*Result*: Fragmented plates in Sumatra–Andaman subduction zone revealed by distinct mantle anisotropic directions.

*Subduction zones are tectonic plate boundaries where one tectonic plate is being forced beneath another and are known for producing some of the most powerful earthquakes in history. Understanding the regional-scale structural heterogeneity in the subduction zones is crucial for deciphering the genesis of megathrust earthquakes and the factors that control rupture dynamics. The rupture dynamics of the earthquakes in the Andaman-Sumatra region have been primarily hindered by a lack of velocity and anisotropy information at the lithospheric level. Compressional Pn waves propagate through the uppermost mantle and provide constraints on velocity and anisotropy along their ray paths. We generated the first comprehensive high-resolution Pn wave tomography, restricting the turning of diving ray paths less than 50 km depth in the collision zones, to map lithospheric velocity and anisotropy by inverting 65,297 Pn arrivals extracted from 6,958 regional events recorded at 384 stations. The results show a strong variations in Pn wave velocity and fast polarization directions (FPDs) across the entire study region that may be an essential factor for earthquake nucleation. We observed an abrupt change in the Pn FPDs from the Andaman to Sumatra and Sumatra to Java regions. We suggest that well-defined plate boundaries between the Indian and Capricorn Plates, that was earlier reported to be diffused one. Such observation is also supported by the lower spreading rate and higher crustal age of the old Indian oceanic plate in the Andaman region, compared to the faster spreading rate and lower crustal age of the Capricorn oceanic plate in the northern Sumatra region. We suggest that the Sunda plate strongly coupled with subducting oceanic slab causes the mega events of 2004 and 2005 supported by trench-normal Pn FPDs along with rigid-slab nature, and both the rupture propagation terminate near Simeulue Island region due to abrupt material changes beneath it. Our study also reveals the presence of low Pn -velocity anomalies in the west of the Andaman Sea, suggesting the existence of a magma reservoir pouring out lava through the steeply torn Indian oceanic slab. [ABSTRACT FROM AUTHOR]*