Kinematic boundary between Burma and Sumatra at the Nicobar Trench

The Andaman–Nicobar segment of the Sunda subduction zone hosts some of the world’s highest tsunamigenic hazards, exemplified by the 2004 Mw 9.1-9.3 Sumatra–Andaman event. Highly oblique convergence promotes slip partitioning between the megathrust and upper-plate strike-slip faults; however, the detailed fault architecture of the Andaman arc remains uncertain. To better constrain the neotectonic framework, we perform regionally comprehensive kinematic block modelling using the most up-to-date geodetic velocities and earthquake slip vector azimuths, allowing us to quantify slip rates on major crustal faults from southern Myanmar to Java and assess along-strike variations in deformation style. Our results reveal several new features of the tectonic system. 1) A distinct transition in trench behaviour occurs near the Nicobar Islands, separating independently moving Andaman (Burma plate) and Sumatran segments of the rigid forearc. This boundary coincides with pronounced changes in slip magnitude, rake and rupture velocity during the 2004 great Andaman–Sumatra earthquake, implying that the rupture spanned two kinematically distinct plate boundaries that are interseismically loaded at different rates and in different directions. This boundary is expressed by sharp changes in gravity, bathymetry, and trench obliquity gradient, analogous to those observed at the Sunda Strait, particularly marked by a negative residual Bouguer anomaly indicative of a mechanically weak zone capable of accommodating differential block motion. 2) The strike-slip Andaman-Nicobar Fault, the offshore continuation of the Sumatran Fault, has a slip rate of >30 mm/yr, about twice that of the Sumatran Fault. The lack of recorded large earthquakes along this system and abundant swarm seismicity, imply that deformation may be accommodated by a combination of fault creep and localized locked patches, and/or by distributed slip across multiple structures. Our results have important implications for seismic hazard assessment and for future tectonic and tsunami-generation models, which must account for the structural barrier and complex strain accommodation in this part of the subduction system.