Zuer Li
1 and
Qihang Li
School of Marxism, Fujian Normal University, Fuzhou 350108, ChinaState Key Laboratory of Coal Mine Disasters Dynamics and Control, Chongqing University, Chongqing 400044, China
Author to whom correspondence should be addressed. Sustainability 2024, 16(15), 6490; https://doi.org/10.3390/su16156490Submission received: 13 June 2024 / Revised: 19 July 2024 / Accepted: 26 July 2024 / Published: 29 July 2024
(This article belongs to the Special Issue Remote Sensing in Geologic Hazards and Risk Assessment)To proactively respond to the national fourteenth Five-Year Plan policy, we will adhere to a comprehensive land and sea planning approach, working together to promote marine ecological protection, optimize geological space, and integrate the marine economy. This paper provides a comprehensive review of the sustainable development of marine geological hazards (MGHs), with a particular focus on submarine landslides, the marine environment, as well as the marine economy. First, the novelty of this study lies in its review and summary of the temporal and spatial distribution, systematic classification, inducible factors, and realistic characteristics of submarine landslides to enrich the theoretical concept. Moreover, the costs, risks, and impacts on the marine environment and economy of submarine engineering activities such as oil and gas fields, as well as metal ores, were systematically discussed. Combined with the current marine policy, an analysis was conducted on the environmental pollution and economic losses caused by submarine landslides. Herein, the key finding is that China and Mexico are viable candidates for the future large-scale offshore exploitation of oil, gas, nickel, cobalt, cuprum, manganese, and other mineral resources. Compared to land-based mining, deep-sea mining offers superior economic and environmental advantages. Finally, it is suggested that physical model tests and numerical simulation techniques are effective means for investigating the triggering mechanism of submarine landslides, their evolutionary movement process, and the impact on the submarine infrastructure. In the future, the establishment of a multi-level and multi-dimensional monitoring chain for submarine landslide disasters, as well as joint risk assessment, prediction, and early warning systems, can effectively mitigate the occurrence of submarine landslide disasters and promote the sustainable development of the marine environment and economy.
The ocean harbors abundant marine mineral, chemical, and biological resources, presenting immense potential for development. By means of exploring, exploiting, and applying marine geology technology, the full utilization of these resources can be achieved [1,2,3]. Nevertheless, a deficient understanding of marine geology or the disturbance to its equilibrium state will inevitably result in the occurrence of marine geological hazards (MGHs) [4]. For instance, in 1977, a submarine landslide occurred in the port of Gioia Tauro, Italy, involving an approximate volume of 5.5 × 10 6 m 3 . This event caused severe damage to the port infrastructure and resulted in significant economic losses [5]. In 1979, a landslide occurred in Nice, France, resulting in the tragic death of seven workers and causing water to surge inland by approximately 150 m [6]. In 1994, an oil spill occurred offshore at a distance of 10 nautical miles from the coast of the United Arab Emirates (UAE), resulting in a black oil sheen that extended for 40 km and causing severe pollution to the region’s water quality and environment [7]. The Solwara 1 cuprum and gold mine project in Papua New Guinea (PNG), which involved drilling, blasting, and excavation activities in 2014, resulted in significant marine pollution and severe damage to the living environment of marine organisms [8]. To conclude, the occurrence of MGHs is inevitably accompanied by economic losses and environmental degradation [9,10]. Moreover, during the 14th Five-Year Plan Period (2021–2025), China aims to proactively expand the development space of its marine economy through strategic initiatives [11]. In other words, we will adhere to the land and marine comprehensive planning, and jointly promote the construction of marine ecological protection, geological spatial optimization, as well as the integration of marine economic development. Accordingly, examining the correlation between MGHs and the marine economy is crucial for achieving the sustainable development of the marine ecological environment.
