Fractured Bedrock Research Articles

First Insight into the Mobilization and Sequestration of Arsenic in a Karstic Soil during Redox Changes.

Karst terrains provide drinking water for about 25% of the people on our planet, particularly in the southwest of China. Pollutants such as arsenic (As) in the soil can infiltrate groundwater through sinkholes and bedrock fractures in karst terrains. Despite this, the underlying mechanisms responsible for As release from karst soils under redox changes remain largely unexplored. Here, we used multiple synchrotron-based spectroscopic analyses to explore As mobilization and sequestration in As-polluted karstic soil under biogeochemical conditions that mimic field-validated redox conditions. We observed that As in the soil exists primarily as As(V), which is mainly associated with Fe(oxyhydr)oxides. The concentration of the dissolved As was high (294 μM) and As(III) was dominant (∼95%) at low Eh (≤-100 mV), indicating the high risk of As leaching under reducing conditions. This As mobilization was attributed to the fact that the dissolution of ferrihydrite and calcite promoted the release and reduction of associated As(V). The concentration of the dissolved As was low (17.0 μM) and As(V) was dominant (∼68%) at high Eh (≥+100 mV), which might be due to the oxidation and/or sequestration of As(III) by the recrystallized ferric phase. Our results showed that the combined effects of the reductive release of As(V) from both ferric and nonferric phases, along with the recrystallization of the ferric phase, govern the redox-induced mobilization and potential leaching of As in soils within karst environments.

Late Cretaceous and Early Paleogene Fluid Circulation and Microbial Activity in Deep Fracture Networks of the Precambrian Basement of Western Greenland

AbstractDeep fracture‐hosted fluids of Precambrian bedrock cratons are relatively stagnant over long time spans compared to near‐surface systems. However, episodic events, such as fracture reactivations, transgressions, and deglaciations, may introduce dilute water, replacing, and mixing with the deep continental brines, thereby sparking microbial activity. Secondary minerals that line bedrock fractures serve as important geochemical archives for such episodic events. Here we explore the fracture mineral record of Archean rocks of Western Greenland by analyzing samples from deep boreholes with the aim to trace and characterize episodic paleofluid flow and paleomicrobial activity. A sequence of hydrothermal to low temperature fluid flow events is demonstrated. For the youngest generation, microscale S‐isotope analysis of pyrite reveals substantial 34S‐depletion (minimum δ34S:−58‰V‐CDT) compared to fracture‐hosted barite (δ34S:13‰ ± 2‰) and gypsum (δ34S:2.6‰–10.6‰). This suggests the formation of pyrite following S isotope fractionation during microbial sulfate reduction. This metabolism is further indicated by several methyl‐branched fatty acids preserved in calcite. A general discrepancy between calcite and groundwater δ18O‐values suggests that calcite formed from water different from the presently residing glacial meltwater‐influenced groundwater mix. High spatial resolution U‐Pb carbonate geochronology of the youngest generation of calcite yielded ages for two samples: 64 ± 3, 75 ± 7 Ma (2σ). These ages overlap with tectonic events related to early stages, or prestages, of the opening of the Atlantic and Labrador Seas. This suggests that deep fracture networks in Western Greenland were colonized by microorganisms, such as sulfate reducers, in the course of this extensional event.

Geologic and Tectonic Controls on Deep Fracturing, Weathering, and Water Flow in the Central California Coast Range

AbstractThe creation of fractures in bedrock dictates water movement through the critical zone, controlling weathering, vadose zone water storage, and groundwater recharge. However, quantifying connections between fracturing, water flow, and chemical weathering remains challenging because of limited access to the deep critical zone. Here we overcome this challenge by coupling measurements from borehole drilling, groundwater monitoring, and seismic refraction surveys in the central California Coast Range. Our results show that the subsurface is highly fractured, which may be driven by the regional geologic and tectonic setting. The pervasively fractured rock facilitates infiltration of meteoric water down to a water table that aligns with oxidation in exhumed rock cores and is coincident with the adjacent intermittent first‐order stream channel. This work highlights the need to incorporate deep water flow and weathering due to pervasive fracturing into models of catchment water balances and critical zone weathering, especially in tectonically active landscapes.

