Postseismic deformation of the Andaman Islands following the 26 December, 2004 Great Sumatra–Andaman earthquake

Abstract

Two years after the Great Sumatra‐Andaman earthquake the 3.1 m WSW coseismic displacement at Port Blair, Andaman Islands, had increased by 32 cm. Postseismic uplift initially exceeded 1 cm per week and decreased to <1 mm/week. By 2007 points near Port Blair had risen more than 20 cm, a 24% reversal of coseismic subsidence. Uplift at eight GPS sites suggests a gradual eastward shift of the coseismic neutral axis separating subsidence from uplift. Simulations of the GPS postseismic displacements as viscoelastic relaxation of coseismic stress change and as slip on the plate interface indicate that slip down‐dip of the seismic rupture dominates near‐field deformation during the first two years. Postseismic slip beneath the Andaman Islands released moment equivalent to a magnitude M w ≥ 7.5 earthquake, and the distribution suggests deep slip in the stable frictional regime accelerated to catch up to the coseismic rupture.