Constraints on 2004 Sumatra–Andaman earthquake rupture from GPS measurements in Andaman–Nicobar Islands

The 26 December 2004 Sumatra–Andaman earthquake (Mw 9.0–9.3) is the greatest earthquake of the modern seismological era. The rupture characteristics of the earthquake, particularly in the Andaman–Nicobar region, are not well resolved from seismological or far-field geodetic data. Here, in this article we present, campaign mode Global Positioning System (GPS) measurements of coseismic displacements at 13 sites in the Andaman–Nicobar Islands before and after the 2004 Sumatra–Andaman earthquake. These measurements provide improved estimates of rupture characteristics in the region. Coseismic horizontal ground displacement of 1.5–6.5 m towards the southwest and coseismic vertical displacement, mostly subsidence, of 0.5–2.8 m occurred along the Andaman–Nicobar Islands with maximum displacements in the Nicobar Islands. We estimate coseismic slip under the Andaman and Nicobar Islands as 3.8–7.9 m and 11–15 m, respectively. The length of the rupture is estimated to be about 1500 km with a width varying from 120 km under Middle Andaman Island to 160 km under Great Nicobar Island. GPS measurements during January 11–22, 2005 from Port Blair suggest rapid afterslip in the postseismic period. Limited GPS data available from 1995 measurements at two sites in Andaman provide evidence of strain accumulation that varied significantly in the 10 yr preceding the earthquake.