Slow-creeping landslides may fail catastrophically, posing significant threats to infrastructure and lives. Landslides weaken over time through rock mass damage processes that may occur by slow steady-state creep or transient accelerations of slip, called creep bursts. Creep bursts may control landslide stability by inducing short-term damage and strain localization. This study focuses on the Åknes landslide in Norway, which moves up to 6 centimetres per year and could potentially trigger a large tsunami in the fjord lying below. Here, an eleven-year dataset is compiled and analyzed, including kinematic, seismic, and hydrogeological data acquired at the landslide surface and in a series of boreholes. Creep bursts with millimetre amplitude are detected in the landslide’s shear zone. An annual average of two creep burst events have been recorded within the shear zone in each borehole, accounting for approximately 11% of the total displacement. Creep bursts phased over multiple boreholes are preceded by increased seismic activity and water pressure increase. However, most creep bursts are observed in only one or a few boreholes. Creep bursts often occur during the seasonal high and low levels of groundwater, correlating with local peaks in water pressure, but no such correlation is observed during summer. We propose that on one side, the progressive wear of asperities leads to creep bursts being uncorrelated to water pressure changes. Conversely, enhanced stress corrosion causes creep bursts to correlate to water level fluctuations. Our findings offer unique insights into landslide mechanics, correlating shear zone dynamics with surface displacement and environmental parameters.

Advances in micro-scale imaging techniques, such as X-ray microtomography, have provided new insights into a broad range of porous media processes. However, direct imaging of flow and transport processes remains challenging due to spatial and temporal resolution limitations. Here, we investigate the use of dynamic three-dimensional neutron imaging to image flow and transport in Bentheim sandstone core samples before and after in-situ calcium carbonate precipitation. First, we demonstrate the applicability of neutron imaging to quantify the solute dispersion along the interface between heavy water and a cadmium aqueous solution. Then, we monitor the flow of heavy water within two Bentheim sandstone core samples before and after a step of in-situ mineral precipitation. The precipitation of calcium carbonate is induced by reactive mixing of two solutions containing CaCl2 and Na2CO3, either by injecting these two fluids one after each other (sequential experiment) or by injecting them in parallel (co-flow experiment). We use the contrast in neutron attenuation from time-lapse tomograms to derive three-dimensional fluid velocity field by using an inversion technique based on the advection-dispersion equation. Results show mineral precipitation induces a wider distribution of local flow velocities and leads to alterations in the main flow pathways. The flow distribution appears to be independent of the initial distribution in the sequential experiment, while in the co-flow experiment, we observed that higher initial local fluid velocities tended to increase slightly following precipitation. These findings suggest that neutron imaging is a promising technique to investigate dynamics processes in porous media.

The spatial organization of deformation may provide key information about the timing of catastrophic failure in the brittle regime. In an ideal homogenous system, deformation may continually localize toward macroscopic failure, and so increasing localization unambiguously signals approaching failure. However, recent analyses demonstrate that deformation, including low magnitude seismicity, and fractures and strain in triaxial compression experiments, experience temporary phases of delocalization superposed on an overall trend of localization toward large failure events. To constrain the conditions that promote delocalization, we perform a series of X-ray tomography experiments at varying confining stresses (5-20 MPa) and fluid pressures (zero to 10 MPa) on Westerly granite cores with varying amounts of preexisting damage. We track the spatial distribution of the strain events with the highest magnitudes of the population within a given time step. The results show that larger confining stress promotes more dilation, and promotes greater localization of the high strain events approaching macroscopic failure. In contrast, greater amounts of preexisting damage promote delocalization. Importantly, the dilative strain experiences more systematic localization than the shear strain, and so may provide more reliable information about the timing of catastrophic failure than the shear strain.

The question “what arrests an earthquake rupture?” sits at the heart of any potential prediction of earthquake magnitude. Here, we use a one-dimensional, thin-elastic-strip, minimal model, to illuminate the basic physical parameters that control the arrest of large ruptures. The generic formulation of the model allows for wrapping various earthquake arrest scenarios into the variations of two dimensionless variables $\bar \tau_k$ (initial pre-stress on the fault) and $\bar d_c$ (fracture energy), valid for both in-plane and antiplane shear loading. Our continuum model is equivalent to the standard Burridge-Knopoff model, with an added characteristic length scale, $H$, that corresponds to either the thickness of the damage zone for strike-slip faults or to the thickness of the downward moving plate for subduction settings. We simulate the propagation and arrest of frictional ruptures and derive closed-form expressions to predict rupture arrest under different conditions. Our generic model illuminates the different energy budget that mediates crack- and pulse-like rupture propagation and arrest. It provides additional predictions such as generic stable pulse-like rupture solutions, stress drop independence of the rupture size, the existence of back-propagating fronts, and predicts that asymmetric slip profiles arise under certain pre-stress conditions. These diverse features occur also in natural earthquakes, and the fact that they can all be predicted by a single minimal framework is encouraging and pave the way for future developments of this model.

The creation of new fractures controls fault slip, produces rock damage, and contributes to the dissipation of energy during earthquakes. Pulverized rocks around faults contain grains with a wide range of sizes, but the mechanisms that produces nanoscale grains remain elusive. Using molecular dynamics simulations, we model tensile rupture propagation in alpha-quartz under conditions of stress that occur during earthquake propagation. Our results show that for rupture speeds below 15 % of the Rayleigh wave speed, the fracture propagates straight. At higher speeds, fracture propagation undergoes path instabilities with crack oscillations and microbranching. We show that microbranching can lead to nanoscale fragments. Energy dissipation occurs by the creation of fracture surfaces and material damage; the dissipated energy increases with rupture speed. This nanoscale mechanism of irreversible deformation during earthquake propagation conditions contributes to the energy budget of earthquakes, and damage production in fault zones.

Approximating the three-dimensional structure of a fault network at depth in the subsurface is key for robust estimates of fluid flow. However, only observations of two-dimensional outcrops are often available. To shed light on the relationship between two- and three-dimensional measurements of fracture networks, we examine data from a unique set of eleven X-ray synchrotron triaxial compression experiments that reveal the evolving three-dimensional fracture network throughout loading. Using machine learning, we derive relationships between the two- and three-dimensional measurements of three properties that control fluid flow: the porosity, and volume and tortuosity of the largest fracture at a particular differential stress step. The models predict the porosity and volume of the largest fracture with R2 scores of >0.99, but predict the tortuosity with maximum R2 scores of 0.68. To test the assumption that different rock types may require different equations between the two- and three-dimensional properties, we develop models for both individual rock types (granite, monzonite, marble, sandstone) and all of the experiments. Models developed using all of the experiments perform better than models developed for individual rock types, suggesting fundamental similarities between fracture networks in rocks often analyzed separately. Models developed with several parallel two-dimensional observations perform similarly to models developed with several perpendicular two-dimensional observations. When the models are developed with statistics of the two-dimensional observations, the models primarily depend on the mean and median when they predict the porosity, and minimum when they predict the volume and tortuosity.