Seismotectonics in Northeast India: a stress analysis of focal mechanism solutions of earthquakes and its kinematic implications

In Northeast India, three major plates interact along two convergent boundaries: the Himalayas and the Indo-Burma Ranges, which meet at the Assam Syntaxis. To clarify this tectonic interaction and the underlying dynamics, we determine the regional seismotectonic stress from the stress inversion of 285 double couple focal mechanism solutions of earthquakes with an average magnitude of 5. We then compare the reconstructed stress regimes with the available information about geodetically determined relative displacements. North-south compression, in a direction consistent with India-Eurasia convergence, prevails in the whole area from the Eastern Himalayas to the Bengal Basin, through the Shillong-Mikir Massif and the Upper Assam Valley. E-W extension in Tibet is related to this N-S India-Eurasia convergence. Not only does the major N-S compression affect the outer segments of the Indo-Burma Ranges, it also extends into the descending slab of Indian lithosphere below these ranges, although stresses at depth are controlled by bending of the slab beneath the Burmese arc. The existence of widespread N-S compression in the Bengal Basin, far away from the Himalayan front, is compatible with the previously proposed convergence between a Shillong-Mikir- Assam Valley block and the Indian craton. E-W compression inside this block supports the hypothesis of a component of eastward extrusion. Stress inversion of focal mechanism solutions in the Indo-Burma Ranges reveals a complex stress pattern. The Burmese arc and its underlying lithosphere experience nearly arc-perpendicular extension with ESE-WNW trends in the northernmost, NE-trending segment and ENE-WSW trends in the main N-S arc segment. Such extensional stress, documented from many arcs, is likely a response to pull from and bending of the subducting plate. At the same time, the Indo-Burma Ranges are under compression as a result of oblique convergence between the Sunda and Indian plates. The maximum compressive stress rotates from NE-SW across the inner and northern arc to E-W near the Bengal Basin. This rotation is consistent with the deformation partitioning reflected in the rotation of relative displacement vectors, from a SSW-directed Sunda-Burma motion to a WSW-directed Burma-India motion. As a consequence of this partitioning, the major belt-parallel fault zones show a variety of movements across the main N-S arc segment, from right-lateral slip in the inner ranges to oblique reverse-dextral slip in the outer ranges and pure thrusting in the westernmost foreland belt.