The 1881 Nicobar Islands earthquake occurred at about 07:49 local time (01:49 UTC) on 31 December, with an epicentre beneath Car Nicobar. It occurred as two separate ruptures, the largest of which had an estimated magnitude of 7.9 on the moment magnitude scale and triggered a tsunami that was observed around the Bay of Bengal. It is probably the earliest earthquake for which rupture parameters have been estimated instrumentally.^[1]

Tectonic setting

The Nicobar Islands are a series of volcanic islands that are part of an active volcanic arc, formed above the subduction zone where the Indo-Australian Plate passes beneath the Burma Plate. 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, which passes northwards into the Andaman Sea spreading centre.^[1] This plate boundary has been the location of many historical megathrust earthquakes. The 1881 rupture area lies almost entirely within that for the M_w = 9.2 2004 Indian Ocean earthquake, while rupturing less than a third of its down-dip width.^[2]

Damage

The Andaman and Nicobar Islands suffered some damage, although the only masonry buildings affected were in Port Blair, 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 volcanoes were observed.^[1] There were no reported casualties associated with either the earthquake or the subsequent tsunami.^[3]

Characteristics

Earthquake

The earthquake was very widely felt with reports from many parts of mainland India, Burma and Sumatra, a total area of .^[1] 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.^[2] 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 M_w = 7.9 event and the smaller a M_w = 7.0 event.^[2]

Tsunami

The tsunami was recorded by eleven of a series of continuous tide gauges around the Bay of Bengal that had recently been deployed by the Great Trigonometric Survey of India.^[1] The ten gauges on the Indian mainland were synchronised using a telegraph to Madras (Chennai) time, while that at Port Blair was set by a chronometer linked to local time. The maximum recorded wave height was at Nagapattinam.^[1]

References