<P> 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 . The earthquake produced its own aftershocks (some registering a magnitude of as great as 6.1) and presently ranks as the third largest earthquake ever recorded on the moment magnitude or Richter magnitude scale . </P> <P> Other aftershocks of up to magnitude 6.6 continued to shake the region daily for three or four months . 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 . </P> <P> 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 . However, the U.S. Geological Survey sees no evidence of a causal relationship between these events . </P> <P> The December earthquake is thought to have triggered activity in both Leuser Mountain and Mount Talang, 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 . </P>

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