Sinkhole Collapse Research Articles

Basic processes and factors determining the evolution of collapse sinkholes – A sensitivity study

Collapse sinkholes are closed depressions appearing at the surface as a consequence of a complex interplay between dissolution, erosion and mechanical stability in karst aquifers. Sinkholes are difficult to study and their locations are almost impossible to predict because of the multiple parameters involved and complex mechanisms controlling their evolution and collapse. With this work we present a systematic theoretical research focused on the processes responsible for the development of collapse sinkholes. Splitting the evolution time in different stages we propose a conceptual model allowing decoupling of the main processes and a sensitivity study on the controlling parameters. The conceptual framework is implemented with the help of a high resolution 2D karst evolution model and a simplified 3D DEM mechanical model.Our results demonstrate that the development of a collapse sinkhole is controlled by dissolution throughout most of its evolution, while the mechanical collapses occur during short time intervals. The karstification is extremely sensitive to variations in the hydraulic conductivity or the amount of water flowing through the aquifer. Finally, we discuss the hydro-geological conditions, which allow or not for systematic sensitivity studies.

A preliminary sinkhole raveling chart

Sinkholes are a major geohazard in karst areas where soluble carbonate bedrock exists. Downward seepage resulting from the groundwater recharge continuously erode cover soils into cavities within the underlying bedrock, resulting in raveling of cover soils. Identifying these raveled soils (or zones) is critial in preventing and mitigating the sinkhole collapse. In this study, a proof-of-concept was performed to demonstrate the concept of a sinkhole raveling chart that can quantify the severity levels of raveling. The raveling chart is constructed based on Cone Penetration Test (CPT) data collected through multiple sinkhole active sites in Central Florida. A preliminary analysis (e.g. data preparation, grouping, and zoning) was carried out to construct the prototype sinkhole raveling chart. This prototype raveling chart, as a function of tip resistance (qc) and sleeve friction (fs), could delineate not only the raveled and non-raveled zones but also the areas that correspond to different levels of raveling severity.

Closed depressions in Kotido crater, Arabia Terra, Mars. Possible evidence of evaporite dissolution-induced subsidence

The identification of karst sinkholes in Mars may provide evidence of dissolution processes caused by liquid water and information on paleoclimatic and paleohydrological conditions. This work presents a comprehensive cartographic inventory of 513 closed depressions developed on evaporite-bearing Equatorial Layered Deposits (ELDs) within Kotido crater, Arabia Terra. Detailed mapping, morphometric analyses and spatial distribution relationships reveal a number of features supporting that the depressions correspond to collapse sinkholes related to evaporite dissolution: (1) suitable topographic and litho-structural conditions for the development of a fracture-controlled epigene evaporite karst; (2) presence of open fissures at the foot of the scarped margins; (3) dimensions and frequency-size distributions comparable with those reported on Earth; (4) spatial association with high-permeability zones (i.e., fractures). Some characteristics of the depressions indicate that they have been re-shaped and enlarged by wind erosion: (1) dominant orientation consistent with the prevalent one-directional winds; (2) differing morphological characteristics on the downwind- and upwind-sides; and (3) nested depressions associated with the upwind sector. The relatively fresh appearance of the depressions and the lack of impact craters suggest a poorly constrained Amazonian karstification phase in the region.

Sinkhole occurrence monitoring over shallow abandoned coal mines with satellite-based persistent scatterer interferometry

