Active faulting, seismicity and their relationship with megathrust ramps and inherited structures in Western Nepal: insights from a three-dimensional model



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Earthquakes and related landslides are a common geologic hazard for Himalayan countries. In April of 2015, the country of Nepal was reminded of their susceptibility to great earthquakes when a magnitude 7.2 earthquake struck the region. This study focuses on an area where it is hypothesized that an active fault either extends toward the large city of Surkhet or merges with another fault that passes to the south of the city. Previous studies suggest that the western Nepal Himalaya is currently at a high risk of a great earthquake striking the region. Earthquake hazard assessment requires an understanding of the geometry and distribution of active faults. We integrated published active fault maps, geologic maps, microseismicity data, cross-sections, a megathrust model, and a digital elevation model, into a three-dimensional database using MOVE software. By interpreting the data, we constructed additional geological cross-sections trending in both NE-SW and NW-SE directions. Using the 3D kriging method to build the three-dimensional geologic model, exhibiting major faults and horizons from cross-sections and traces of contacts that are projected to the digital elevation model in MOVE. Also, from the seismicity data obtained, there is a noticeable pattern that suggests a segment of the megathrust is generating a new front ramp by incorporating a slice of lower plate to the upper plate. By integrating our observations and interpretations with the three-dimensional database, we also discuss the potential effects of megathrust ramps, active faults, and potential reactivation of old thrusts on seismic hazard in the region.