GRAVITY DATA ANALYSIS OF THE NEPAL HIMALAYA CRUST

The Nepal Himalayas are located in the central portion of the Himalaya which is geologically segmented by four major fault systems: Main Frontal Thrust (MFT), Main boundary thrust (MBT), Main Central Thrust (MCT), and the South Tibetan Detachment System (STDS). The MFT, MBT, and MCT are part of the basal decollement (Main Himalayan Thrust (MHT)) which accommodates the convergence between India and Asia. Despite the 2015 Gorkha earthquake which occurred within the MHT system, the MHT system itself has been seismically locked since the 1934 Nepal-Bihar Earthquake. As the MHT system has been locked for such a long time, the geometry of the fault system at depth has been difficult to determine using earthquake data. Thus, the available land and airborne gravity data will be used to help to determine the general crustal structure of Nepal as well as the geometry of the fault system. The gravity data has been processed into a series of gravity anomaly maps including complete Bouguer, isostatic residual, wavelength filtered, and derivative gravity anomalies. The derivative analysis and the 3D Euler analysis have outlined the main thrust systems and have indicated that these faults continue into regions not previously mapped. The 3D Euler deconvolution has estimated that the density source associated with MHT beneath the MCT to be 20-25 km. Within the region of the Gorkha earthquake, the density sources are shallower (8-10 km) and are above the estimated focus of the event (30-40 km). The results of the above analysis indicates the crust in the region of the Gorkha earthquake is less dense that the region to the west and east suggesting that large scale density differences may have played a roll in isolating stresses in this region.