Collaborative Research: Filling in the Central Himalayan Seismic Gap: A Structural, Neotectonic, and Paleoseismic Investigation of the Western Nepal Fault System

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The 2015 Gorkha Mw 7.8 earthquake, which caused 9,000 fatalities, resulted from rupture along a megathrust - the Main Himalayan thrust - where the Indian sub-continent subducts beneath deformed Himalayan belt and the Asian continent. Most megathrust fault systems, where one tectonic plate is being forced underneath another tectonic plate, occur deep beneath the world's oceans where observations are limited due to difficult submarine access. These systems produce the largest recorded earthquakes in the world and typically consist of a primary fault and additional splay faults that work together to accommodate collision between the tectonic plates. A recently discovered complex fault system west of the Gorkha epicentral region, the Western Nepal Fault System (WNFS), appears to play a major role in accommodating the Himalayan collision and poses a major seismic hazard for a large region of Nepal. This project will document the long-term rate of motion, determine the timing and location of prehistoric earthquakes, and map out the full extent of the WNFS. The data will be used to develop constraints on the seismic hazard exposure of regional populations and provide recommendations for low-cost, sustainable, and culturally sensitive earthquake risk mitigation strategies. The project would advance other desired societal outcomes such as full participation of women in STEM, increased public scientific literacy and public engagement with STEM through participation in local public outreach activities, development of a diverse, globally competitive STEM workforce through undergraduate and graduate student training, and fostering international collaboration.Geologic and geophysical observations have long shown that areas of oblique plate convergence tend to form slip partitioned systems, where the basal megathrust accommodates margin-perpendicular convergence, and a strike-slip splay or backarc fault accommodates margin-parallel motion. Given the curvature of most convergent margins, convergence obliquity often varies along strike, becoming zero at some point. However, because these zones are usually inaccessible in offshore subduction zones, the kinematics and earthquake cycle behavior of these splay faults are poorly known. This project targets the newly discovered Western Nepal Fault System as a major, subaerial, well-exposed splay fault system within the Himalayan thrust wedge that appears to transfer dextral strain from the Karakoram Fault in the obliquely convergent northwest Himalayan backarc to the central Himalayan forearc where convergence is primarily margin-normal. The objective of this project is to test multiple working hypotheses that use the deformation patterns of the WNFS to constrain models of fault segmentation and linkage for regional slip-partitioning and splay faulting models: (1) strong strain partitioning hypothesis in which the WNFS is a well-connected system of active faults that transfer slip from the Karakoram- Gurla Mandhata/Humla fault system in the northwest, across the thrust wedge, and branches with the Main Frontal Thrust at the front of the wedge; (2) weak strain partitioning hypothesis in which the WNFS is a collection of disconnected faults that broadly accommodate arc-parallel strike-slip faulting and northwest translation of the Himalayan arc sliver; and (3) thrust faulting and oblique ramp hypothesis in which the WNFS is a collection of faults that accommodate shortening within the thrust wedge. To address these hypotheses and leverage new constraints on regional seismic hazard, the research team will carry out a three year multidisciplinary investigation consisting of three primary components: (1) structural geology, neotectonics, geomechanics, and geochronology with paleoseismology to constrain (a) the geometries and kinematics of active fault systems, (b) displacement magnitude, (c) fault slip rates, and (d) historical patterns of strain release using paleoseismology; (2) an updated probabilistic seismic hazard and risk analysis integrating the results of the geologic studies; and (3) a benefit-cost analysis of structural and non-structural vulnerability-reducing strategies at household to community scales to provide recommendations for reducing risk and promoting resilience through mitigation and recovery. The active fault mapping, slip rate, and paleoearthquake data are also fundamental components of seismic hazard analyses. Working with Nepali collaborators, the results will be used to provide updated probabilistic seismic hazard analysis and develop practical mitigation strategies for a region widely thought to be at heightened seismic risk.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.