Collaborative Research: Deformation Processes in the Central Andaman Islands

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The Mw=9.3 Dec 2004 Sumatra earthquake ruptured 1000 miles of India's eastern plate boundary causing damage in the Nicobar and Andaman islands, shifting them bodily 3m-7m southwestward, and tilting them down to the east. Although shaking damage in the Andaman and Nicobar islands was minor, the tilting caused massive damage to coastline ecologies. Thousands of hectares of agricultural land were flooded, and erosion of forested shorelines has commenced in regions that sank. Hundreds of km of coral reefs were killed where the land rose, with attendant loss of nearshore fishing grounds. Yet clearly this was not the first time that this has occurred, and it is certain that it will occur again.Several societal questions arise. How frequently do these catastrophic tsunami earthquakes occur? Are the flooded islands permanently drowned or will they slowly recover their former levels? Will the saline intrusion and flooding of shoreline farmland be reversed or worsened in the coming decades. Will harbor defenses and docks require extended construction to accommodate additional submergence or emergence. In what way does the Sumatra earthquake resemble a similar Mw=9 expected in the Pacific NW of the US? What can be learned from the Sumatra earthquake that has application to the coasts of Oregon and Washington?These questions can be answered from a geophysical knowledge of the subsurface process involved in the Sumatra/Andaman event. Near Port Blair, where most of the people of the Andaman/Nicobar islands live, the plate boundary accomodates both northward motion of India and the westward motion of the Andaman plate: every 100 years it accommodate s roughly1 m of northward motion and 4 m of convergence. All this motion occurs infrequently in earthquakes, yet it is driven by slow interseismic processes.Scientists from Memphis and Colorado are investigating continued movements in the Andaman islands using Global Positioning geodesy and sea level monitoring devices to find out whether these catastrophic changes are increasing or decreasing. The spatial variation of these effects with time provides unique constraints on the continuing processes in the islands. Their initial results indicate that although the islands continue to move SW some of the subsidence has been reversed. In this two year project they hope to provide many hundreds of points to constrain future motions, and to answer how frequently these earthquakes occur.One of their objectives is to show local scientists how to undertake these kinds of measurements. Strong collaborative agreements have been established with scientists affiliated with the Society for Andaman and Nicobar Ecology (SANE) who have access throughout most of the islands.