Transtension to transpression: A study of strain evolution in Andaman Islands based on GPS measurements following Great Sumatra–Andaman Earthquake, 2004

We use campaign mode and continuous GPS data from 2005 to 2008 to examine the evolving strain pattern in Andaman Islands after the Great Sumatra–Andaman earthquake. Two and half years after the earthquake, the post-seismic period changes to inter-seismic period. At Port Blair, post-seismic displacement rate reduced to about 95% with clockwise rotation of azimuths and towards North Andaman, velocity vector gradually rotates towards north with drop in magnitude. The inter-seismic displacement was dominated by trench parallel movement. Strain calculations on inclined transpession model shows deformations at Andaman were highly heterogeneous both in post and inter-seismic periods with dominant strike-slip and dip-slip shearing along with layer parallel shortening and extension. Post-seismic and inter-seismic periods had suffered simultaneous subduction perpendicular extension and contraction leading to transtension and transpression deformational environments respectively. Occurrence of earthquakes during post and inter-seismic periods were results of interplay between simple shear and pure shear components of the strain matrix.

► Andaman takes 2.5 years to attain inter-seismic period after the 2004 mega-earthquake.
► Post-seismic period shows subduction perpendicular extension leading to transtension.
► Transpressional deformation characterize inter-seismic period.
► Interplay between simple and pure shear results earthquakes during both the periods.
► Adopted strain analysis can explain better present strain evolution at subduction zone.