Coseismic offsets due to the 11 April 2012 Indian Ocean earthquakes (M_w 8.6 and 8.2) derived from GPS measurements

Abstract

The 11 April 2012 earthquake (M_w 8.6) in the Indian Ocean, about 100 km west off the Sumatra subduction zone, is the largest intraplate strike-slip earthquake in the known history. Two hours later, it triggered another great earthquake of M_w 8.2 in its vicinity. The earthquakes reflect the internal deformation of the diffused plate boundary between India and Australia caused by the differential plate motion between them. The slip occurred on conjugate planes, and the presence of some of them has been reported from the swath bathymetry and satellite magnetic anomalies. We estimate coseismic offsets due to these earthquakes at continuous GPS sites in the Andaman-Nicobar region and at other International GNSS Service (IGS) sites around the earthquakes source region. The sites on the Andaman Islands, which are about 900–1200 km to the north of the earthquake epicenters, experienced predominantly southward coseismic offset of up to 3 cm. The nearest site, Campbell Bay, on Great Nicobar Island, about 500 km to the north of the earthquake, documented an ESE offset of about 4 cm. The coseismic offsets are consistent with the finite-fault slip models derived from back projection of the seismic waves recorded by the global networks.