Date of Graduation

Summer 2021

Degree

Master of Natural and Applied Science in Geography, Geology, and Planning

Department

Geography, Geology, and Planning

Committee Chair

Kevin Mickus

Keywords

Main Himalayan Thrust, Nepal Himalaya, 2015 Gorkha Earthquake, Bouguer gravity anomaly, crustal structure

Subject Categories

Geology | Geophysics and Seismology

Abstract

Nepal Himalaya is situated in the central portion of the seismically active Himalayan Orogenic Belt, which is geologically segmented by four major mega fault structures: 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 the Indian and Eurasian Plate. The 2015 Gorkha Earthquake occurred within the MHT system, MHT system itself had been seismically locked since the 1934 Nepal-Bihar Earthquake, complicating the geometry depth of MHT. Available land and airborne gravity data were used to determine the general crustal structure of Nepal. The gravity data was processed into a series of gravity anomaly maps such as complete Bouguer gravity, isostatic residual gravity, wavelength filtered gravity, and derivative gravity anomalies which revealed major tectonic boundaries, and the gravity anomalies showed the density variation of the underlying rocks. The derivative map, upward continued map and Euler’s deconvolution map indicated the depth of the MHT system ranges from 3 km and 15 km before the locking zone. The less dense area is surrounded by large dense anomaly bodies in the 2015 Gorkha Earthquake region, which can be interpreted as the causes of stress within the region. The north to the south steeper gradient indicates a steeper and deeper Moho towards the north. The geometry and exact location of the MHT profile can be revealed from a 2-D forward gravity model.

Copyright

© Kapil Prasad Phuyal

Open Access

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