Geological modeling of a tectonically controlled hydrothermal system in the southwestern part of the Pannonian basin (Croatia)

Geothermal energy is an important resource in the green economy transition. For the preservation of a geothermal resource it is crucial to assess its renewability and the sustainability of the exploitation. These aspects are influenced by the interaction among the physical, chemical, geological, and hydrogeological processes. The reconstruction of the geological assemblage allows the detailing of the geometries of the reservoir and fracture systems that influence the fluid flow and the water/rock interaction. The control of regional/local scale fault and fold systems on the development of the Daruvar hydrothermal system (DHS), located in Croatian part of the Pannonian basin, is detailed in this work. Field investigations were conducted to collect structural data on strata orientation and fault/fracture systems. The dataset was integrated with geological and geophysical data to develop composite geological profiles and a 3D geological model. Results display a pattern of generally N-S and E-W striking folds and cogenetic fracture systems with orientations parallel to the fold axes. The geological reconstruction was integrated with geophysical, hydrogeological, and geochemical data to propose a conceptual model of the DHS. The DHS is a topographically driven system hosted in a Mesozoic carbonate reservoir where E-W striking fracture systems are regional flow paths that enable infiltration of meteoric water to 1 km depth and its reheating in its reservoir area. In Daruvar, an anticline and fault/fracture systems accommodate the uplift of reservoir to shallow depths, promoting the bedrock fracturing and increase of the permeability field. These conditions favor the localized upwelling of thermal water resulting in four thermal springs (38°C and 50°C) in Daruvar city area. This work highlights the importance of employing a multidisciplinary approach to detail the complex interaction among the processes driving the geothermal resource.

GEOCRYOLOGICAL CONDITIONS OF THE FORMATION OF GIANT SPRING AUFEIS AT THE ANMANGYNDA RIVER (MAGADAN REGION) ACCORDING TO GEOPHYSICAL DATA

Giant aufeis fields, common in the Northeast of Russia, are the indicators of water exchange processes in cryosphere. The development of ideas about icing processes is relevant both from the fundamental point of view of studying the permafrost evolution, and from the practical point of view – for the development of aufeis hazard measures. The aufeis in the Anmangynda River basin (aufeis glade area 7 km2) is considered representative of the region, and its studies have been carried out since 1962. In 2022, during the period of maximum thawing of the active layer Electrical Resistivity Tomography (ERT) soundings were carried out at the aufeis glade aiming to identify underchannel taliks and flooded fault zones in bedrock, including local areas of groundwater discharge. It was found that within the main river channels there are underchannel taliks up to 30 m deep. According to the results of 2D inversion, local anomalies of low electrical resistivity mark groundwater filtration channels. In 3D geoelectrical models, pipe-like anomalies of low resistivity are identified in the areas of groundwater discharge, interpreted as filtration channels in the alluvium and the zone of exogenous fracturing in bedrock formed by sandy-clay shales, as well as linear vertical anomalies of low resistivity, interpreted as faults. On vertical sections of 3D resistive models, a connection between faults and filtration channels in alluvium and a layer of exogenous fracturing is traced. In the right bank of the valley, geoelectric signs of taliks in the bedrock, presumably associated with fault tectonics, have been established. It is assumed that the identified faults are the additional transit routes for groundwater in the Anmangynda River valley, along with the alluvial aquifer and the zone of exogenous fracturing of bedrock.