Unexpectedly occurring sinkholes caused by shallow submerged voids are one of the key problems of intensively built-up post-mining areas. Five percent of Poland’s territory is still under the influence of post-mining deformation. Impending sinkholes, which tend to develop randomly over very large areas, cannot be detected by using traditional geophysical methods. Moreover, geodetic measurement methods like Global Positioning System (GPS) analyses, tachymetry, laser scanning, or photogrammetry may be useful only for the registration of the dimensions and locations of sinkholes that have already occurred. Here, we investigate an area in Upper Silesia, Poland, where 345 sinkholes were recorded over a period of more than 20 years (1992–2013). Most of the events occurred in intensively built-up areas, and thus, these sinkholes posed a direct threat to the population. In the test area, 11 sinkholes were detected with a maximum depth of 16 m and maximum dimension of 25 m. The root cause of sinkhole formation was the collapse of post-mining shallow voids. We used satellite radar technology to detect and monitor ground movements potentially associated with the impending sinkholes. The findings showed that the application of Persistent Scatterer Interferometry can support the identification of zones where sinkholes will occur, given adequate spatio-temporal sampling. A stack of Envisat Synthetic Aperture Radar (SAR) images acquired between March 2003 and August 2010 was used in the analysis, and the results confirmed that precursory ground movements were detectable at an early stage of sinkhole development. The land subsidence rate observed prior to sinkhole collapse was not constant in time. Accelerated ground movements within 100 m of an observed sinkhole were detected. We conclude that satellite measurements may provide significant support in the early identification of areas prone to sinkhole occurrence.

Cover collapse sinkhole over a deep buried carbonate bedrock: The case study of Fossa San Vito (Sarno - Southern Italy)

Sinkholes are widely found in Italy in both carbonate slopes and alluvial plain settings. Since they may occur catastrophically, without showing premonitory signs and resulting in severe economic losses and casualties, they constitute a significant risk in many areas, and even pose a threat to human life. In this paper, we carried out a detailed multidisciplinary study of the Fossa San Vito sinkhole (southern Italy) that is a particular type of cover collapse sinkhole. The latter is a sub-circular, closed depression, with a diameter of ~200 m and with inner walls up to 25 m high, located in the piedmont area placed along the inner border of the Sarno River alluvial plain. The study entailed morphological and stratigraphic analysis by means of field surveys, including nine trenches, four boreholes, and the reinterpretation of stratigraphical data from the literature. In addition, hydrogeological and soil gas analysis were carried out to investigate the possible role of deep-water uprising. The study attempted to identify the genetic mechanism and include the reconstruction of a detailed subsurface stratigraphic model, showing that the infilling of the depression is about 45 m thick and consists of well-laminated, silty-clay lacustrine sediments interbedded with tephra layers. 14C dating of fossil wood found at the base of the lacustrine deposits constrains the age of the collapse at 5483 cal. years B.P. The collapse of the topographic surface involved a volume of about 400,000 cubic metres and originated at depth >80 m.

Subsidence mechanisms and sedimentation in alluvial sinkholes inferred from trenching and ground penetrating radar (GPR). Implications for subsidence and flooding hazard assessment

Sinkholes function as small sediment traps that may host valuable stratigraphic records for paleoenvironmental reconstructions and hazard assessments (e.g., subsidence, floods, hurricanes, tsunamis, tephra fall-out). The sedimentological features and completeness of such archives are influenced by the sedimentation and subsidence patterns and rates. However, karst depressions are frequently treated as static basins unaffected by settlement. This work illustrates the practicality of integrated studies combining trenching, numerical dating and shallow geophysical techniques (GPR) for characterizing the subsurface subsidence structure associated with sinkholes and reconstructing their deformational and sedimentary history. The approach is applied in two collapse sinkholes located in contrasting geomorphic settings (relict terrace and floodplain) related to deep-seated interstratal karstification of evaporites. The analysis of the sinkholes, particularly the trenching technique, provides practical information for assessing the associated subsidence hazard, including the presence of larger cavities at depth, the kinematic regime (episodic versus progressive), evidence of catastrophic displacement (fluidization structures) and the magnitude and timing of collapse events, especially the most recent one. The sinkhole located in the floodplain offers the opportunity of analyzing the possibilities and limitations of subsidence sinkholes as recorders of past floods in alluvial environments. This depression shows a largely incomplete record attributable to the high frequency of flood events compared to that of the collapse events, which create the accommodation space for sediment deposition. These limitations could be partially overcome by selecting old sinkholes situated in low terraces and/or affected by rapid subsidence.