We investigated the direct financial losses (DFLs) and the number of dead (including missing) persons (DPs) resulting from MGHs in China between 2010 and 2022 (see Figure 1a). The investigation data indicates that the mean number of DFLs and DPs amounts to CNY 8.715 billion and 52 individuals, respectively. Among them, all exceeded the average between 2010–2013, possibly attributable to the rapid development of the marine and tourism industries that resulted in significant human and property losses [12,13,14]. Subsequently, the DFLs caused by MGHs have been decreasing year by year as science and technology continue to advance [15]. Nevertheless, in 2019, storm surges (11 cases), sea waves (39 cases), as well as red tides (38 cases) were the primary causes of MGHs, resulting in a DFL totaling CNY 11.703 billion [16,17,18]. Herein, the DFLs resulting from the Typhoon Lekima storm surge were particularly severe in many coastal provinces [19]. Finally, the period of 2020–2022 witnessed a significant and stable decline in both DFLs and DPs, which could be attributed to the impact of the COVID-19 pandemic that resulted in industrial stagnation and social isolation [20,21,22]. The main factors contributing to MGHs can be categorized into four causal aspects: (1) caused by strong atmospheric disturbances, such as typhoons and huge waves [23]; (2) caused by a disturbance or sudden change in the state of the sea, such as storm surges and sea ice [24]; (3) caused by submarine earthquakes, submarine or island volcanic eruptions, submarine collapses, landslides, ground cracks, and other lithosphere movements, such as tsunami disasters [25]; and (4) caused by human activities, such as red tides, marine pollution, etc. [26]. Therefore, the primary manifestations of MGHs are succinctly summarized in Figure 1b. MGHs are the result of submarine geological processes, which can trigger a series of secondary disasters and create a chain reaction. The illustration in Figure 1c depicts the evolution of submarine landforms and typical MGHs. Due to the diverse geological backgrounds and environmental conditions across different sea areas, the locations, causes, and hazards of MGHs are complex. MGHs can be classified into two types, sudden and delayed, based on various factors such as the characteristics of disaster evolution, disaster-causing factors, scope of impact, and degree of harm [27]. The occurrence of sudden-marine geological hazards (S-MGHs), such as submarine landslides, turbidity currents, shallow gas eruptions, active faults, earthquakes, and tsunamis, is characterized by a low frequency and harmful effects that can manifest within days, hours, or even minutes or seconds [28]. On the contrary, delayed-marine geological hazards (D-MGHs) primarily encompass erosions and sedimentations, seabed deformations and settlements, as well as active sand slopes and seawater intrusions. These phenomena demonstrate gradual changes over an extended period of time, spanning several years or even longer [29].
MGHs have the potential to trigger a cascade of destructive phenomena, exhibiting a chain-like structure and an amplification effect [30,31,32]. A typical chain structure cons-ists of earthquake-triggered undersea landslides, which result in tsunamis and turbidity currents that pose a threat to coastal cities [33]. In addition, turbidity currents have the potential to cause damage to undersea pipelines [34]. Nevertheless, mounting evidence suggests that many tsunamis associated with earthquakes are not triggered by the seis-mic activity itself, but rather by undersea landslides caused by the earthquakes [35,36]. The chain structure of MGHs can be triggered by natural factors or exacerbated by human activities, as exemplified by the 2011 Fukushima nuclear accident in Japan (earthquake/tsunami/nuclear leakage) [37]. The Fukushima nuclear power plant in Japan was struck by a 14–15 m high tsunami generated by the magnitude 9.0 earthquake on 11 March, resulting in the failure of emergency generators [38]. The subsequent nuclear meltdown, hydrogen explosion, as well as radioactive contamination caused economic damage totaling JPY 5.8 trillion [39]. The impact of a tsunami triggered by submarine landslides is severe and incalculable [40,41]. The Grand Banks earthquake on 18 November 1929 triggered a submarine landslide of 20 km 2 , resulting in the loss of 27 lives and the deposition of approximately 200 km 3 of debris into deep water. Subsequently, this gravity flow caused the transatlantic telegraph cable to be severed [42]. On 26 December 2004, a tsunami (accompanied by an earthquake) caused by a submarine landslide hit the Indian Ocean, causing 292,000 casualties and an estimated economic damage of more than EUR 10 billion [43]. Therefore, it is imperative to conduct a comprehensive review and forward-looking analysis of the causes, effects, and remedies for submarine land-slide disasters.
