Postseismic Deformation Measurements from the Andaman and Nicobar Islands

We carried out campaign GPS measurements at several sites in the Andaman and Nicobar Islands (India) 3-5 weeks after the December 26, 2004 Sumatra-Andaman Islands earthquake. In April 2004, we installed two continuous GPS sites in Diglipur, northern Andamans, and Car Nicobar. The continuous sites were installed near campaign sites, and survey ties are being carried out that will allow the continuous data to be combined with the January 2005 campaign data to provide a fuller record of postseismic deformation following this earthquake. Based on our preliminary results, postseismic displacements are large. Over a 19 day period in January 2005, a site in Port Blair moved ~3.5 cm westward, 1 cm southward and ~5 cm up (relative to the Indian plate). Longer-term displacements at Port Blair are not yet available. During April to August, the Diglipur continuous site moved ~5 cm westward, ~1 cm southward, but with no significant vertical motion, and displacements over this time period show a nearly linear rate of motion that is roughly an order of magnitude faster than the pre-earthquake velocity. Results for Car Nicobar are not yet available. Afterslip and viscoelastic relaxation are both expected to contribute to the observed postseismic deformation. We will present updated postseismic displacements and discuss their relevance to the mechanisms of postseismic deformation.