Distinct element geomechanical modelling of the formation of sinkhole clusters within large-scale karstic depressions

Abstract. The 2-D distinct element method (DEM) code (PFC2D_V5) is used here to simulate the evolution of subsidence-related karst landforms, such as single and clustered sinkholes, and associated larger-scale depressions. Subsurface material in the DEM model is removed progressively to produce an array of cavities; this simulates a network of subsurface groundwater conduits growing by chemical/mechanical erosion. The growth of the cavity array is coupled mechanically to the gravitationally loaded surroundings, such that cavities can grow also in part by material failure at their margins, which in the limit can produce individual collapse sinkholes. Two end-member growth scenarios of the cavity array and their impact on surface subsidence were examined in the models: (1) cavity growth at the same depth level and growth rate; (2) cavity growth at progressively deepening levels with varying growth rates. These growth scenarios are characterised by differing stress patterns across the cavity array and its overburden, which are in turn an important factor for the formation of sinkholes and uvala-like depressions. For growth scenario (1), a stable compression arch is established around the entire cavity array, hindering sinkhole collapse into individual cavities and favouring block-wise, relatively even subsidence across the whole cavity array. In contrast, for growth scenario (2), the stress system is more heterogeneous, such that local stress concentrations exist around individual cavities, leading to stress interactions and local wall/overburden fractures. Consequently, sinkhole collapses occur in individual cavities, which results in uneven, differential subsidence within a larger-scale depression. Depending on material properties of the cavity-hosting material and the overburden, the larger-scale depression forms either by sinkhole coalescence or by widespread subsidence linked geometrically to the entire cavity array. The results from models with growth scenario (2) are in close agreement with surface morphological and subsurface geophysical observations from an evaporite karst area on the eastern shore of the Dead Sea.

Remotely sensing large- and small-scale ground subsidence: A case study of the Guangdong–Hong Kong–Macao Greater Bay Area of China

As the world's largest city cluster, the Guangdong–Hong Kong–Macao Greater Bay Area (GBA) is vulnerable to significant subsidence. The widely distributed sediments and rapid urbanization in the GBA result in the concurrence of large- and small-scale subsidence. Mono-sensor synthetic aperture radar (SAR) images usually suffer from the limitation of either low resolution or small coverage, and thus, are not applicable to completely monitoring the GBA. In this study, we used Sentinel-1 (S1), COSMO-SkyMed (CSK) and TerraSAR-X (TSX) images jointly to reveal multi-scale subsidence of the GBA. The overall subsidence ranged from 0 to 112.3 mm/yr derived from the 2015–2017 S1 images. Three regional subsidence bowls (Zhuhai–Zhongshan, Jiangmen, and Guangzhou–Zhongshan) formed in the western alluvial plain. The high correlation between regional subsidence and Quaternary sediments confirms that sediment consolidation is the main cause of subsidence. The land use and numerical modeling results suggest that groundwater extraction and artificial loading are the triggering factors. Two representative local subsidence cases were analyzed using high-resolution images: reclamation settlement at the Hong Kong International Airport (HKIA) and sinkhole subsidence due to the excavation of the Shenzhen Wenbo skyscraper foundation pit. The validation at the HKIA showed that the measurements from the CSK and S1 data both agreed well with the leveling data. However, CSK outperformed S1 in the sense that it increased the point density by a factor of 5, improved the height precision by a factor of 4, and showed fewer false alarms. CSK is therefore more applicable to monitoring the local subsidence of key infrastructures. The cross-heading tracks of TSX and CSK images detected precursory subsidence before the sinkhole collapse from two sides, indicating that the cross-heading tracks benefit the comprehensive monitoring of local subsidence in high-rise and high-density built environments. In summary, the synergistic use of multi-sensor SAR images demonstrates the practicability of the operational surveillance of multi-scale subsidence from regional surveying to the fine monitoring of local areas in the GBA.