Shifting groundwater fluxes in bedrock fractures: Evidence from stream water radon and water isotopes

Geologic features (e.g., fractures and alluvial fans) can play an important role in the locations and volumes of groundwater discharge and degree of groundwater-surface water (GW-SW) interactions. However, the role of these features in controlling GW-SW dynamics and streamflow generation processes are not well constrained. GW-SW interactions and streamflow generation processes are further complicated by variability in precipitation inputs from summer and fall monsoon rains, as well as declines in snowpack and changing melt dynamics driven by warming temperatures. Using high spatial and temporal resolution radon and water stable isotope sampling and a 1D groundwater flux model, we evaluated how groundwater contributions and GW-SW interactions varied along a stream reach impacted by fractures (fractured-zone) and downstream of the fractured hillslope (non-fractured zone) in Coal Creek, a Colorado River headwater stream affected by summer monsoons. During early summer, groundwater contributions from the fractured zone were high, but declined throughout the summer. Groundwater contributions from the non-fractured zone were constant throughout the summer and became proportionally more important later in the summer. We hypothesize that groundwater in the non-fractured zone is dominantly sourced from a high-storage alluvial fan at the base of a tributary that is connected to Coal Creek throughout the summer and provides consistent groundwater influx. Water isotope data revealed that Coal Creek responds quickly to incoming precipitation early in the summer, and summer precipitation becomes more important for streamflow generation later in the summer. We quantified the change in catchment dynamic storage and found it negatively related to stream water isotope values, and positively related to modeled groundwater discharge and the ratio of fractured zone to non-fractured zone groundwater. We interpret these relationships as declining hydrologic connectivity throughout the summer leading to late summer streamflow supported predominantly by shallow flow paths, with variable response to drying from geologic features based on their storage. As groundwater becomes more important for sustaining summer flows, quantifying local geologic controls on groundwater inputs and their response to variable moisture conditions may become critical for accurate predictions of streamflow.

The grain size of sediments delivered to steep debris‐flow prone channels prior to and following wildfire

AbstractDebris flows are powered by sediment supplied from steep hillslopes where soils are often patchy and interrupted by bare‐bedrock cliffs. The role of patchy soils and cliffs in supplying sediment to channels remains unclear, particularly surrounding wildfire disturbances that heighten debris‐flow hazards by increasing sediment supply to channels. Here, we examine how variation in soil cover on hillslopes affects sediment sizes in channels surrounding the 2020 El Dorado wildfire, which burned debris‐flow prone slopes in the San Bernardino Mountains, California. We focus on six headwater catchments (<0.1 km2) where hillslope sources ranged from a continuous soil mantle to 95% bare‐bedrock cliffs. At each site, we measured sediment grain size distributions at the same channel locations before and immediately following the wildfire. We compared results to a mixing model that accounts for three distinct hillslope sediment sources distinguished by local slope thresholds. We find that channel sediment in fully soil‐mantled catchments reflects hillslope soils (D50 = 0.1–0.2 cm) both before and after the wildfire. In steeper catchments with cliffs, channel sediment is consistently coarse prior to fire (D50 = 6–32 cm) and reflects bedrock fracture spacing, despite cliffs representing anywhere from 5% to 95% of the sediment source area. Following the fire, channel sediment size reduces most (5‐ to 20‐fold) in catchments where hillslope sources are predominantly soil covered but with patches of cliffs. The abrupt fining of channel sediment is thought to facilitate postfire debris‐flow initiation, and our results imply that this effect is greatest where bare‐bedrock cliffs are present but not dominant. A patchwork of bare‐bedrock cliffs is common in steeplands where hillslopes respond to channel incision by landsliding. We show how local slope thresholds applied to such terrain aid in estimating sediment supply conditions before two destructive debris flows that eventually nucleated in these study catchments in 2022.

Quantifying seabed geodiversity of the Archipelago Sea, Baltic Sea, Finland

This study investigated the geodiversity of the Archipelago Sea in the northern Baltic Sea, focusing on geological features and their spatial distribution. By adapting methods used in previous Baltic Sea studies, we conducted spatial analyses of geological data sets including bedrock type, seabed substrates and seabed structures. Bedrock and substrate data were freely available, while seabed structures were modelled from bathymetry data. Geodiversity was quantified using a geodiversity index, which considers the variety of physical elements, roughness and area of the unit. The analyses revealed a diverse seabed environment in the Archipelago Sea with varying geodiversity throughout the study area. Significant features contributing to geodiversity included bedrock fracture and fault zones and large end-moraine formations. Similar patterns have been observed in terrestrial areas of Finland. The analyses also detected relations between archipelago zonation and geodiversity with areas of open sea more homogeneous than the middle and inner archipelago. This study formally recognises the complexity of the seabed in the Archipelago Sea and highlights the importance of understanding the geological processes shaping the region. The results can inform maritime spatial planning and sustainable resource management.