A submarine landslide is a type of mass transport deposit (MTD) that occurs at the boundary between the continental shelf and slope, resulting in damage to shallow geological structures and significant impacts on deep-sea oil and gas hydrate drilling, as well as engineering activities [44,45,46]. It can be categorized into two types: shallow-sea landslides and deep-sea landslides. Typically, submarine landslides with water depths exceeding 1000 m are classified as deep-sea landslides [47]. Deep-sea landslides are a significant process in the migration of seafloor sediments, transporting clay and sand from the shelf slope break zone to the continental slope and deep-sea basin [48,49]. This includes various transformations such as creep, slide, slump, clastic flow, and turbidity currents. Compared to shallow-sea landslides, deep-sea landslides are characterized by a low slope angle (less than 2°), high velocity (35 m/s), long sliding distance (up to hundreds of kilometers) and large sliding volume (up to hundreds of millions of cubic meters). This makes them one of the most harmful deep-sea geological disasters, and they play an important role in the evolution of submarine canyons [50,51,52]. At present, the primary research methods for deep-sea landslides are focused on geological surveys, physical explorations, laboratory testings, numerical simulations, in situ long-term observations and monitoring, as well as early warning technologies [53,54]. In China, the study of geological hazards caused by deep-sea landslides began in the 1980s [55]. Since then, significant breakthroughs have been achieved in identifying, classifying, and determining influencing factors, as well as monitoring/warning about submarine landslide elements (Table 1). Nevertheless, these studies in Table 1 still have some shortcomings, and no systematic analysis has been conducted on the formation mechanism, type, distribution characteristics, scale, and scope of submarine landslides. Moreover, over the past decade, research on submarine landslides in China has primarily focused on laboratory tests and numerical simulations, with few studies conducted on the environmental degradation and economic evaluation resulting from submarine landslides.
Accordingly, to prevent submarine landslide disasters and achieve the sustainable development of the marine environment and economy, it is necessary to take appropriate measures. Herein, (1) the distribution, classification, triggering mechanisms, and characteristics of submarine landslides are thoroughly discussed; (2) the environmental pollution and economic losses caused by submarine landslides are systematically analyzed; and (3) the current theories and technologies applied to submarine landslides are comprehensively introduced. In this study, the key finding is that China and Mexico are viable candidates for the future large-scale offshore exploitation of oil, gas, nickel, cobalt, cuprum, manganese, and other mineral resources. Compared to land-based mining, deep-sea mining offers superior economic and environmental advantages. In addition, it provides a comprehensive answer on how to achieve sustainable development among submarine landslide disasters of the marine environments and economies, while also considering the prospects for the future based on current trends.
The oceanic region constitutes approximately 71% of the earth’s total area, while coastal areas concentrate around 60% of the global population within two-thirds of large and medium-sized cities [82]. Submarine landslides, being highly destructive MGHs, can result in immeasurable losses to both the economic development of coastal regions and the safety of people’s lives and property. A previous study has identified areas with a high susceptibility to submarine landslides [80]. The construction of Figure 2 involves integrating global submarine landslide locations and frequencies, along with their associated tsunami occurrences. As depicted in Figure 2, submarine landslide hotspots are primarily concentrated in the Pacific, Atlantic, and Indian Oceans. Notably, submarine landslides occur more frequently in the Atlantic Ocean, where secondary disasters such as tsunamis are more prominent. Submarine landslides are primarily concentrated in the northern region of Brazil and eastern Venezuela, as well as the eastern, western, and southern regions of the United States. Additionally, they occur in the western areas of Gabon and Congo; the western areas of the Western Sahara and Morocco; Norway’s western region; the northwestern region of the Mediterranean Sea; southeastern India; most waters surrounding Japan; and China’s southeastern region.