Time-lapse gravity and levelling surveys reveal mass loss and ongoing subsidence in the urban subrosion-prone area of Bad Frankenhausen, Germany

Abstract. We present results of sophisticated, high-precision time-lapse gravity monitoring that was conducted over 4 years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Mainly in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes, occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore focus on the associated deformation processes. In this study, we combine levelling and highly precise time-lapse gravity observations. Our investigation area is the urban area of Bad Frankenhausen in central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravimetry and 18 levelling campaigns were carried out in quarterly intervals within a local monitoring network. This network covers historical sinkhole areas but also areas that are considered to be stable. Our results reveal ongoing subsidence of up to 30.4 mm a−1 locally, with distinct spatiotemporal variations. Furthermore, we observe a significant time-variable gravity decrease on the order of 8 µGal over 4 years at several measurement points. In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the Global Land Data Assimilation System (GLDAS) Noah model to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity monitoring in urban areas for sinkhole investigations. Although the observed rates of gravity decrease of 1–2 µGal a−1 are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks.

Identification of gypsum karstification using an electrical resistivity tomography technique: The case-study of the Sivas gypsum karst area (Turkey)

The Sivas district is the most important gypsum karstified area in Turkey. The oldest evaporites in the region were formed during the sedimentation process of the Oligocene period. Generally, these units extend along the southern part of the Sivas basin. After the Early Miocene Sea drained from this region, extensive evaporite deposition developed. An electrical resistivity tomography (ERT) study was carried out between the Hafik and İmranlı districts of the Sivas gypsum karstified area. In this research, the prior aim was to determine the general characterization, geometry, sedimentary cover and resistivity properties of gypsums. In addition, the study includes the synthetic modelling of different gypsum karstification types. The evaluation and interpretation were based on ERT data that was collected from the Hafik-İmranlı region. First, the different karstifications were identified, together with the characteristic features of gypsum in the area. The ERT studies showed that there are large and old karst structures between the Hafik and Zara district. However, the karst structures that are located between the Zara and İmranlı regions were interpreted as smaller and younger. The characteristics of both of the regions are different in terms of the morphometric properties. It is certain that the Kızılırmak River has an important impact on the formation of these structures. ERT studies have shown that the cover- and bedrock-type collapse sinkholes are very spread out across the area, and their geometries are different. Throughout time, the suffosion type of cavities formed inside the cover-type sinkholes as a consequence of geological and geomorphological events. The ERT sections showed that certain parts of these structures collapsed, and then alluvial materials filled in. The shallow groundwater level in the drilling results and the conductive zone in the ERT results are compatible with each other. ERT studies clearly determined the various karstification typologies of gypsums in the region, which also contributed to the creation of a detailed conceptual geological model.

Evolution of sinkholes over Wink, Texas, observed by high-resolution optical and SAR imagery

Two caprock and cover collapse sinkholes, Wink Sinks #1 and #2, in Winkler County, Texas, collapsed in 1980 and 2002, respectively. They are located in a region underlain by the Salado Formation on the eastern edge of the Delaware Basin. The sinkhole collapses were associated with intense hydrocarbon drilling and production activities in the Hendrick oilfield. Production records and prior investigations imply the sinkholes developed after salt dissolution due to freshwater intrusion, cavity formation, roof failure, and successive upward cavity migration. High-resolution (1 m) aerial photography shows the ensuing expansion of Wink Sink #2 from 2002 through 2010 and relative stability of the ground surface after 2010. Very high-resolution (25 cm) staring spotlight mode TerraSAR-X reveals the spatiotemporal details of Wink sinkhole deformations. The vicinities around the two existing sinkholes (Wink Sinks #1, #2) deformed at a rate of 5 cm/yr in line-of-sight (LOS) direction from October 2015 to March 2016, and the deformation was possibly caused by the gradual deposit of the debris from overlying unconsolidated Alluvium and Santa Rosa sandstones infilling the cavity after the sinkhole collapse. Specifically, the largest deformation of 53 cm/yr in LOS direction was found 1 km east of Wink Sink #2. Our study suggests that severe drought in 2011 widened and created fractures as well as ground fissures in the rocks overlying and underlying the salt formation due to the increased effective stress. Fractures, ground fissures, and the abandoned unplugged boreholes or improperly completed wells opened one or more vertical pathways of freshwater from groundwater flow and precipitation toward the salt bed, finally causing the rapid salt dissolution and the opening of new cavities. To explain the onset of a rapid subsidence near the Wink sinkholes, our study considers the combined effect of natural perturbation (i.e., drought, rock fractures, ground fissures, subsurface drainage, precipitation) and anthropogenic effects (i.e., improper well control). Our method to explore high-resolution imagery can be applied to characterize the sinkhole deformation and mitigate sinkhole-related geohazards.