2D Near‐Surface Full‐Waveform Tomography Reveals Bedrock Controls on Critical Zone Architecture

AbstractFor decades, seismic imaging methods have been used to study the critical zone, Earth's thin, life‐supporting skin. The vast majority of critical zone seismic studies use traveltime tomography, which poorly resolves heterogeneity at many scales relevant to near‐surface processes, therefore limiting progress in critical zone science. Full‐waveform tomography can overcome this limitation by leveraging more seismic data and enhancing the resolution of geophysical imaging. In this study, we apply 2D full‐waveform tomography to match the phases of observed seismograms and elucidate previously undetected heterogeneity in the critical zone at a well‐studied catchment in the Laramie Range, Wyoming. In contrast to traveltime tomograms from the same data set, our results show variations in depth to bedrock ranging from 5 to 60 m over lateral scales of just tens of meters and image steep low‐velocity anomalies suggesting hydrologic pathways into the deep critical zone. Our results also show that areas with thick fractured bedrock layers correspond to zones of slightly lower velocities in the deep bedrock, while zones of high bedrock velocity correspond to sharp vertical transitions from bedrock to saprolite. By corroborating these findings with borehole imagery, we hypothesize that lateral changes in bedrock fracture density majorly impact critical zone architecture. Borehole data also show that our full‐waveform tomography results agree significantly better with velocity logs than previously published traveltime tomography models. Full‐waveform tomography thus appears unprecedentedly capable of imaging the spatially complex porosity structure crucial to critical zone hydrology and processes.

Imaging freshwater and saline aquifers beneath Bradford County, Pennsylvania, USA, using Audio-Magnetotelluric (AMT) data

We investigate the utility of audio-magnetotelluric (AMT) data for detecting high electrical conductivity zones indicative of fresh and saline groundwater lenses at depths of < ∼1000 m at seven survey sites in the southwestern corner of Bradford County, PA. Dimensionality analysis indicates the AMT data can be interpreted using two-dimensional (2-D) models. 2-D electrical resistivity models obtained from inverting the AMT data reveal conductive zones at depths of <30 m at all sites, between depths of 50–150 m at six sites, and between depths of 400–600 m at one site. The models show slightly lower resistivities (∼15–30 Ω m) in the deeper zones compared to the shallower zones (>30 Ω m). Consistent with well log data from two boreholes in the study area and salinity estimates obtained from the imaged resistivities, we interpret the <30 m deep conductive zone as the freshwater aquifer, and the two deeper conductive zones as salty/briny groundwater lenses. Our salinity estimates for the deeper zones vary from 1 to 380 (unitless Practical Salinity Scale), which agrees well with previous estimates of Appalachian brine salinity of 10–343. We attribute the high-salinity groundwater lenses to Appalachian Basin brine migrating upwards from deeper in the basin via bedrock fracture and fault systems. Our findings indicate AMT surveying can be useful for imaging fresh and saline aquifers at shallow depths (< ∼1000 m), at least within the northeastern section of the Appalachian Basin in Pennsylvania.

