Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya
Near‐Surface Geomechanical Properties and Weathering Characteristics Across a Tectonic and Climatic Gradient in the Central Nepal Himalaya
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DOI：
10.1029/2021jf006240
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发表时间：
2021-12
期刊：
影响因子：
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通讯作者：
William G. Medwedeff;Marin Clark;D. Zekkos;A. West;D. Chamlagain
中科院分区：
文献类型：
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作者：
William G. Medwedeff;Marin Clark;D. Zekkos;A. West;D. Chamlagain
浅层基岩强度控制着滑坡灾害以及侵蚀的速度和形式，但由于收集现场测量的挑战，近地表力学特性的区域模式很少被定量地了解。在这里，我们介绍了尼泊尔喜马拉雅山脉中部浅层地下的地震和地质力学特征。通过根据地质强度指数分类系统将广泛分布的一维剪切波速度调查和工程露头描述配对，我们评估了近地表力学特征的景观尺度模式及其与已知影响岩石强度的环境因素的关系。我们发现浅层基岩强度更多地取决于化学和物理风化的程度，而不是喜马拉雅山中部发现的变质岩性之间的矿物和结构差异。此外，风化作用随地形而系统地变化。基岩山脊顶部地点风化程度高，具有更典型的土壤 S 波地震速度和剪切强度特征，而靠近谷底的地点往往风化程度较低，具有岩石典型的高 S 波速度和剪切强度估计特征。山坡上的风化变化明显更大，导致横波速度介于山脊和河道端元之间。我们推测，山坡环境的变异性可能部分是由于大规模浪费的偶发性造成的，这种浪费清除了最近出现滑坡疤痕的风化物质。这些结果强调了浅层地下的机械不均匀性，并强调在估计区域滑坡灾害分析的强度参数时需要考虑可变的基岩风化。
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.