Mechanism of sinkhole formation during groundwater-level recovery in karst mining area, Dachengqiao, Hunan province, China

The Meitanba Coal Mine area in Hunan province, China, had been impacted by severe cover collapse sinkholes since 1982 due to mine dewatering. After the coal mine was closed in February 2015, the groundwater level has increased significantly. A series of sinkholes were recorded in the study area during groundwater-level recovery. Analysis of monitoring results and in-situ investigation indicated that 13 sinkhole collapses were more likely induced by abrupt change of groundwater–air pressure in response to heavy rainfall from March 2015 to July 2016 when the groundwater level increased by as much as 76 m. When the karst conduit was flooded, a relatively sealed environment was formed between saturated sediments and flooded karst conduit. Implosion of entrapped air might have caused the cave roof to collapse followed by surface collapses in a short time. On the other hand, four sinkholes occurred in November 2016 when the groundwater levels were near the soil–bedrock interface at elevations between 52.5 and 58.9 m amsl and the groundwater-level increase was at slower paces. Field measurements indicate that the groundwater-level fluctuation at the soil–bedrock interface could enlarge the soil cavity and accelerate the subsoil erosion process.

Geomechanical modelling of sinkhole development using distinct elements: model verification for a single void space and application to the Dead Sea area

Abstract. Mechanical and/or chemical removal of material from the subsurface may generate large subsurface cavities, the destabilisation of which can lead to ground collapse and the formation of sinkholes. Numerical simulation of the interaction of cavity growth, host material deformation and overburden collapse is desirable to better understand the sinkhole hazard but is a challenging task due to the involved high strains and material discontinuities. Here, we present 2-D distinct element method numerical simulations of cavity growth and sinkhole development. Firstly, we simulate cavity formation by quasi-static, stepwise removal of material in a single growing zone of an arbitrary geometry and depth. We benchmark this approach against analytical and boundary element method models of a deep void space in a linear elastic material. Secondly, we explore the effects of properties of different uniform materials on cavity stability and sinkhole development. We perform simulated biaxial tests to calibrate macroscopic geotechnical parameters of three model materials representative of those in which sinkholes develop at the Dead Sea shoreline: mud, alluvium and salt. We show that weak materials do not support large cavities, leading to gradual sagging or suffusion-style subsidence. Strong materials support quasi-stable to stable cavities, the overburdens of which may fail suddenly in a caprock or bedrock collapse style. Thirdly, we examine the consequences of layered arrangements of weak and strong materials. We find that these are more susceptible to sinkhole collapse than uniform materials not only due to a lower integrated strength of the overburden but also due to an inhibition of stabilising stress arching. Finally, we compare our model sinkhole geometries to observations at the Ghor Al-Haditha sinkhole site in Jordan. Sinkhole depth ∕ diameter ratios of 0.15 in mud, 0.37 in alluvium and 0.33 in salt are reproduced successfully in the calibrated model materials. The model results suggest that the observed distribution of sinkhole depth ∕ diameter values in each material type may partly reflect sinkhole growth trends.

