Current understanding of the seismotectonics of Western Nepal Himalaya and vicinity

The study of seismic activity at some stage in 1963 to 2006 in the Western Nepal Himalaya and its adjoining regions (28–31°N and 79–82.3°E), reveal that seismicity is non-uniform in space and time. The analyses of fault-plane solutions of twenty-four earthquakes inferred that the Western part of Nepal Himalayan frontal arc is in compressed state in which seismic activity is dominated by thrust faulting. Based on orientation of P-axes, compressive stress directed north-south to northeast-southwest approximately perpendicular to the prevailing stress along the major trend of the Himalaya. Thrust faulting coupled with shallow dip of nodal planes reflects that the Indian continental lithosphere is under-thrusting at a shallow angle. This information suggests crustal shortening in north-south direction in which earthquakes are generated due to northward compression. In the adjoining Tibet parts earthquake activity is due to normal faulting with east-west extension. These might be due to the presence of a relatively strong Main Himalayan Thrust, the plate boundary fault below the Himalayas, would have favored the occurrence of thrusting. While, a weak Main Himalayan Thrust below Tibet along with initiation of the Main Central Thrust can explain South Tibetan Detachment (geodynamic process) and associated stress field in Western Nepal Himalaya and its adjoining regions