Change of stress patterns during 2004 MW 9.3 off‐Sumatra mega‐event: Insights from ridge–trench interaction for plate margin deformation

Abstract

The Andaman–Sumatra subduction margin is identified as a complex tectonic transition zone between the Myanmar and Java regions. The Late Cenozoic evolution of various basins, faults, plate slivers, and the Andaman back‐arc rift‐transform system altered the boundary. The sharp changes in free air gravity anomaly, north‐eastward swing of the Andaman Sea spreading ridge, increased dip of the subducting plate and the decrease in seismicity towards the north of the study area possibly account for increased interaction of the Ninetyeast Ridge (NER) against the Andaman Trench. Stress fields of the subducting plate were modified from semi‐radial compression (SRC) during pre‐seismic deformation to pure extension (PE), semi‐radial extension (SRE), and strike‐slip extension (SSE) during post‐seismic deformation phase in the central segment. The co‐seismic second energy burst, subsidence in the overriding plate, and subsequent reactivation of the Barren and Narcondum volcanoes also corroborate the transformation of the stress fields in the overriding plate into predominant extension. The dominant SRC stress in the subducting Indo‐Australian lithosphere in the south changed to pure compression (PC) prior to the 2004 MW 9.3 mega‐event. While the changes in stress field from SRC to pure strike‐slip (PSS) in the north account for stress relaxation after the occurrence of the mega‐event and the co‐seismic ~1,300‐km rupturing, the occurrences of uplift and subsidence in the northern part of the study area were possibly caused by inherited stress field from the Eastern Himalaya. The continued overall compression in the subducting Indo‐Australian Plate in the south, under pre‐ and post‐seismic deformation phases, is presumably correlated with the focal mechanism stress parameters of the 2004 mega‐event, whereas the strike‐slip‐dominated movement in the overriding plate remained almost unchanged. The stress perturbations in the central segment are interpreted to be the result of interaction between the NER and Andaman–Sumatra Trench.