Tectonism and fracture-related dissolution induced the formation of the large-scale metamorphic granite reservoir: Implications from Bozhong 26–6 Precambrian bedrock trap, offshore Bohai Bay basin, Northern China

Crystalline rocks in basins are unconventional sites for hydrocarbon exploration. Owing to the lack of primary pores in the bedrock, it remains unclear how later tectonism and dissolution alter crystalline rock within large-scale reservoirs. Notably, the recent discovery of a 200 million-ton oil field of Precambrian metamorphic granite in the Bonan buried hill, Bohai Bay Basin, China provides an important opportunity for exploring this mechanism. Herein, the relationship between fractures and the regional tectonic evolution of this buried metamorphic granite mass, as well as the accompanying phenomena of dissolution and vein filling (which record related fluid interactions contributing to second pore formation from tight crystalline rocks) were investigated. The results indicated that the reservoir space was primarily composed of tectonic fractures and associated dissolution pores (maximum porosity ≤18%). The densely interlaced fractures with dominant NWW-trending flat surfaces indicated that the majority of Precambrian bedrock fractures were induced by Indosinian tectonic compression. The Cenozoic north–south tectonic extensional fault obliquely intersects these NWW-trending fractures, accounting for 73% of the previous dense shearing fractures transformed into open fractures. The dissolution pores were primarily distributed along the fractures and increased the porosity of these fracture-related reservoirs by ≤ 12%. The carbon and oxygen isotopes of the calcite fillings within the fractures suggested that atmospheric freshwater was the primary dissolution fluid, whereas some pores were formed by the dissolution action of deep hydrothermal and organic acid fluids. Thus, regional compressive stress is a necessary tectonic mechanism for the formation of densely interlaced fractures in tight crystalline rocks. The later extensional setting contributing to fracture release and fracture dissolution in the epi-diagenesis stage are two immediate factors in the formation of permeability and storage capacity within buried bedrock traps.

Stiff, Smooth, and Solid? Complex Fracture Hydraulics' Imprint on Oscillatory Hydraulic Testing

AbstractFractured bedrock aquifers, especially deep aquifers, represent increasingly common targets for waste storage and alternative energy development, necessitating detailed quantitative descriptions of fracture hydraulic properties, geometry, and connectivity. Yet, multi‐scale characterization of the physical properties that govern fluid flow through and storage in fractured bedrock remains a fundamental hydrogeologic challenge. Oscillatory hydraulic testing, a novel hydraulic characterization technique, has been showing promise in field experiments to characterize the effective hydraulic properties of bedrock fractures. To date, these characterization efforts utilize simplified diffusive analytical models that conceptualize a non‐deforming, parallel‐plate fracture embedded within impermeable host rock, and have found that the returned fracture hydraulic parameter estimates exhibit an apparent period‐dependence. We conduct synthetic experiments using three different numerical models to examine proposed mechanisms that might contribute to the observed period‐dependence including heterogeneous flow and storage within the fracture (i.e., aperture heterogeneity), fracture‐host rock fluid exchange, and fracture hydromechanics. This work represents the first systematic analysis that seeks to understand the process(es) occurring within a bedrock fracture that might be contributing to this apparent period‐dependence. Our analysis demonstrates that all investigated mechanisms generate period‐dependent effective hydraulic parameter estimates, each with their own potentially diagnostic trends; however, fracture hydromechanics is the only explored mechanism that consistently reproduces period‐dependent trends in parameter estimates that are consistent with existing field investigations. These results highlight the need to develop more complex numerical modeling approaches that account for this hydromechanical behavior when characterizing fractured bedrock aquifers.

Three-dimensional electric-field vector resistivity imaging for deep subsurface fractures network in heterogeneous crystalline rocks

SUMMARY Crystalline rocks, exposed in different parts of the world and over about one-third of India, have complex aquifer systems, which pose a challenge to mapping groundwater dynamics. Electrical resistivity tomography in quasi-3-D and electrical logging of some borewells carried out in an experimental ‘hydrogeological park’ in southern India, which has numerous boreholes and other geophysical information for validation, were unable to map the existence of deeper bedrock fractures and their connectivity. We have attempted electric-field vector resistivity imaging (EVRI), a new tool, to resolve the possible fracture-induced deep interconnectivity in a hard rock aquifer system. In the experiment, multiple two-orthonormal-channels independent receiver nodes for potential measurements are deployed and illuminated with several current injections between ∼ 0.9–3.7 A in full 3-D fashion, which allowed for improved mapping of resistivity variation than earlier approaches. The EVRI-derived full 3-D model shows the presence of fractures for depths between 20 and 70 m with substantial resistivity variations, supported by some borewells hydraulic investigations. It has also enhanced lateral resolution for depths &amp;gt; 30 m and almost doubled the depth of investigation than earlier electrical models. EVRI results revealed unweathered/unfractured granitic rock with no significant signature of fractures beyond 70 m depth that corroborates with existing borehole logs and hydrogeological conceptual model. Therefore, this study demonstrates the potential of EVRI for 3-D mapping of heterogeneous crystalline rocks, which would greatly help in groundwater management.

