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Seasonal deformation of mountain rock slopes can be driven by groundwater infiltration and depletion. Such processes could explain our field observation in the Aletsch Valley, Switzerland, where GNSS‐derived 3D annual displacement amplitudes reach 3.4 cm. However, the physical mechanisms behind such groundwater‐driven surface displacements are not well understood. Here, we develop a fully coupled hydromechanical model to simulate the relevant processes in a valley slope embedded with numerous fractures of variable sizes. The magnitude and orientation of transient annual slope surface displacement obtained from our model are in overall agreement with the field observations. The key geological factors controlling the type and magnitude of reversible mountain slope deformations are fracture network geometry, fracture aperture, and regional stress field. We show that the heterogeneity and anisotropy of bedrock hydromechanical responses, originating from depth‐dependent variations of fracture properties, play a critical role in groundwater recharge and valley slope deformation. During recharge events, pore pressure perturbations migrate downward from the groundwater table and toward the receiving stream and the deep subsurface. This process driven by pressure diffusion and poroelastic stressing develops in the subsurface with a great reach of up to a few kilometers, called critical hydromechanical response zone, and controls surface deformation patterns. During groundwater recession, this hydromechanical response zone expands downward and ground surface displacement vectors rotate upwards. Our results suggest that slope surface deformation can inform about subsurface permeability structures and pore pressure fluctuations, which have important implications for understanding groundwater flow in fractured bedrock slopes. Plain Language Summary: Groundwater recharge in alpine valleys mainly occurs during snowmelt and/or rainstorm events. When recharge raises groundwater table in the slope, the bedrock can deform, and centimeter‐scale surface displacements can be observed. Here, we model this process in an alpine valley and investigate how bedrock fractures and regional stresses affect the surface displacement pattern. We show that slope surface deformation is controlled by a so‐called hydromechanical response zone reaching up to a few kilometers below the valley ridges. The shape and location of this zone change throughout the year. We compare our modeling results with long‐term deformation records from the lower Aletsch Valley, Switzerland, and show that they are in general agreement. The results of this work and insights obtained advance our fundamental understanding of the mechanisms of groundwater‐driven deformations in fractured rock slopes. Our research opens an avenue to use the information of surface deformation patterns to infer subsurface hydrogeological processes. Key Points: Coupled hydromechanical simulations are compared to surface displacement magnitude, orientation and hysteresis observed at Aletsch, SwitzerlandGroundwater recharge‐related pressure diffusion and poroelastic stressing in subsurface rock drive slope surface displacementCentimeter‐scale slope surface displacements occur in fractured crystalline rock with permeability in the narrow range of 10−16–10−14 m2 [ABSTRACT FROM AUTHOR]

Cross‐hole fluid injection tests were performed on shallow (0.5 km depth) crustal fault zones to characterize their internal flow structures on scales of 3–30 m. The data were acquired at the Grimsel Rock Laboratory, Switzerland, after the zonal isolation of damage zones in boreholes equipped with multipacker systems. We show that 80% of the pressure responses detected evolved as a power function of time due to fracture‐controlled flow and were best described by a fractional model with a flow dimension (n) in the range of [1.3,1.5]. The scaling of characteristic times (tc) and intermediate transitions in the flow dimension is shown to be inconsistent with the trend expected for normal diffusion, indicating a diffusion slowdown process caused by the spatial integration of structurally different damage zones. An analysis of the entire data set points to a subdiffusive regime where the mean squared displacement of pressure fronts, ⟨r2(t)⟩, scales as ⟨r2(t)⟩∝t0.59, similar to an anomalous diffusion process on synthetic fractal systems. We characterize this diffusion slowdown by calculating a network connectivity exponent (θ) of 1.4, from which we estimate a fractal dimension of df=2.23 for the tested fault zones. Such a value close to 2.0 supports the view of fault damage zones as hierarchical systems promoting flow channeling. Our results provide further support to theoretical models of fractional flow and their application to fractured media in nature. Key Points: We report direct, high‐resolution observations of pressure diffusion from fluid injection tests in shallow crustal fault zonesFractional flow regimes systematically emerge in response to spatial integration of structurally different damage zones during injectionThe fractal structure of damage zones may explain the emergence of anomalously slow pressure diffusion around faults [ABSTRACT FROM AUTHOR]

Deep-seated gravitational slope deformations (DSGSDs) are complex slope instabilities commonly involving long-term evolution with significant spatial and temporal variations in slope kinematics. The Cerentino compound landslide in southern Switzerland exemplifies the long-term evolution of DSGSDs as well as short-term movements causing substantial damage to infrastructure. Using an integrated analysis of surface geomorphology, displacement monitoring data, borehole core and cosmogenic nuclide dating, we discuss the preconditioning and driving factors, kinematic behaviour, and long-term evolution of the instability. We demonstrate that foliation orientation and lithological boundaries, together with slope geometry and alpine faults, likely played significant roles in preconditioning the slope for failure. Based on morphology mapping and hierarchical clustering analysis of 2007–2018 detrended surface displacement data, we show that the landslide is separated into up to eight compartments at surface, revealing differences in kinematic behaviour from very active at the toe to insignificant movements in the upper region of the landslide above the village of Cerentino. These surficial compartments likely correspond to secondary sliding zones within the landslide mass. The analysis of data from inclinometer and fibre optic strain sensors installed in a new deep borehole (230 m) provided insight into movement within the landslide mass. Results show vertical heterogeneity in total displacement, which appears to be controlled by an active discrete shear zone located at 107 m, by shallower secondary sliding surfaces, and by distributed movement, particularly in the upper 20 m of the instability. Extrapolation of cosmogenic nuclide dating results from five samples taken on the exposed upper sliding scar reveals a possible age of failure initiation of 5–6 ka, with long-term decelerating movement trends. These generally slow averaged movements show multi-annual responses to long-term changes in precipitation and locally rapid responses to strong recharge events. The rare combination of dating and detailed analysis of displacement allows us to demonstrate the behaviour of DSGSDs at long and short timescales, with implications for hazard assessment and mitigation of these complex failures. • Integrated approach to deep-seated failure behaviour and geometry, combining geomorphology, engineering geology, and statistics • Analysis of landslide compartments, combining statistical analysis (HCA) and morphological interpretation • Multitemporal analysis of landslide dynamics and evolution, combining cosmogenic dating with 40 years of surface displacement monitoring • Demonstration of importance of preconditioning and driving factors for landslide mitigation planning [ABSTRACT FROM AUTHOR]