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Seasonal freeze–thaw cycles pose major challenges to underground infrastructures in cold regions, where rock fractures exert strong control over subsurface fluid flow, heat transfer, and ice formation. However, the role of fracture–matrix interactions during freezing and thawing remains poorly understood. In this study, we develop a coupled thermo–hydraulic (TH) model with phase change and local thermal non-equilibrium (LTNE), formulated within a finite element framework, to investigate these processes in fractured porous media. The model is based on the discrete fracture network approach, where fractures are represented as lower-dimensional entities embedded within a permeable porous matrix. It can capture ice–water phase change, LTNE heat exchange between fracture fluid and the surrounding rock matrix, as well as latent heat effects and phase-dependent permeability in both fracture and matrix domains. Model verification is conducted against four benchmark problems, addressing fracture–matrix heat exchange under LTNE conditions, ice–water phase change under freezing and melting, and lower-dimensional fracture representation implemented in the proposed TH modeling framework. The model is further applied to problems of increasing complexity, from a single fracture to intersecting fractures and ultimately to a fracture network, to systematically elucidate the roles of freeze–thaw processes in fracture-scale permeability evolution and network-scale flow reorganization. Our results reveal a self-reinforcing feedback mechanism in which freezing blocks secondary pathways, focusing flow into major channels, which in turn suppress further freezing and sustain preferential flow. These findings have important implications for many hydrological, geophysical, and geotechnical applications in cold regions.

Rock elasticity varies with both humidity and water saturation, yet their combined effects remain poorly understood, although in nature vapor adsorption and liquid infiltration occur simultaneously. Here, we present experimental data of P-wave velocity and volume expansion in a free-standing sandstone subject to progressive wetting. Elastic softening, evidenced by P-wave velocity reduction, precedes the wetting front, followed by stiffening as liquid infiltration reverses this trend. To reconcile these softening/stiffening behaviors, vapor migration ahead of the wetting front is captured by numerical simulation of moisture transport constrained by experimental data. Initial softening is explained by a micromechanical model governed by surface energy reduction at grain contacts and validated by independent vapor adsorption tests. Subsequent stiffening is attributed to water infiltration, consistent with patchy saturation theory. We propose softening and stiffening are transitional processes governed by the advancing wetting front, with implications for seismic imaging of progressive wetting processes in crustal rocks.

Fault-slip rockbursts, triggered by seismic rupture of nearby or remote faults, constitute a significant geohazard during deep underground excavations. Although these events occur frequently in underground projects, their underlying mechanisms are not yet fully understood. Most studies tacitly assume dynamic stress waves as the main triggering factor, often disregarding the role of coseismic static stress changes associated with fault slip. This paper introduces a novel analytical framework to diagnose both static and dynamic coseismic stress perturbations and quantify their contributions to fault-slip rockburst around a circular tunnel. Building on linear elastic fracture mechanics, seismic source theory, and the Kirsch solution, the model assesses whether coseismically elevated maximum tangential stress on the tunnel boundary under static and dynamic triggering effects is sufficient to induce failure around the tunnel. We extensively test our framework using synthetic case studies that represent typical fault-slip rockburst scenarios. Our results yield a rockburst hazard map that delineates regions of elevated triggering potential in the near-field and far-field of the seismogenic fault, and classify the triggering types as static, dynamic, or dual. We perform a comprehensive parametric sensitivity analysis to investigate how key factors, including seismic source characteristics, rock mass properties, and in-situ stress conditions, influence the spatial distribution of rockburst susceptibility. The model is further applied to a historical fault-slip rockburst event at the Gotthard Base Tunnel, effectively capturing the triggering mechanism of the observed failure. Our research provides a physically grounded and computationally efficient analytical framework with the results carrying significant implications for rockburst hazard assessments during deep underground excavations.

Study region

Shentou springs, China, are one of the typical karst springs in northern China.

Study focus This study focuses on Shentou springs discharge prediction based on the multi-scale precipitation. A framework called MCT that is composed of Multi-scale Convolutional network and Transformer is proposed. First, an adaptive denoising method for precipitation data is put forward. Second, a multi-scale convolutional network is designed to extract multi-temporal scale features of precipitation. Then, the Transformer is used to explore the spatial relationship between spring discharge and precipitation. Finally, the output of the coupled model is used to predict spring discharge.

New hydrological insights for the region

Results show that (1) denoising precipitation data can improve the accuracy of spring discharge prediction. (2) Precipitation and spring discharge exhibit obvious multi-temporal scale relationships, precipitation typically affects spring discharge after a lag of 6 months, and precipitation has a temporally persistent influence on spring discharge. (3) Precipitation in areas closer to the spring have an intense impact on spring discharge, while areas far from the spring but located in topographic depressions have volume-driven influence on spring discharge.

In their recent contribution, Erharter and Elmo (Rock Mech Rock Eng, 2025) proposed to resolve the long-standing continuum vs. discontinuum question in rock engineering by characterising rock mass complexity. While their attempt to introduce quantitative metrics into this debate is commendable, their treatment tends to apply the concepts of complexity and emergence in ways that do not align with their established meanings. In this Discussion, I address these issues by providing several points of clarification, particularly concerning how the authors have positioned their work in relation to complexity science and to my earlier article (Lei, Rock Mech Bull, 2023).

