Seismic characterization of the Critical Zone in the Nepal Himalaya: a regional perspective

Understanding the dynamics that govern the chemical alteration, detachment, and transport of fresh bedrock represents a 'grand challenge' in Earth surface process science (National Research Council, 2010). A recent surge of studies following the establishment of the US Critical Zone Observatories has contributed detailed characterizations of the architecture and variability within the deep weathering zone. However most observatory sites are in areas with limited modern tectonic activity, and particularly little is known about how regolith thickness and the rate of bedrock weathering respond to the gradients in climate and tectonics across convergent plate boundaries. Here we present 1D shear wave velocity profiles from across the central Nepal Himalaya which constrain regolith mechanical properties in the upper ~20m of the subsurface. We also supplement our seismic observations with characterizations of exposed outcrops following the Hoek-Brown rock mechanical criteria. Our data reflect a strong geomorphic control on the thickness of the weathered zone, and on the associated intensity of weathering. Ridge sites exhibit both intense chemical alteration and dense fracturing, resulting in shear wave velocities below the engineering classification for a stiff soil (<500m/s, International Building Code) to depths beyond the limit of investigation. Where hillslopes are not mantled in colluvium, depth averaged shear wave velocities are highly variable (>200m/s, <900m/s) and do not correlate strongly with lithology, slope gradient, or slope aspect. We hypothesize this variability may be partly explained by the stochastic nature of mass wasting events, which clear away weathered material to exhume fresh bedrock in punctuated events. Adjacent to bedrock channels, we find low fracture densities and average shear wave velocities characteristic of intact rock (~1000 m/s), which is consistent with both catchment hydrologic models of CZ evolution (e.g. Rempe & Dietrich, 2014) and topographic-stress-induced bedrock fracturing (e.g. St. Clair et al., 2015). These results provide a snapshot view of the Critical Zone in a tectonically active landscape, with implications both for seismic/landslide hazard assessment and for the interplay between regolith thickness and topographic structure.