Active faulting in the upper plate of the Himalayas: Paleoseismic insights from the Western Nepal fault system

Abstract

Understanding the role of upper-plate faults in obliquely convergent margins is essential for assessing regional strain distribution and seismic hazards. In the Himalayas, paleoseismic research has focused on the Main Frontal thrust as the primary surface expression of the plate boundary, leaving the seismic potential of upper-plate faults like the Western Nepal fault system largely unstudied. This study presents new paleoseismic evidence from seven trench sites across five mapped fault segments of the ~250-km-long Western Nepal fault system, providing the first direct constraints on its earthquake history. Stratigraphic and geochronologic data reveal at least three Holocene surface-rupturing earthquakes, with age constraints indicating that at least one of these events overlapped in time with a major Himalayan earthquake within the past ~800 yr. This demonstrates that the Western Nepal fault system is a significant, cross-orogen fault system, playing an active role in accommodating strain in the region. These findings establish the Western Nepal fault system as an active seismogenic system that accommodates oblique plate convergence and contributes to regional strain partitioning. This challenges the prevailing view that seismic deformation in the Himalayas is primarily confined to the Main Frontal thrust and highlights the need to incorporate upper-plate fault systems into seismic hazard assessments.