Fluid injection-induced fault activation and seismicity pose significant risks to the integrity of CO2 geological sequestration projects. This study develops a computational model integrating two-phase fluid flow and geomechanics to investigate the influence of secondary fractures on fault activation and induced seismicity. The frictional strength variation of the fault and fractures is captured by a slip weakening model that incorporates healing mechanisms. A dynamic time-marching scheme is implemented to efficiently capture both slow aseismic slips and rapid seismic ruptures in a densely faulted/fractured reservoir, enabling detailed analysis of energy release during long-term CO2 injection. Our results indicate that secondary fractures facilitate early-stage pressure dissipation, delaying fault slip and reducing seismic events. However, at later stages, secondary fractures contribute to increased seismicity, characterized by larger magnitudes and more extensive rupture zones. Critical pressure analysis reveals that seismicity propagates ahead of the fluid-pressurized zone, indicating that stress transfer plays a key role in triggering induced seismicity. Furthermore, we document a novel mechanism of seismic slip cascades developing in the fracture population, where fracture interactions boosted by stress transfer and aseismic deformation activate a series of fracture clusters to rupture in a bursting manner, promoting the spatial migration of seismic events beyond the fluid pressurization front. These findings provide new insights into the mechanisms of injection-induced seismicity, with far-reaching implications for CO2 sequestration in fractured geological media.

Weathering is a fundamental driver of landslide evolution over geological timescales. Despite its ubiquity and importance, quantifying how weathering drives the progressive destabilization of rock slopes remains challenging. In this work, we develop a unified computational framework based on the particle finite element method to investigate the evolution of weathering-induced landslides, from long-term weathering to short-term slope failure and runout dynamics. The framework integrates key processes, including weathering front propagation, time-dependent strength degradation, rupture surface development, and post-failure runout dynamics. Through numerical simulation experiments, we elucidate how interactions among weathering characteristics (type, intensity, and rate law), bedrock strength, fracture distribution, and slope geometry govern the failure modes and kinematics of weathering-induced landslides. Simulations show that matrix-dominated weathering leads to shallow translational failures, whereas fracture-dominated weathering produces deep-seated rotational and compound landslides. Pre-existing fractures and slope morphology also strongly influence the movement of destabilized landmasses, affecting the failure pattern (e.g., kinematic mode and rupture surface geometry) and post-failure behavior (e.g., runout velocity). We further demonstrate that the failure time and volume of weathered slopes are governed by the competition between gravitational driving forces and cohesive resisting forces during progressive destabilization. These findings provide new insights into the fundamental mechanisms that drive the emergence of diverse failure patterns of weathering-induced landslides with important implications for landslide hazard assessment.

We develop a discrete event modelling framework that captures the progression of geophysical systems toward catastrophic failure through sequences of distinct damage events. By representing geophysical system evolution as a succession of temporally accelerating and amplitude-varying events, the framework reveals how finite-time singularities, both logarithmic and power-law types, naturally emerge from the interplay between shrinking interevent intervals and growing event magnitudes. This event-based perspective, which can be viewed as a discrete representation of progressive damage accumulation in heterogeneous geomaterials, provides an intuitive physical understanding of rupture processes, highlighting how precursory signals such as accelerating strain rate, event frequency and energy release can be traced back to simple underlying mechanisms. A mean-field damage-based formulation further links the observed power-law exponents to the evolving stiffness of the geophysical system under constant or time-varying stress. Incorporating stochastic fluctuations, the model captures the inherent randomness of natural systems leading to the emergence of stochastic finite-time singular behaviour. Together, these results establish a simple yet powerful framework for interpreting the dynamics of catastrophic events, providing a common event-based perspective on observations from landslides, glacier breakoffs, volcanic eruptions and other related processes, and strengthening the physical foundations of early warning and hazard forecasting.

Upscaling unsaturated flow in fractured rock remains challenging because fractures and matrix often exhibit sharply contrasting hydraulic behaviors across saturation states. Here, we demonstrate that unsaturated flow undergoes a transition between matrix- and fracture-dominated regimes. Three-dimensional direct numerical simulations reveal that this bimodal retention behavior emerges naturally from saturation-dependent partitioning of flow between fractures and matrix. We analytically derive a generalized retention formulation that identifies a critical saturation marking the transition between the two distinct retention regimes and reproduces the bimodal behavior captured in the numerical simulations. An analytical expression for the critical pressure head is derived to represent the limiting case of fully connected fracture networks, providing a physically based transition criterion and showing good agreement with the numerical results for systems above the percolation threshold. Our results provide a mechanistic framework for understanding and upscaling unsaturated flow in fractured rock, with broad implications for hydrology and geophysics.

Fluid injection into fractured rocks can trigger seismicity through hydro-mechanical interactions. We perform 2D fully coupled hydro-mechanical simulations, based on the finite element method and the discrete fracture network approach, to quantify how fracture network geometry controls pressure diffusion, damage growth, fracture slip, and induced seismicity. Two parameters are used to constrain the generation of fracture networks: the power law length exponent a and the fracture intensity γ. Together they set the connectivity, measured by the percolation parameter χ. The generated networks span the disconnected (χ < χ_c) to well-connected (χ > χ_c) regimes, where the percolation threshold χ_c ≈ 6 for 2D fracture networks. Our results reveal that well-connected systems accommodate coherent pressure fronts, broad damage halos, and a radially migrating seismic band; disconnected systems produce intermittent pressure pockets, localized damage, and clustered seismic events. Fracture length-magnitude scaling is steeper for sparse networks, where slip localizes along individual long fractures, whereas dense networks partition slip across multiple fractures. Radial pressure profiles reveal a rapid decay of mean pressure elevation near the injection well, but persistent peak perturbations at fracture cluster boundaries. These peaks coincide with Coulomb failure stress hotspots and delayed seismic events. Fracture network connectivity governs how far induced seismicity extends from the injection point and how it is partitioned between matrix damage and fracture slip. These findings link fracture network structures to the spatiotemporal evolution of induced seismicity, with implications for enhanced geothermal systems and other injection related subsurface applications.