12-7 Across the Grain: A Seismically-Active, Oblique Megathrust Splay Fault System in Western Nepal

Session: The Geodynamic Evolution of the Himalaya: From Mountain Building to Modern Seismicity and Climate Change

Presenting Author:

Sean Bemis

Authors:

Bemis, Sean P.¹, Curtiss, Elizabeth Rose², Hoxey, Andrew KR³, Daniel, Michael⁴, Kafle, Manoj⁵, Murphy, Michael A.⁶, Taylor, Michael H.⁷, Styron, Richard⁸, Sutley, Elaina⁹, Chamlagain, Deepak¹⁰, Fan, Suoya¹¹

(1) Geosciences, Virginia Tech, Blacksburg, VA, USA, (2) Geosciences, Virginia Tech, Blacksburg, VA, USA, (3) George Mason University, Fairfax, VA, USA, (4) University of Houston, Houston, TX, USA, (5) Tribhuvan University, Kathmandu, Nepal, Nepal, (6) University of Houston, La Canada Flintridge, CA, USA, (7) Univ. of Kansas, Lawrence, KS, USA, (8) GEM Foundation, Portland, OR, USA, (9) University of Kansas, Lawrence, KS, USA, (10) Tribhuvan University, Kathmandu, Nepal, (11) University of Pittsburgh, Pittsburgh, PA, USA,

Abstract:

Through studies spanning geoscience subdisciplines, the wedge of a subaerial subduction zone, change in convergence obliquity, high Himalayan topography, and the Covid-19 pandemic, we examine the evolution and earthquake potential of a prospective megathrust splay in the Himalayan orogen. The Western Nepal fault system was previously documented only through remote observations and limited field study, leaving the extent, connectivity, slip sense and rate, and earthquake behavior unconstrained. We conducted field traverses along 170 km of the fault system targeting the along-fault bedrock geology, tectonic geomorphology, paleoearthquake records, and infrastructure vulnerability. Our mapping documents a continuous dextral-normal fault system, including a major right-step with connecting fault segments, extending from Simikot in far northwestern Nepal to Tansen, near the Himalayan range front in central Nepal. Mapped bedrock offsets near Talphi indicate 3.9 to 6.8 km of cumulative slip across the fault system, with displaced geomorphic markers in the same area indicating a minimum slip rate of 2.7 ± 0.6 mm/yr since ca. 130 ka. Similar slip rates also occur southwest along the fault system with offset landforms exhibiting a persistent pattern of dextral-normal offset. Paleoseismic investigations at seven sites demonstrate youthful surface rupture, with the most recent earthquake at several sites constrained within the past 700 years and earthquake timing at all sites overlapping with the 1505 Great Himalayan earthquake. We also documented the construction typology of a new rural school building to model the seismic performance of buildings constructed to newer (post-Gorkha earthquake) standards. Our study demonstrates that the Western Nepal fault system is a major fault that cuts obliquely across the older Himalayan thrust zones and forms the boundary of an orogenic wedge sliver. The dextral-normal sense of slip illustrates how the fault system accommodates orogen-parallel extension and the partitioning of oblique plate boundary strain. Characterization of the Western Nepal fault system provides a significant contribution of new parameters for probabilistic seismic hazard assessment of Nepal with the addition of new faults, controls on fault geometry, slip rates, paleoearthquake timing, and scenarios for potential along-fault earthquake rupture behavior.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-4302

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