].

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 [19].

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 [20].

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 at 1729 local time (1029 GMT) about 120km (75 miles) south-west of the city of Padang. See BBC News: Sumatra shaken by new earthquake - also see Wikinews

Some scientists warn that geological stresses caused by the recent quakes may even have increased the possibility that the Lake Toba supervolcano could erupt. [21] 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. [22]

Coincidentally, Mount Talang has since erupted[23] and is now on top alert. [24]

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Power of the earthquake
The total energy released by the earthquake in the Indian Ocean has been estimated as 4.3 exajoules (4.3×1018 joules) [25]. This is equivalent to 100 gigatons of TNT, or about as much energy as is used in the United States in 6 months. It is estimated to have resulted in an oscillation of the Earth's surface of about 20-30 cm (8 to 12 inches), equivalent to the effect of the tidal forces caused by the Sun and Moon [26]. The shock waves of the earthquake were felt across the planet; as far away as Oklahoma, vertical movements of 3 mm (0.12 inches) were recorded [27]. 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) [28] 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 inch) in the direction of 145° east longitude [29], [30] or perhaps by up to 5 or 6 cm (2.0 to 2.4 inches) [31]. 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 feet) movement laterally and 4 to 5 m (13 to 16 feet) 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 feet). 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 [32]. The Andaman and Nicobar Islands appear to have shifted southwest [33] by around 4 m, 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 feet) and 5,000 m (16,500 feet) 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 feet) high and 2 km (1.25 miles) 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 miles) across the sea bed. An newly-formed oceanic trench several kilometres wide was also found in the earthquake zone [34].

By a beneficial and remarkable coincidence, satellites TOPEX/Poseidon and Jason 1 happened to pass over the tsunami as it was crossing the ocean [35]. These satellites carry radars that measure precisely the height of the water surface; anomalies of the order of 50 cm (20 inches) were measured. Measurements from these satellites may prove invaluable for the understanding of the earthquake and tsunami [36]. 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.