Near-field observations on the co-seismic deformation associated with the 26 December 2004 Andaman-Sumatra earthquake

We have been studying the tectonics of the Andaman and Nicobar region since 2001, for mapping geomorphological features such as elevated and subsided landforms, both of which are associated with subduction tectonics. We had also established eight GPS control points along the arc and had completed four GPS campaigns, the last pre-earthquake survey having ended three months prior to the great earthquake of 26 December 2004. During the post-earthquake surveys, we mapped permanent changes in the landforms at many locations, subsided coastal tracts and emerged colonies of corals, all of which are indicative of co-seismic deformation. GPS data indicate that the co-seismic offsets along the arc are nonuniform, the southern islands having been displaced by > 6 m in the southwest direction. Our observations indicate two regions characterized by higher and lower slips, the Nicobar and the Andaman segments respectively. Geomorphological observations indicate widespread uplift along the west coast of the Andaman segment and subsidence on the eastern part. The little Andamans and the northern part of the Andaman region generally show uplift. The GPS measurements indicate vertical displacement of -1.36 to +0.63 m along the arc.