Stress regimes analysis in Northeast India, Indo-Burma Ranges: Stress field implications based on Moment Tensor solution data

The Northeast India plate boundary is a globally significant convergence zone where the Indian, Eurasian, and Burmese plates interact. This area comprises two tectonic regions: the Himalayan collision zone in the north and the Indo-Burma Ranges to the east. Numerous major earthquakes have struck this region, such as the 1897 Shillong event (Mw ≥ 8.1) and 1950 Assam-Tibet earthquake (Mw ≥ 8.6). Despite its high seismicity, a comprehensive depth resolved stress analysis, across the area remains poorly defined. This research fills the gap by performing seismotectonic stress analysis using 377 focal mechanism solutions (Mw ≥ 4.0) between 1950 and 2025 gathered from global earthquake catalogues and major published sources. To identify lateral and vertical variations in the stress field, the study region (85°E-98°E, 13°N-31°N) was spatially subdivided into 21 seismotectonic zones based on seismicity clustering, focal depth distribution, slab geometry, and structural boundaries. The Hardebeck-Michael method is applied for linear stress tensor inversion, resolving fault plane uncertainty by rotational optimization and Mohr-Coulomb instability criteria. Iterative inversion was performed with Shape ratio (R)=0-1 and Friction coefficient (μ)=0.2-0.8, retaining only solutions where misfit angles are less than 45°, ensuring accurate determination of principal stress axes and Maximum horizontal compressive stress (SHmax) directions. The results indicate a N-S compressional stress regime extending from the Eastern Himalayas to the Bengal Basin aligning with the India-Eurasia convergence. This stress state is associated with major tectonic structures including the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), the Dauki Fault, and Brahmaputra Fault. However, the Indo-Burma Ranges show strong depth-dependent stress heterogeneity. Shallow to intermediate depth earthquakes exhibit arc-perpendicular extension (ENE-WSW to ESE-WNW), interpreted as a response to slab pull and upward convex bending of the subducting Indian lithosphere. Deep focus events (>70 km) indicate slab parallel N-S compression, which shows lithospheric shortening within the descending plate rather than solely due to India-Eurasia collision. A separate NE-SW compressional regime appears in the northern Indo-Burma arc and Sagaing Fault region, indicating stress-strain partitioning between Indian, Burmese, and Sunda plates. The clockwise rotation of SHmax along the arc from NNE-SSW in the inner segment to ENE-WSW in the outer foreland supports a transition from dextral strike slip motion to arc-perpendicular shortening. In the Shillong Plateau and Assam Valley, the coexistence of N-S and E-W compression indicates eastward extrusion of a crustal block, consistent with geodetic measurements and borehole breakout results. The results indicate that the stress regime is influenced not only by India-Eurasia convergence, but also by slab geometry, crust-mantle interaction, and block extrusion processes. These insights will be helpful for seismic hazard assessment and tectonic modelling in one of the most seismically active complex convergent plate boundary zones.