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The intraplate region of the northern Bay of Bengal has occasional moderate-to-strong earthquakes ranging in depth from 10 to 60 km. This region is known to have a very thick sedimentary cover (~20 km) underlain by the oceanic plate. These intraplate earthquakes occur both within the sedimentary layers and the basement. In order to understand the seismotectonics of this region, we study the source mechanism of recent moderate-to-strong earthquakes (Mw>5.5). We model teleseismic body waveforms (P and SH) to obtain the best fit double couple solution and well constrained centroid depth. The dataset has good azimuthal coverage and clear depth phase. We use a simple source time function which reproduces the main features of the observed waveforms and invert the dataset using linearized least squares algorithm. We also perform uncertainty tests on the modeled source parameters by fixing each parameter to a range of values and allowing the inversion to find the best fit solution. Through this we are able to ascertain the ±1 sigma bound on the modeled parameters. Our results reveal strike-slip faulting in the northernmost Bay of Bengal and normal faulting to its east, close to the Indo-Burman subduction zone. The strike-slip faults originates at depth between 20-30 km, while the normal faulting are shallower than 10 km. We conjecture that the strike-slip faulting occurs on passive margin rift faults within the basement, which are reactivated under oblique N20E motion of India. Whereas the shallow normal faulting close to the Indo-Burman subduction zone occurs due to flexural extension of the top of the downgoing Indian plate. [ABSTRACT FROM AUTHOR]

Northeast India is sandwiched between the seismically active plate boundaries of the Eastern Himalaya to the north and the Indo-Burman subduction zone to the east. This plate boundary system is different from the central Nepal Himalaya, due to oblique convergence across two orthogonal plate boundaries, resulting in a zone of distributed deformation both within and away from the plate boundary. Previous studies of spatial distribution and source mechanism of earthquakes have shown that the N20E convergence between India and Tibet is partitioned into N-S underthrusting and E-W subduction of the Indian plate beneath the Himalaya and Burma micro-plate, respectively. However, south-west of the plate boundaries, the middle-to-lower crust deforms by strike-slip faulting in the Kopilli Fault Zone (KFZ); upper-to-mid crustal strike-slip faulting along the western Brahmaputra Valley; and lower crustal strike-slip faulting beneath the Bengal Basin. Strike-slip faulting in the KFZ also extends northwards beneath the Eastern Himalaya and southeastward beneath the Naga fold-thrust belt. In order to understand the role of these strike-slip faults in accommodating the GPS derived convergence across northeast India, we study the source mechanism of recent earthquakes using teleseismic (for earthquakes with Mw>5) and local (4.0