Active Faulting, Seismicity and Their Relationship With Megathrust Ramps and Inherited Structures in Western Nepal: Insights From a Three-Dimensional Model

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 wedge-normal and wedge-parallel directions. Invoking the 3D kriging algorithm, we constructed a three-dimensional model of major faults and horizons from cross-sections and traces of contacts that are projected to the digital elevation model. The model shows an along-strike change in depth of major structures, generally from deep to shallow from west to east, which suggests the potential existence of lateral or oblique ramps in the megathrust or along-strike change in structural pattern. The Bari Gad fault, previously interpreted as an active normal-sense strike-slip fault and partly trending obliquely to the wedge, was excluded in a recent interpretation, but its location partially coincides with a contact between different rock units in the new interpretation. This raises the possibility that the active Bari Gad Fault branches to a wedge-parallel E-W trending segment by reactivating an old thrust fault that juxtaposes different units. This structural relationship may explain previous observations of a brittle phase of strike-slip along thrust faults at the front of the wedge, including the Main Boundary Thrust. The along-strike changes in both microseismicity distribution pattern and relative depth between the seismicity and the floor of the Lesser Himalaya duplex suggest a segment of the megathrust is generating a new front ramp by incorporating a slice of lower plate to the upper plate. This process is interpreted as a mechanism of the development of duplex structure and lateral or oblique ramps in the megathrust. 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.