Periglacial resurfacing of hillslopes and channels with large boulders in the Virginia Appalachians

AbstractLarge, resistant, quartz‐rich boulders deposited on hillslopes and in channels armour the landscape, trap sediment and influence hillslope angle and erodibility. In the Virginia Appalachians, such boulders are a significant component of hillslopes and channels. Establishing the timing of and processes responsible for bedrock fracture and boulder deposition is a critical piece of understanding the landscape as a system. In this study, we use cosmogenic 10Be exposure dating to resolve the timing of boulder deposition at three sites in the Virginia Valley and Ridge province: Gap Mountain, Brush Mountain and Little Stony Creek, and at one site in the Virginia Blue Ridge: Devil's Marbleyard. The correlation between measured boulder exposure ages (101.7 ± 6.9 ka to 10.8 ± 0.8 ka; n = 23) and the Wisconsin Glacial Stage and subsequent Laurentide Ice Sheet (LIS) deglaciation (~115–11.7 ka) suggests a periglacial origin for deposition of large hillslope and channel boulders in the Virginia Appalachians. The lack of boulder exposure ages corresponding to the Last Interglacial Stage or following Wisconsin LIS retreat suggests interglacial non‐deposition and stability. The absence of exposure ages from the penultimate Illinoian or older Quaternary Glacial Stages suggests that periglacial hillslope processes allow the landscape to be resurfaced with large boulders during each return to cold climate conditions. This cyclic resurfacing of hillslopes and channels is an example of how climatic oscillations insert disequilibrium into the landscape cycle and contributes to our appreciation of the timescales over which contemporary climate change may impact boulder dominated landscapes in rapidly warming alpine and arctic environments.

Automated mapping of bedrock-fracture traces from UAV-acquired images using U-Net convolutional neural networks

This contribution presents a novel U-Net convolutional neural network (CNN)-based workflow for automated mapping of bedrock fracture traces from 0.55 cm spatial resolution aerial photographs, acquired by unmanned aerial vehicles (UAV), over the Wiborg Rapakivi granite outcrops in the islands off the coast of the Loviisa Region in Southern Finland. The workflow comprised training a U-Net CNN using a small subset of photographs with manually traced fractures for optimizing the network parameters using the root mean squared propagation optimizer and sigmoidal focal loss function for semantic segmentation of input images and pixel-wise identification of fracture traces. The ridge detection algorithm was then applied to the U-Net prediction results, followed by vectorization of the fracture-traces pixels as vector polylines representing the traces of fractures. Both intensity values of the pixels and topological connectivity were used in the process of vectorization. Quantitatively the results were assessed using various accuracy assessment metrics. Qualitative evaluations of the results were implemented by comparisons of orientations and length-frequency distributions of automatically- and manually-mapped traces.The results show that the model has the class-balanced accuracy score 0.945, predicting 88.79% of the fracture-trace pixels. Bedrock outcrops with well-exposed surfaces present high true positive rates (>99%). The demonstration (test) site has the class-balanced accuracy score of 0.873 and 75% true positive rate. Additionally, the fracture trace networks replicate the orientation distributions of the manually digitized traces. The length distributions of automatic traces, however, differ with varying intensity from the manual trace length distributions. However, this study demonstrates that the workflow can be successfully applied to UAV-acquired images for fast and efficient automated mapping of bedrock-fracture traces during the initial phases of structural characterization of a region.