Deep seated fault‐related volcanogenic H2S as the key agent of high sinkhole concentration areas

AbstractKarst collapse sinkholes (KCS) are a peculiar karst morphology resulting from the collapse of the topographic surface caused by subsurface karstification. In the Southern Apennines these phenomena are not randomly distributed but concentrate in several zones, named High Sinkhole Concentration Areas (HSCA), showing peculiar geological, structural and hydrogeological conditions. Gas vents and mineral springs occur widely in these HSCA, and are often stricty related to the KCS. Starting from four representative areas, the aim of this study is to ascertain if there is a genetic link between peculiar mineralogical vs geochemical features of densely fractured/altered rocky masses in KCS, gas vents/springs occurrences and diffuse hypogenic karstification. By means of a multidisciplinary approach (geological/geomorphological, mineralogical and geochemical) we selected and analysed four different kinds of samples related to KCS, gas vent and spring occurrences: (i) altered to deeply altered limestones sampled in the sinkholes; (ii) unaltered limestones sampled in close proximity to sinkhole areas; (iii) gypsum‐rich crusts/patinae, precipitated near to gas vents; (iv) gypsum‐ and calcite‐rich precipitates occurring at springs or inside caves. Among neoformed non‐carbonate minerals, gypsum is virtually ubiquitous, halides (fluorite, halite, sylvite) also occur in small to trace amounts. The mineralogical assemblages of the different samples show similarities and may also be compatible with hypogenic speleogenesis and with a process of alteration of the carbonate bedrock by means of uprising mineralizing fluids along structural discontinuities. Stable isotopic compositions (S, O) display strong variability in δ34S and δ18O for sulfate in the different areas, but a deep‐seated sulfur source can be hypothesized for many of the studied KCS‐related samples. This study has important implications for the relationships between areas of high concentration of sinkholes, regional fault systems, mechanical characteristics of rocks and the high seismicity typical of these areas of the Southern Apennines. © 2018 John Wiley & Sons, Ltd.

THREE‐DIMENSIONAL SEISMIC GEOMORPHOLOGY OF PALEOKARST IN THE CRETACEOUS MACAÉ GROUP CARBONATES, CAMPOS BASIN, BRAZIL

Three‐dimensional seismic data from an oilfield in the Campos Basin, SE Brazil, was used to characterize a karstified unconformity surface at the top of the Albian – Cenomanian Macaé Group. Macaé Group carbonates underwent intense karstification associated with subaerial exposure during a period of some 10 to 15 Ma resulting in the development of canyons, valleys, sinkholes and cave systems. The carbonates host commercial oil accumulations at a number of important oil fields. Understanding how the karstification process has affected the carbonates' reservoir properties is essential for future exploration, not only to improve recovery rates but also to avoid drilling‐related issues such as thief zones. In this context, this study aims to characterize the top‐Macaé Group paleokarst system by investigating the morphology of the associated reflectors recorded on seismic data, together with endokarst features in the underlying carbonate succession such as cave systems and collapse structures. The top‐Macaé Group seismic horizon can be divided into two principal geomorphological domains: highlands, characterized by abrupt relief with well‐developed erosional features; and lowlands, marked by a smoother topography. Collapse sinkholes occur in both domains and take the form of closed circular depressions. The study of endokarst features from an analysis of amplitude anomalies (bright spots) indicates the presence of heterogeneous cave systems. The interpreted data contributes to a better understanding of the spatial distribution of the paleokarst system in the Macaé Group carbonates and may assist with future hydrocarbon exploration in the Campos Basin.

