At 16:09 UTC on 28 March 2005 (23:09 local time), a magnitude 8.6 earthquake struck the Sunda megathrust off the Banyak Islands, near Nias and Simeulue off the west coast of northern Sumatra, at a depth of about 30 km. It ruptured barely three months after the catastrophic magnitude-9.1 Sumatra-Andaman earthquake of December 2004, and is considered the tenth-largest earthquake ever recorded by modern instruments.
More than 1,000 people were killed, most of them on Nias island, with further deaths on Simeulue and the Banyak Islands and over 9,000 injured. Roughly 13,000 households were destroyed on Nias alone, along with 720 schools and hundreds of government buildings, while some coastal sections of Nias were permanently uplifted by up to 2 metres.
Unlike the 2004 quake, this one produced only a modest tsunami, with waves up to about 4 metres at Simeulue, sparing the region a second wave of mass casualties. Seismologists concluded the rupture was most likely triggered by stress transferred from the neighbouring segment that broke in 2004 — a textbook case of one great earthquake loading a fault and nudging it toward failure.
Because international aid teams and equipment were already in the region for the 2004 tsunami response, relief reached Nias faster than most comparable disasters, though reconstruction still took years. The event pushed Indonesian and international seismologists to treat the Sunda megathrust as a chain of interlinked segments, where a major rupture on one section measurably raises the short-term risk on its neighbours.