Streamflow Composition and Water “Imbalance” in the Northern Himalayas

AbstractThe Yarlung Zangbo River (YZR) is the largest river in the northern Himalayas, providing crucial water resources for downstream. A full understanding of the streamflow dynamics and regional water budget is critical to secure water security of the Himalayan water tower. Here we establish a comprehensive hydrological model to simulate the precipitation‐runoff‐evapotranspiration‐groundwater‐streamflow complex in the YZR basin. We decipher contributions of different water sources (e.g., precipitation, meltwater, groundwater) to YZR's streamflow and estimate that groundwater sustains ∼36% of annual streamflow in the YZR, while precipitation and melt surface runoff contribute 40% and 24%, respectively. Combining modeling, observation and reanalysis data, our results reveal a water “imbalance” that ∼31% of annual precipitation and meltwater (∼333 mm yr−1 or ∼85 km3 yr−1) is unaccounted for in the YZR basin. We propose that the “excess water” discharges to deep fractured bedrock aquifers, which is promoted by widespread permeable active structures (e.g., faults, fractures). This hypothesis is supported by groundwater storage (GWS) estimates where inclusion of the deep groundwater bridges the discrepancy between baseflow‐derived (shallow) GWS and those derived from the Gravity Recovery and Climate Experiment satellite data. The deep groundwater most likely flows across basins, bypasses streams, and finally discharges to downstream aquifers in the Indo‐Gangetic Plain as mountain block recharge. This study not only provides a comprehensive analysis of the streamflow composition in the YZR, but also contributes to shaping a more complete picture of the functionality of the Himalayan water tower, highlighting the importance of groundwater in regional water transfers.

Formation mechanism and quantitative risk analysis of the landslide-induced hazard chain by an integrated approach for emergency management: A case study in the Bailong River basin, China

The landslide–debris flow–river blockage hazard chain is one of the most common types of geological hazard chain in mountainous areas, seriously threatening human life and property across an extensive geographical area. The development of approaches for the rapid investigation and prediction of multi-hazard chains can play a significant role in reducing the number of casualties and economic losses. Here, taking the Lijie landslide–debris flow–river blockage in the Bailong River basin as a case study, we used remote sensing methods, geophysical technology, numerical simulations and empirical formulas to develop an integrated approach for analyzing the mechanisms, multi-hazard prediction and quantitative risk assessment induced by this geological hazard chain. The reactivation process of the Lijie landslide is composite retrogressive sliding. The steep slopes and highly fractured bedrock with low strength are the principal internal factors, and heavy rainfall in 2020 and spring freeze–thaw effect are the principal triggering factors. The deposited landslide material is eroded and transformed into a channel-type debris flow under high rainfall conditions. The steep terrain and abundant fine-grained material provides favorable conditions for a viscous debris flow with a long runout, which is usually followed by secondary disasters associated with river blockage. Our results indicate that under rainfall conditions with a 100-year return period, the Bailong river will be completely blocked by the landslide-induced debris flow. And there will be an estimated 5.84 million CNY of economic losses caused by the multi-hazard chain, which is 2.6 times the losses directly caused by the landslide. Our integrated approach is practicable in an emergency context and useful for local administrators to rapidly develop strategies for multi-hazard chain prevention and mitigation, based on information about the transformation mechanisms of the multi-hazard chain, potential hazard-affected areas, damaged degree of elements at the risk, and incurred economic losses.

Spatial variation of surface erosion rate in a fault zone and its controlling factors

