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Plate motion, crustal deformation, and earthquake occurrence processes in the northwest Sunda Arc, which includes the Indo-Burmese wedge (IBW) in the forearc and the Sagaing fault in the backarc, are very poorly constrained. Plate reconstruction models and geological structures in the region suggest that subduction in the IBW occurred in the geological past, but whether it is still active and how the plate motion between the India and Sunda plates is partitioned between motion in the IBW and Sagaing fault is largely unknown. Recent GPS measurements of crustal deformation and available long-term rates of motion across the Sagaing fault suggest that ~20 ± 3 mm/yr of the relative plate motion of ~36 mm/yr between the India and Sunda plates is accommodated at the Sagaing fault through dextral strike-slip motion. We report results from a dense GPS network in the IBW that has operated since 2004. Our analysis of these measurements and the seismicity of the IBW suggest that the steeply dipping Churachandpur-Mao fault in the IBW accommodates the remaining motion of ~18 ± 2 mm/yr between the India and Sunda plates through dextral strike-slip motion, and this motion occurs predominantly through velocity strengthening frictional behavior, i.e., aseismic slip. The aseismic motion on this plate boundary fault signifi cantly lowers the seismic hazard due to major and great interplate earthquakes along this plate boundary.

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We report GPS measurements of postseismic deformation from 22 campaign‐mode and one continuous GPS sites in the Andaman‐Nicobar region following the great Sumatra‐Andaman earthquake of 26 December 2004. Large horizontal displacements toward west to southwest, varying in magnitude from 10 to 40 cm, and with uplift reaching 16 cm, occurred in the region in the first year after the earthquake. The observed motion decreased logarithmically in the subsequent year. We suggest that in the Andaman region, frictional afterslip occurred farther downdip of the coseismic rupture, while in the Little Andaman and Nicobar regions, the coseismic rupture and afterslip patch partly overlapped. The afterslip was mostly aseismic and did not contribute to the aftershocks. The aftershocks and postseismic displacements appear to follow a similar relationship, although with different decay times. The temporal dependence of the two differs only by a term linear in time. Thus the temporal evolution of the afterslip seems to be consistent with a mechanism governed by frictional afterslip. Available rates of interseismic and postseismic deformation and coseismic static offsets allow us to approximately estimate a return period of about 400 years for great earthquakes in the Andaman region.

The 11 April 2012 earthquake (Mw 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 Mw 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.

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