Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya

Abstract

Shallow bedrock strength controls both landslide hazard and the rate and form of erosion, yet regional patterns in near‐surface mechanical properties are rarely known quantitatively due to the challenge in collecting in situ measurements. Here we present seismic and geomechanical characterizations of the shallow subsurface across the central Himalayan Range in Nepal. By pairing widely distributed 1D shear wave velocity surveys and engineering outcrop descriptions per the Geological Strength Index classification system, we evaluate landscape‐scale patterns in near‐surface mechanical characteristics and their relation to environmental factors known to affect rock strength. We find that shallow bedrock strength is more dependent on the degree of chemical and physical weathering, rather than the mineral and textural differences between the metamorphic lithologies found in the central Himalaya. Furthermore, weathering varies systematically with topography. Bedrock ridge top sites are highly weathered and have S‐wave seismic velocities and shear strength characteristics that are more typical of soils, whereas sites near valley bottoms tend to be less weathered and characterized by high S‐wave velocities and shear strength estimates typical of rock. Weathering on hillslopes is significantly more variable, resulting in S‐wave velocities that range between the ridge and channel endmembers. We speculate that variability in the hillslope environment may be partly explained by the episodic nature of mass wasting, which clears away weathered material where landslide scars are recent. These results underscore the mechanical heterogeneity in the shallow subsurface and highlight the need to account for variable bedrock weathering when estimating strength parameters for regional landslide hazard analysis.Plain Language SummaryRock strength controls the occurrence of landslides and the relationship between topography and erosion, but it is rarely known in natural environments because the most common measurement techniques are designed for individual site characterization and they are time‐consuming to apply over broad spatial scales. In this paper, we apply a unique sampling strategy consisting of two field‐based measurement techniques that are relatively portable and quick to apply repeatedly, and provide rock strength information over the scale of an outcrop (tens of meters). We apply our approach to an area of central Nepal where rock strength estimates are particularly important to know due to the high landslide hazard in this region. Our results show that rock strength is strongly controlled by the degree of weathering (i.e., the breakdown of rock by chemical and physical processes), which varies widely in Nepal depending on the local climate and topographic characteristics. In particular, we find that weathering is systematically greater (and thus rock strength is lower) on ridges compared to valley bottoms. On hillslopes between ridges and valleys, both weathering and rock strength are highly variable and difficult to predict, highlighting the need to further investigate regional variability in rock strength for future landslide hazard assessment.Key PointsWe employ seismic surveys and geomechanical rockmass characterizations to investigate regional patterns in rock strengthNo single variable explains the observed patterns in near‐surface mechanical properties within this tectonically active terrainWeathering characteristics associated with ridge‐to‐channel topography contributes order‐of‐magnitude variations in near‐surface strength