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How much did tsunami really move Andaman?

Subhra Priyadarshini

doi:10.1038/nindia.2008.228 Published online 18 June 2008

A Global Positioning System set up in the Andaman islands.

© NGRI

Four years after the Indian Ocean tsunami, fresh data emerging from geological calculations have suggested that the killer seaquake did not actually displace the Andaman and Nicobar islands by over a meter as earlier reported but by just around seven centimeter.

New data collated by a group of Indian geologists1 overrides earlier calculations of the Survey of India posing another significant question: is there a need to make a fresh assessment of the magnitude of the 2004 seaquake? The finding means all data pertaining to the magnitude and displacement in government records will have to be refreshed.

Soon after the tsunami, the Survey of India (SOI) had estimated that Port Blair, the capital of Andaman and Nicobar islands, had shifted by over one meter. The team of surveyors compiled scientific readings from across the Andaman and Nicobar islands to discover that its Port Blair control point had shifted towards the south-eastern side by 1.15 meter from its earlier position.

The sea had recorded a not-so-significant drop of 25 cm from its earlier level at the Port Blair control point, which led the team to infer that there could be some subsidence also. The mean sea level had also increased by 1.5 m post-tsunami. The seven-member team of the survey body studying the latitude, longitude and height at 20 of its control points in the islands released this data at an international conference on disaster management in 2005.

However, a joint study by geologists of the National Geophysical Research Institute (NGRI), Hyderabad, Centre for Mathematical Modelling and Computer Simulation, Bangalore, National Centre for Antarctic and Ocean Research, Goa and the India Meteorological Department, Port Blair says the movement was between seven and 12 centimeter.

"There is a difference in the figures since these are two estimates of two different processes," explains lead researcher Vineet Kumar Gahalaut from NGRI, who was also part of the team studying the earlier SOI data.

The estimate derived by SOI pertained to co-seismic deformation or deformation accompanying the earthquake. At Port Blair, the co-seismic deformation was about 3.5 meter towards the west south west and subsidence of about one meter. However, the latest data talks of post-seismic deformation meaning deformation subsequent to the earthquake.

"Since the observations of post-seismic deformation started only after about 16 days of the earthquake, we do not know how much post-seismic deformation occurred during that period," says Gahaluat's colleague Rajender Kumar Chadha. Earlier estimates of this post-seismic deformation from various worldwide groups range between 0.25 m and 1.5 m. However, the new study has estimated it to be only about 7-12 cm.

"This has strong implications in estimating the magnitude of the earthquake, the deformation processes, rheology or physical properties of the medium and earthquake recurrence interval in the region," Gahalaut says.

The team has suggested a revised magnitude for this earthquake at 9.23. Earlier estimates were ranging from 9.0 to 9.3. "The magnitude scale is a logarithmic scale. So an increment of 0.3 increases the energy about 10 times!" he explains the significance of the seemingly insignificant range.

Incidentally, India had also lost precious strategic land to the tsunami as large stretches in its southernmost tip, about 120 km from the Indonesian shores, were devoured by the waves. Just 125 km away from the epicentre of the earthquake that jolted the Indian Ocean, this was the first landmass hit by the waves in the northwest direction gulping about 20 km of land. Indira Point, the most prominent landmark at the southernmost tip of the Republic of India, went under water with the famous red and white lighthouse, earlier on the shore, now standing halfway in the sea.


References

  1. Gahalaut, V. K. et al. No evidence of unusually large postseismic deformation in Andaman region immediately after 2004 Sumatra-Andaman earthquake. Geophys. Res. Lett. 35, L10307 (2008) | Article |