Shear wave reflection seismic yields subsurface dissolution and subrosion patterns: application to the Ghor Al-Haditha sinkhole site, Dead Sea, Jordan

Abstract. Near-surface geophysical imaging of alluvial fan settings is a challenging task but crucial for understating geological processes in such settings. The alluvial fan of Ghor Al-Haditha at the southeast shore of the Dead Sea is strongly affected by localized subsidence and destructive sinkhole collapses, with a significantly increasing sinkhole formation rate since ca. 1983. A similar increase is observed also on the western shore of the Dead Sea, in correlation with an ongoing decline in the Dead Sea level. Since different structural models of the upper 50 m of the alluvial fan and varying hypothetical sinkhole processes have been suggested for the Ghor Al-Haditha area in the past, this study aimed to clarify the subsurface characteristics responsible for sinkhole development.For this purpose, high-frequency shear wave reflection vibratory seismic surveys were carried out in the Ghor Al-Haditha area along several crossing and parallel profiles with a total length of 1.8 and 2.1 km in 2013 and 2014, respectively. The sedimentary architecture of the alluvial fan at Ghor Al-Haditha is resolved down to a depth of nearly 200 m at a high resolution and is calibrated with the stratigraphic profiles of two boreholes located inside the survey area.The most surprising result of the survey is the absence of evidence of a thick (> 2–10 m) compacted salt layer formerly suggested to lie at ca. 35–40 m depth. Instead, seismic reflection amplitudes and velocities image with good continuity a complex interlocking of alluvial fan deposits and lacustrine sediments of the Dead Sea between 0 and 200 m depth. Furthermore, the underground section of areas affected by sinkholes is characterized by highly scattering wave fields and reduced seismic interval velocities. We propose that the Dead Sea mud layers, which comprise distributed inclusions or lenses of evaporitic chloride, sulfate, and carbonate minerals as well as clay silicates, become increasingly exposed to unsaturated water as the sea level declines and are consequently destabilized and mobilized by both dissolution and physical erosion in the subsurface. This new interpretation of the underlying cause of sinkhole development is supported by surface observations in nearby channel systems. Overall, this study shows that shear wave seismic reflection technique is a promising method for enhanced near-surface imaging in such challenging alluvial fan settings.

Growth of a sinkhole in a seismic zone of the northern Apennines (Italy)

Abstract. Sinkhole collapse is a major hazard causing substantial social and economic losses. However, the surface deformations and sinkhole evolution are rarely recorded, as these sites are known mainly after a collapse, making the assessment of sinkhole-related hazard challenging. Furthermore, more than 40 % of the sinkholes of Italy are in seismically hazardous zones; it remains unclear whether seismicity may trigger sinkhole collapse. Here we use a multidisciplinary data set of InSAR, surface mapping and historical records of sinkhole activity to show that the Prà di Lama lake is a long-lived sinkhole that was formed in an active fault zone and grew through several events of unrest characterized by episodic subsidence and lake-level changes. Moreover, InSAR shows that continuous aseismic subsidence at rates of up to 7.1 mm yr−1 occurred during 2003–2008, between events of unrest. Earthquakes on the major faults near the sinkhole do not trigger sinkhole activity but low-magnitude earthquakes at 4–12 km depth occurred during sinkhole unrest in 1996 and 2016. We interpret our observations as evidence of seismic creep at depth causing fracturing and ultimately leading to the formation and growth of the Prà di Lama sinkhole.

Fate and transport of radioactive gypsum stack water entering the Floridan aquifer due to a sinkhole collapse

