Discourse on Seismotectonics of Nepal Himalaya and Vicinity:Appraisal to Earthquake Hazard

Abstract

An extremely complex geotectonic framework coupled with high seismic status has made the Central part of the Himalayas, a destination to study the complex inter-continental collision processes. The collision also caused large scale deformation and high seismicity of vast region of colliding continents. This region displays all major tectonic features of the Himalayan mobile belt and is seismically one of the active regions in the Himalayan arc. Focal mechanism solutions bear out a multifaceted pattern. Thrust environment is dominant in the Western and Central Nepal region, whereas, in the Eastern Nepal, it is a amalgamation of thrust and strike-slip with large thrust mechanism. In western region thrust faulting coupled with shallow dip nodal planes reflects the Indian lithosphere is under-thrusting at a shallow angle. Here, the crustal shortening in north- south direction in which earthquakes is generated due to northward compression. The shortening was accommo- dated by development of various NW-SE trending structures like Himalayan Arc MCT (Main Central Thrust), MBT (Main Boundary Thrust); MFT (Main Frontal Thrust). The observed change in the faulting pattern in the eastern parts of the thrust zone may indicate substantial movement along the transverse faults, as compared to that of the western region with the changes in the deep crustal structure. The thrusting decreases rapidly with increasing focal depth and deformation occur due to strike-slip motion at greater depths. This may investigative of an unstable state of the upper mantle leading to a rapid deformation in the presence of high degree of thermal regime. The composite stereographic projection of the compression and tension axes suggest a shallow compressive stress, dipping N-S to NE-SW in Western Nepal whereas it is N-S to NNE-SSW direction of compression at a shallow angle in Eastern Nepal. The region produced a number of devastating events in the past. Central Himalaya comprising Nepal and its adjoining region in which different types of faulting patterns exist have signatures of a great earthquake in 1934 and a number of large events thereafter, advocate serious seismic hazard in the region.