Topographic relief and bedrock fractures caused by fault activity can accelerate surface erosion, suggesting that erosion rates increase with decreasing distance to the fault core. To test this hypothesis, we used 10Be cosmogenic nuclides to quantify catchment-wide erosion rates at various distances from a fault in the footwall of the Daqing Shan fault system, a 200-km-long active normal fault in Inner Mongolia, northern China. In addition, in order to examine the influence of topography and bedrock fractures on the surface erosion processes in the fault-weakened zone, their relationships with hillslope erosion, fluvial sediment grain size and transport efficiency, erosion rates, and erosion coefficients were studied. The resulting catchment-wide erosion rates ranged from 0.01 ± 0.00–0.13 ± 0.01 mm yr−1. These erosion rates and coefficients were unaffected by distance from the fault. The analysis of topography, bedrock fractures, and hillslope erosion yielded distinct hillslope erosion processes at different distances from the fault. Rapid mass wasting events, such as landslides, occurred at <15 km from the fault in steep, rocky hillslopes with abundant bedrock fractures. In contrast, at >15 km from the fault, hillslopes gradually become more gentle and soil-mantled. These spatial-slope configuration differences result in a decrease in sediment grain size delivered to river channels with increasing distance from the fault. Once sediments enter river channels, they impact fluvial transport capacity by influencing the initial motion threshold of sediment. Correlation analyses revealed that sediment transport efficiency in channels directly controls the catchment-averaged erosion rate and coefficient. This mechanism explains why the erosion rate remains insensitive to variations in topography and bedrock fractures in fault-weakened zones.

Detailed investigation of multi-scale fracture networks in glacially abraded crystalline bedrock at Åland Islands, Finland

Abstract. Using multiple scales of observation in studying the fractures of the bedrock increases the reliability and representativeness of the respective studies. This is because the discontinuities, i.e. the fractures, in the bedrock lack any characteristic length and instead occur within a large range of scales of approximately 10 orders of magnitude. Consequently, fracture models need to be constructed based on representative multi-scale datasets. In this paper, we combine a detailed bedrock fracture study from an extensive bedrock outcrop area with lineament interpretation using light detection and ranging (lidar) and geophysical data. Our study offers lineament data in an intermediary length range (100–500 m) missing from discrete fracture network modelling conducted at Olkiluoto, a nuclear spent-fuel facility in Finland. Our analysis provides insights into multi-scale length distributions of lineaments and fractures and into the effect of glaciations on lineament and fracture data. A common power-law model was fit to the lineament and fracture lengths with an exponent of −1.13. However, the fractures and lineaments might follow distinct power laws or other statistical distributions rather than a common one. When categorising data by orientation, we can highlight differences in length distributions possibly related to glaciations. Our analysis further includes the topological, scale-independent fracture network characteristics. For example, we noticed a trend of decreasing apparent connectivity of fracture networks as the scale of observation increases.

RQD modeling using statistical-assisted SRT with compensated ERT methods: Correlations between borehole-based and SRT-based RMQ models

In complex tropical granitic terrains, foundation rocks’ bearing strength is important in infrastructure design, while bedrock fractures that adversely impact engineering structures can boost groundwater productivity. Consequently, rock quality designation (RQD) can characterize these features effectively. This technique is difficult to determine in boreholes and relatively costly for a large area. As a result, novel approaches were employed to effectively determine RQD over a large aerial extent using statistically optimized borehole and seismic P-wave velocity (Vp) data with compensated resistivity modeling. This study was carried out in the typical granitic terrain of Penang Island, Malaysia. The first stage involves evaluating and developing borehole-based RQD models, and the results were correlated with Vp data and were regressively analyzed to develop lithology-based empirical relations. These empirical relations were used for developing 2D/3D SRT (seismic refraction tomography) based RQD models and were treated as novel models. Integrating the borehole-based and SRT-based RQD models with compensated resistivity models effectively delineate the surficial-to-subsurface soil-rock profiles based on their rock mass quality (RMQ) and conditions, as residual soil, completely weathered rock, relatively weathered rock, and integral/fresh granitic bedrock, including different directional and multiple axial fractures. Based on these results, the highlighted suitable sections for founding the foundations of infrastructure are pinned on the fresh bedrock at the depths of 8–25 m, with >2400 m/s, >2000 Ω m, and RQD >90%. The deep-weathered/fractured zones, with depth >35 m at localized sections, are water-bearing proposed for groundwater development. Overall, the methods and lithology-based empirical relationships can be adopted in granitic terrains to rapidly estimate RQD for a vast area with few and no borehole data.