The 1881 Nicobar Islands earthquake occurred at about 07:49 local time (01:49 UTC) on 31 December, with an

epicentre The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

beneath

Car Nicobar Car Nicobar ( in Car language) is the northernmost of the Nicobar Islands. It is also one of three local administrative divisions of the Indian district of Nicobar, part of the Indian union territory of Andaman and Nicobar Islands. Annual ra ...

. It occurred as two separate ruptures, the largest of which had an estimated magnitude of 7.9 on the

moment magnitude scale The moment magnitude scale (MMS; denoted explicitly with or Mw, and generally implied with use of a single M for magnitude) is a measure of an earthquake's magnitude ("size" or strength) based on its seismic moment. It was defined in a 1979 pape ...

and triggered a

tsunami A tsunami ( ; from ja, 津波, lit=harbour wave, ) 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 other underwater exp ...

that was observed around the

Bay of Bengal The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line betwee ...

. It is probably the earliest

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

for which rupture parameters have been estimated instrumentally.

Tectonic setting

The Nicobar Islands are a series of volcanic islands that are part of an active

volcanic arc A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate, with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...

, formed above the subduction zone where the

Indo-Australian Plate The Indo-Australian Plate is a major tectonic plate that includes the continent of Australia and the surrounding ocean and extends northwest to include the Indian subcontinent and the adjacent waters. It was formed by the fusion of the Indian an ...

passes beneath the

Burma Plate The Burma Plate is a minor tectonic plate or microplate located in Southeast Asia, sometimes considered a part of the larger Eurasian Plate. The Andaman Islands, Nicobar Islands, and northwestern Sumatra are located on the plate. This island arc ...

. The convergence along this plate boundary is highly oblique, with the plate vector running at a low angle to the boundary. Most of the strike-slip component of the convergence is accommodated by the

Great Sumatran fault The Indonesian island of Sumatra is located in a highly seismic area of the world. In addition to the subduction zone off the west coast of the island, Sumatra also has a large strike-slip fault, the Great Sumatran Fault also known as Semangko ...

, which passes northwards into the Andaman Sea spreading centre. This plate boundary has been the location of many historical

megathrust earthquake 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 ...

s. The 1881 rupture area lies almost entirely within that for the 9.2 2004 Indian Ocean earthquake, while rupturing less than a third of its down-dip width.

Damage

The Andaman and Nicobar Islands suffered some damage, although the only masonry buildings affected were in

Port Blair Port Blair () is the capital city of the Andaman and Nicobar Islands, a union territory of India in the Bay of Bengal. It is also the local administrative sub-division (''tehsil'') of the islands, the headquarters for the district of South An ...

, including an infantry barrack and a chimney, both of which suffered severe cracking. On Car Nicobar, the coconut plantations and native huts were extensively damaged and

sand volcano Sand boils or sand volcanoes occur when water under pressure wells up through a bed of sand. The water looks like it is boiling up from the bed of sand, hence the name. Sand volcano A sand volcano or sand blow is a cone of sand formed by the ...

es were observed. There were no reported casualties associated with either the earthquake or the subsequent tsunami.

Characteristics

Earthquake

The earthquake was very widely felt with reports from many parts of mainland

India India, officially the Republic of India (Hindi: ), is a country in South Asia. It is the seventh-largest country by area, the second-most populous country, and the most populous democracy in the world. Bounded by the Indian Ocean on the so ...

Burma Myanmar, ; UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John Wells explai ...

and Sumatra, a total area of . The rupture area, and therefore the magnitude, has been calculated by modelling the observed tsunami arrival times and heights. Two rupture areas have been identified. The larger, and more southerly, measured 150 km x 60 km dipping about 20° to the east with a displacement of and a smaller area to the north dipping 15° to the east with a displacement of . The larger gives an estimated 7.9 event and the smaller a 7.0 event.

Tsunami

The tsunami was recorded by eleven of a series of continuous

tide gauge A tide gauge is a device for measuring the change in sea level relative to a vertical datum. It its also known as mareograph, marigraph, sea-level recorder and limnimeter. When applied to freshwater continental water bodies, the instrument ma ...

s around the Bay of Bengal that had recently been deployed by the Great Trigonometric Survey of India. The ten gauges on the Indian mainland were synchronised using a telegraph to Madras (

Chennai Chennai (, ), formerly known as Madras ( the official name until 1996), is the capital city of Tamil Nadu, the southernmost Indian state. The largest city of the state in area and population, Chennai is located on the Coromandel Coast of th ...

) time, while that at Port Blair was set by a

chronometer A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and th ...

linked to local time. The maximum recorded wave height was at

Nagapattinam Nagapattinam (''nākappaṭṭinam'', previously spelt Nagapatnam or Negapatam) is a town in the Indian state of Tamil Nadu and the administrative headquarters of Nagapattinam District. The town came to prominence during the period of Medieva ...

References