The 1861 Sumatra earthquake occurred on 16 February and was the last in a sequences of

earthquake An earthquakealso called a quake, tremor, or tembloris the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they ...

s that ruptured adjacent parts of the Sumatran segment of the Sunda megathrust. It caused a devastating

tsunami A tsunami ( ; from , ) is a series of waves in a water body caused by the displacement of a large volume of water, generally in an ocean or a large lake. Earthquakes, volcanic eruptions and underwater explosions (including detonations, ...

which led to several thousand fatalities. The earthquake was felt as far away as the Malay peninsula and the eastern part of

Java Java is one of the Greater Sunda Islands in Indonesia. It is bordered by the Indian Ocean to the south and the Java Sea (a part of Pacific Ocean) to the north. With a population of 156.9 million people (including Madura) in mid 2024, proje ...

. The rupture area for the

2005 Nias–Simeulue earthquake 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. At least 915 people were killed, mostly on the island of Nias. It was among the top 10 mos ...

is similar to that estimated for the 1861 event.

Background

The island of Sumatra lies on the

convergent plate boundary A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a ...

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 In geology, a fault is a Fracture (geology), planar fracture or discontinuity in a volume of Rock (geology), rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust (geology ...

along the

Great Sumatran fault The Great Sumatran fault, also known as Semangko fault, is a large strike-slip fault running the entire length of the island of Sumatra. This Indonesian island is located in a highly seismic area of the world, including a subduction zone off th ...

. The major slip events on the subduction zone interface are typically of

megathrust Megathrust earthquakes occur at convergent plate boundaries, where one tectonic plate is forced underneath another. The earthquakes are caused by slip along the thrust fault that forms the contact between the two plates. These interplate earthqu ...

type. Historically, great or giant megathrust earthquakes have been recorded in

1797 Events January–March * January 3 – The Treaty of Tripoli, a peace treaty between the United States and Ottoman Tripolitania, is signed at Algiers (''see also'' 1796). * January 7 – The parliament of the Cisalpine Repu ...

1833 Events January–March * January 3 – The United Kingdom reasserts British sovereignty over the Falkland Islands in the South Atlantic Ocean. * February 6 (January 25 on the Greek calendar) – Prince Otto Friedrich Ludwig of Bavaria arr ...

, 1861,

2004 2004 was designated as an International Year of Rice by the United Nations, and the International Year to Commemorate the Struggle Against Slavery and Its Abolition (by UNESCO). Events January * January 3 – Flash Airlines Flight 60 ...

2005 2005 was designated as the International Year for Sport and Physical Education and the International Year of Microcredit. The beginning of 2005 also marked the end of the International Decade of the World's Indigenous Peoples, Internationa ...

and

2007 2007 was designated as the International Heliophysical Year and the International Polar Year. Events January * January 1 **Bulgaria and Romania 2007 enlargement of the European Union, join the European Union, while Slovenia joins the Eur ...

, 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 Events January * January 7 – Italian premier Benito Mussolini and French Foreign Minister Pierre Laval conclude an agreement, in which each power agrees not to oppose the other's colonial claims. * January 12 – Amelia Earhart ...

1984 Events January * January 1 – The Bornean Sultanate of Brunei gains full independence from the United Kingdom, having become a British protectorate in 1888. * January 7 – Brunei becomes the sixth member of the Association of Southeas ...

2000 2000 was designated as the International Year for the Culture of Peace and the World Mathematics, Mathematical Year. Popular culture holds the year 2000 as the first year of the 21st century and the 3rd millennium, because of a tende ...

and

2002 The effects of the September 11 attacks of the previous year had a significant impact on the affairs of 2002. The war on terror was a major political focus. Without settled international law, several nations engaged in anti-terror operation ...

Damage

Villages along the seaward side of the Batu Islands were devastated. The combined effects of the earthquake and tsunami caused several thousand fatalities.

Characteristics

Earthquake

There is evidence of coseismic uplift of Nias, with exposure of reefs and rock piers. There were six major

aftershock In seismology, an aftershock is a smaller earthquake that follows a larger earthquake, in Epicenter, the same area of the Mainshock, main shock, caused as the displaced Crust (geology), crust adjusts to the effects of the main shock. Large earthq ...

s over the next seven months, the last of which, on 27 September caused a damaging tsunami. Based on studies of coral growth patterns, researchers have demonstrated that the 1861 event was the culmination of a 32 year long "slow-slip" seismic event, rendering it the longest known example of such a sequence ever recorded.

Tsunami

At least 500 km (310.7 mi) of coastline were affected by the tsunami with run-ups of up to 7m (23 ft) recorded on the southwest side of

Nias Nias (, Nias: ''Tanö Niha'') is an island located off the western coast of Sumatra, Indonesia. Nias is also the name of the archipelago () of which the island is the centre, but also includes the Batu Islands to the southeast and the small ...

References