On 28 March 2005, at 23:09:37 WIB, a Mw 8.6–8.7 earthquake struck the Banyak Islands, located off the coast of Aceh, Indonesia. It was the most powerful earthquake globally since the 2004 Indian Ocean earthquake three months prior, and is tied with the 2012 Indian Ocean, 1957 Andreanof Islands, 1950 Assam–Tibet and 1946 Aleutian Islands earthquakes as the tenth-largest earthquake ever recorded by seismometers.
The earthquake killed at least 1,600 people and injured over 9,300 others. Of the fatalities, 1,300 occurred on the island of Nias, with at least 200 additional deaths in the Banyak Islands and 100 more in Simeulue. These islands were devastated, with the majority of homes damaged or destroyed and entire villages razed. An additional 22 people were killed in mainland Sumatra. A tsunami was also generated, although it only reached up to 4.2 m (14 ft) high and caused moderate damage in the epicentral area.
The island of Sumatra lies on the convergent plate boundary between the Indo-Australian plate and the Eurasian plate. The convergence between these plates is highly oblique near Sumatra, with the displacement being accommodated by near pure dip-slip faulting along the subduction zone, known as the Sunda megathrust, and near pure strike-slip faulting along the Great Sumatran fault. The major slip events on the subduction zone interface are typically of megathrust type. Historically, great or giant megathrust earthquakes have been recorded in 1797, 1833, 1861, 2004, 2005 and 2007, most of them being associated with devastating tsunamis. Smaller (but still large) megathrust events have also occurred in the small gaps between the areas that slip during the larger events, in 1935, 1984, 2000, 2002 and 2004.
The earthquake occurred at 16:09:37 UTC (23:09:37 WIB) on 28 March. The United States Geological Survey and International Seismological Centre reported a moment magnitude of Mw 8.6, while some independent studies put the magnitude at Mw 8.7. Its epicenter was located on the island of Bangkaru, part of the Banyak Islands. It struck 30 kilometres (19 mi) below the surface.
The focal mechanism is consistent with megathrust faulting along a section of the Sunda megathrust. A rupture area of 420 km (260 mi) x 250 km (160 mi) was estimated, extending from Simeulue and Southwest Aceh Regency in the north, to the Batu Islands in the south, with a maximum slip of 9.248 m (30.34 ft) east of Simeulue. The observed source time function gives a 120-second duration for the earthquake, with the greatest phase of seismic moment release occurring about 40 seconds after initiation. Based on surface deformation, it was interpreted that up to 11 m (36 ft) of slip in layered elastic spaces under Sumatra. Because of the elastic environment, the converging oceanic and continental plates build energy and are then locked by friction. The continental plate is forced back, and then shortened, causing uplift. The converging plates exceed frictional forces (deformation), causing slip along the fault. After the March 28 earthquake, afterslips continued to occur for 11 months, exerting the energy equivalent to a single Mw 8.2 earthquake.
The earthquake uplifted the entire 110 km (68 mi) stretch of Nias's western coastline by 2 m (6.6 ft), expanding beaches and exposing coral reefs; up to 6 m (20 ft) of uplift was observed in the town of Lahewa on Nias's northwestern coast. It also caused Simeulue to rise by 2 m (6.6 ft) and the Banyak Islands to sink by 1 m (3.3 ft). Proof of uplift was recorded using cGPS stations, highest level of survival for coral, and coseismic slips. Before the 2005 earthquake, there were already three cGPS sites set up. Three more were set up after the earthquake. These were used to study movement caused by the earthquake, and the Sumatran GPS Array and station were able to record coseismic displacements.
Vertical displacements and substantial coseismic uplift on Nias and Simeulue ranged from 1 to 2.5 m (3.3–8.2 ft). These measurements were derived from continuous GPS stations, with one on Nias recording 2.9 m (9.5 ft) of slip, supplemented by coral micro-atoll data indicating peak uplifts up to 2.8 m (9.2 ft) in coastal areas. Satellite altimetry from Jason-1 also contributed to mapping offshore vertical deformation patterns consistent with this range, validating the tectonic response over the ruptured megathrust segment. Modeling of the fault slip distribution, based on joint inversions of seismic waveforms, GPS, and InSAR data, revealed two primary asperities; a larger patch beneath Nias with maximum slip of 11 m (36 ft) at depths of 15–25 km (9.3–15.5 mi), and a smaller patch under Simeulue with peak slip of around 7–8 m (23–26 ft). The rupture propagated bilaterally from the hypocenter near the Banyak Islands, with overall slip decreasing toward the Sunda megathrust due to the inhibiting effect of accretionary prism sediments. This distribution accounted for the observed surface deformation, with the high-slip zones directly correlating to the elevated coastal uplifts.
In the twenty-four hours immediately after the event, there were eight major aftershocks, measuring between Mw 5.5 and 6.0. By the end of 2005, there were 1,587 aftershocks exceeding Mw 4.0, including 140 events measuring Mw 5.0–5.9 and 13 exceeding Mw 6.0; the largest aftershock measured Mw 6.9, striking on 19 May 2005 with an epicenter close to that of the mainshock's.
Despite the epicenter's proximity 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.
On Nias, the Modified Mercalli intensity was IX–X (Violent-Extreme) along the island's western coastline, VIII (Severe) in Gunungsitoli and VII (Very Strong) in Teluk Dalam. The USGS reported a maximum Modified Mercalli intensity (MMI) of IX (Violent) in Simeulue, the Banyak Islands and the town of Sibolga in North Sumatra. MMI VI (Strong) shaking was recorded in Banda Aceh, while Medan, Padang and Palembang recorded an intensity of MMI V (Moderate). MMI IV (Light) shaking was felt in Bangkok, Thailand and the west coast of Malaysia, while in Singapore and the Maldives, the MMI was III (Weak). Tremors were also felt in Sri Lanka and the Andaman and Nicobar Islands, India.
Due to the earthquake's similar size to the 2004 Indian Ocean earthquake, many people feared a destructive ocean-wide tsunami similar to that of an event. Evacuations were carried out in coastal regions of Thailand, Malaysia, and Sri Lanka. The 2005 earthquake, however, produced a relatively small tsunami. A 3-metre (9.8 ft) tsunami caused moderate damage to the port and airport facilities on Simeulue, with a maximum runup of 4.2 m (14 ft) at the village of Busung, and a 4-metre (13 ft) high tsunami was recorded at Nias and a 3.5 m (11 ft) surge struck the Banyak Islands. Run-ups of 4 m (13 ft) were also reported in coastal areas of northwestern Sumatra, with 2.3 m (7.5 ft) high waves striking Singkil and a 1 m (3.3 ft) wave observed at Meulaboh. Much smaller waves, most detectable only in tide gauge recording systems, were recorded across the Indian Ocean; for example, a 25 cm (9.8 in) wave was recorded at Colombo, Sri Lanka, while a 40 cm (1.3 ft) high surge struck the island of Hanimaadhoo in the Maldives. Seiches were also observed on ponds in West Bengal, India.
Tsunami warnings were issued by the Pacific Tsunami Warning Center, operated by the US National Oceanic and Atmospheric Administration (NOAA), and by the government of Thailand. There were initial concerns that a major tsunami could be generated, particularly travelling south from the event's hypocenter.
Portions of Thailand's southern coast were evacuated as a precaution, and NOAA advised an evacuation of 965 kilometres (600 mi) of coastline in Sumatra. Evacuations occurred in the northern Malaysian states of Penang and Kedah, as well as the eastern coast of Sri Lanka, where ten people died while evacuating. Six people were also killed in traffic accidents associated with tsunami evacuations in Toamasina, Madagascar.