The 2005 Nias–Simeulue earthquake occurred on 28 March off the west coast of northern Sumatra, Indonesia in the subduction zone of the Sunda megathrust.

[8] Despite the proximity of the epicenter to that of the 2004 Indian Ocean earthquake, it ruptured a separate segment of the Sunda megathrust and was most likely triggered by stress changes associated with that earlier event.

[10] The earthquake was strongly felt across the island of Sumatra and caused widespread power outages in the Indonesian city of Banda Aceh, already devastated by the December 2004 tsunami, and prompted thousands to flee their homes and seek higher ground.

Ports, bridges, roads, community health clinics, and Nias's main hospital in Gunung Sitoli were all destroyed.

In response, IOM (International Organization for Migration) focused on expanding road and bridge repair, putting forth plans to be enacted by 2006 that included construction of 75 temporary schools, homes, and child centers.

Plans for permanent schools were underway, 40% of damaged health centers were reconstructed, 25% of mosques and prayer halls were rebuilt, and 80% of fish markets recovered.

A major issue in Nias and Simeulue was the fact that the shop houses are built by practices of foremen and contractors without the use of structural analysis or drawings.

[14] Due to this practice, the building permit system wasn't enforced, leading to no one catching poor material quality or workmanship.

[19] These were used to study movement caused by the earthquake, and the Sumatran GPS Array and station were able to record coseismic displacements.

[19] When coseismic slip occurs, lower parts of the coral heads will remain under water and their topmost living tissues will mark a new highest level of survival.

After the detection of a minor tsunami south of the epicenter, the Indian Ocean island states of Mauritius, Madagascar, and the Seychelles issued warnings to their populations.

Although tsunami warning systems for the region had been actively discussed before the December 2004 earthquake, none had yet been implemented in the Indian Ocean.

The United Nations worked with the Indonesian government to take further actions to prevent a possible catastrophe after the strong earthquake.

[23] Australia announced it would provide A$1 million in emergency aid and, at the request of the Indonesian Government, dispatched Australian Defence Force medical teams and equipment to Nias.

[24] The Australian naval ship HMAS Kanimbla, having only recently left Aceh, was redeployed to the region from Singapore.

At about 09:30 (UTC) 2 April 2005, one of Kanimbla's two Sea King helicopters, Shark 02, crashed on the island of Nias while taking medical personnel to a village.