Constraints on rupture of the December 26, 2004, Sumatra earthquake from far-field GPS observations

We use Global Positioning System (GPS) data from nine permanent GPS sites surrounding the epicentre of the December 26, 2004, Sumatra earthquake to infer coseismic displacements at these sites. The results suggest that GPS site at SAMP, Medan in Sumatra Island, the nearest site from the epicentre, experienced a westward coseismic horizontal displacement of about 14 cm, while sites in southern India, namely, HYDE and IISC experienced predominantly eastward coseismic horizontal displacement of about 6-11 mm. We estimated the slip on the mainshock rupture and rupture extent in a uniform half space by analysing these coseismic displacements. We estimate an average reverse slip of about 11 m on the southern part of the rupture and an oblique slip of about 10 m on the northern part of the 1200 × 100-175-km2 rupture. Our results of estimated coseismic horizontal displacement in the Andaman and Nicobar region using the above rupture parameters are also consistent with the near-field GPS measured coseismic displacements in the region.