Groundwater aquifers are an essential source of drinking water, and must be protected against contamination. Phosphogypsum stacks originating from the processing of phosphate rock contain small amounts of radionuclides, such as 226Ra. In September 2016, a sinkhole located beneath a phosphogypsum stack collapsed under central Florida’s carbonate karst terrain, where the aquifer is mostly confined, raising concern over water quality in the regions nearby. Monitoring and modeling the transport of the contaminated plume is vital to ensure drinking water criteria are met and to improve decision making regarding treatment. To achieve this, a geochemical modeling using PHREEQC software was employed to investigate the trajectory of the plume based on hydraulic and hydrologic conditions. Adsorption was simulated as a removal mechanism that could further reduce the intensity of the plume. The aquifer’s response to the release of contaminated water from the collapsed stack was quantified by simulating a number of scenarios, including variable radionuclide leakage quantities. Results suggest that it may take between 11–17 years and between 5.2 to 8.3 km from the sinkhole leak to reduce radionuclide concentrations to previous levels. Coupling the adsorption effect by minerals in Floridan aquifer (e.g. ferrihydrite, carbonate) can reduce radionuclide migration time to 9–16 years and distances between 4.3 to 7.8 km from the sinkhole leak. It can also reduce the distance needed to lower radionuclide concentrations, though not significantly. Additionally, due to the complexities of soil chemistry, the importance of groundwater remediation is emphasized.

InSAR Reveals Land Deformation at Guangzhou and Foshan, China between 2011 and 2017 with COSMO-SkyMed Data

Subsidence from groundwater extraction and underground tunnel excavation has been known for more than a decade in Guangzhou and Foshan, but past studies have only monitored the subsidence patterns as far as 2011 using InSAR. In this study, the deformation occurring during the most recent time-period between 2011 and 2017 has been measured using COSMO-SkyMed (CSK) to understand if changes in temporal and spatial patterns of subsidence rates occurred. Using InSAR time-series analysis (TS-InSAR), we found that significant surface displacement rates occurred in the study area varying from −35 mm/year (subsidence) to 10 mm/year (uplift). The 2011–2017 TS-InSAR results were compared to two separate TS-InSAR analyses (2011–2013, and 2013–2017). Our CSK TS-InSAR results are in broad agreement with previous ENVISAT results and levelling data, strengthening our conclusion that localised subsidence phenomena occurs at different locations in Guangzhou and Foshan. A comparison between temporal and spatial patterns of deformations from our TS-InSAR measurements and different land use types in Guangzhou shows that there is no clear relationship between them. Many local scale deformation zones have been identified related to different phenomena. The majority of deformations is related to excessive groundwater extraction for agricultural and industrial purposes but subsidence in areas of subway construction also occurred. Furthermore, a detailed analysis on the sinkhole collapse in early 2018 has been conducted, suggesting that surface loading may be a controlling factor of the subsidence, especially along the road and highway. Roads and highways with similar subsidence phenomenon are identified. Continuous monitoring of the deforming areas identified by our analysis is important to measure the magnitude and spatial pattern of the evolving deformations in order to minimise the risk and hazards of land subsidence.

Mapping and modelling of collapse sinkholes in soluble rock: The Münsterdorf site, northern Germany

Münsterdorf is a small village in the north of Hamburg, located along the northern rim of a salt diapir. The Permian rocks are uplifted and overlying rocks such as the Cretaceous limestone, normally at several kilometers depth, have been pushed close to the surface. In Münsterdorf, the Cretaceous limestone can be found at around 20 m depth, and about 2 km further south, Cretaceous limestones are quarried in a large open-pit mine.Since 2004, collapse sinkholes have formed on a sports field in Münsterdorf, with a frequency of about 1 every two years, about 3–5 m in diameter and 3–5 m deep. The collapse sinkholes do not reach the underlying limestone, but seem to be related to accelerated dissolution in that formation. Above the Cretaceous limestone, Quaternary gravels and glacial tills provide a non-soluble, but permeable and heterogeneous cover of about 20 m thickness.We have mapped the sports field and its vicinity with gravity (GRAV), electrical resistivity imaging (ERI), and ground-penetrating radar (GPR). The ERI profiles indicate a significant change in the surface layer, from thin and irregular in the northern part to thicker and more homogeneous in the southern part of the sports field. GPR profiles confirm this result.We numerically model the evolution of flow and porosity in the Cretaceous limestone to estimate the evolution time of subsurface voids in the limestone, and we discuss the potential cause of the sinkhole formation and its sudden onset in light of the hydraulic boundary conditions.