Submarine landslides, as a type of sediment displacement along slopes, represent one of the most significant geological processes in sediment migration [86,87]. The classification of these types is of immense importance in refining the fundamental theory behind submarine landslides and accurately understanding the laws governing the movement instability of submarine rock and soil masses. Figure 3 represents the classification of submarine landslides in different periods. In the classification shown in Figure 3a, submarine landslides are categorized into various types of submarine sediment flows [88]. Nevertheless, due to the continuous movement and gradual disintegration of debris, most submarine landslides ultimately transform into sedimentary flows. As depicted in Figure 3b, this model categorizes submarine landslides into five fundamental types: sliding, tipping, spreading, falling, and flowing [89]. While it essentially encompasses the observed submarine landslide classifications, it does not fully capture the interplay between these types. Masson et al. posit that sliding, debris caving, debris flow, and turbidity currents are the primary modes of failure [90]. Among these, sliding, debris flow, and turbidity currents serve as the principal gravitational driving forces for sediment migration down the slope (Figure 3c). Moscardelli and Wood classified Mass Transport Complexes (MTCs) as MTC and turbidity currents. They further divided MTCs into sliding, slumping, and clastic flows, as illustrated in Figure 3d. The authors also described the main features and seismic identification markers of these complexes [91]. Shanmugam et al. highlighted that landslides encompass all forms of mass-transport deposits (MTDs), including sliding, collapse, debris flow, tipping, creep, and debris avalanche [92]. Since 2013, although some scholars still maintain that submarine landslides and sediment gravity flows are distinct phenomena [93], the prevailing view has shifted towards a more generalized concept of submarine landslides [70,94,95]. On the contrary, with regards to the classification of submarine landslide calculations, Feng et al. categorized submarine landslide types based on factors such as the volume and thickness of the landslide, as well as the relationship between the strata and sliding surface [96]. This classification is presented in Table 2. Overall, the narrow definition of submarine landslides refers to the rapid sliding process of unconsolidated soft sediment or rock with weak structural planes along a slope under the influence of gravity. This includes both translational and rotational landslides. The broad concept of submarine landslides encompasses various sediment transport processes, including creep, collapse, and gravity flow (such as clastic flow, particle flow, liquefaction flow, and turbidity currents).
The instability mechanism of submarine slopes is more intricate than that of terrestrial slopes [97,98]. Submarine landslides are typically triggered by the interaction of internal seabed processes (such as earthquakes, active faults, gas hydrate decomposition, magma volcanoes, mud volcanoes, etc.) and external factors (including storm waves, tides, human activities, tsunamis, sea level fluctuations, etc.) [99]. In previous studies, Prior and Coleman have elucidated the correlation between multiple triggers and submarine landslides (as shown in Figure 4) [100]. Hance conducted a statistical analysis of 534 submarine landslides, revealing that 366 of them were triggered by multiple factors instead of a single specific trigger [101]. As shown in Figure 5, the top three triggers of landslides, in addition to the unidentified circumstance of a submarine landslide disaster (20.50%), include earthquakes and active fault activity (26.82%), rapid deposition (15.61%), and gas hydrate decomposition (7.39%). For deep-sea landslides occurring at depths greater than 1000 m, earthquake and active fault activity, as well as gas hydrate decomposition, are considered the two primary causative factors.
Earthquake and active fault activity: Active fault activity can increase the dip angle of the submarine slope body and transfer energy from the underlying bedrock to the seabed surface sediments, which not only amplifies the shear force of the slope body, but also diminishes the strength of the soil mass due to vibration liquefaction. Mean-while, the active fault serves as a crucial conduit for natural gas migration, enabling deep-seated gases to ascend along the active fault plane and facilitate the development of potential slip surfaces [102]. On the other hand, while submarine earthquakes can directly induce slope instability, they also have the potential to trigger tsunamis that exacerbate such instability. In the northern part of the South China Sea, a multitude of large active faults have developed at the base of the Baiyun seabed landslide, extending vertically for thousands of meters. The seismic reflection characteristics of the strata exhibit polarity reversal and high amplitude anomalies, which are distributed on both sides of or at the top of the active fault plane, potentially serving as a primary trigger for landslide disasters [103].
Natural Gas hydrate decomposition: Natural gas hydrate (NGH) is a crystalline compound composed of water molecules that form cages trapping natural gas molecules. It typically forms under conditions of low temperature and high pressure [104]. The perturbation of external factors, such as fluctuations in the sea level, tidal movements, and earthquake events, can induce the decomposition of gas hydrates and result in the upward migration of reservoir fluids. This process disturbs the original sedimentary state that occurs naturally [105]. NGH serves as an efficient cementing agent among sediment particles, and the decomposition of the hydrate can induce alterations in the local shear stress and trigger instability in submarine slopes. Additionally, at standard temperature and pressure conditions, the decomposition of 1 m 3 of hydrate yields approximately 164 m 3 of methane gas (significantly exceeding its solubility in water) and 0.8 m 3 of water [106]. This can lead to the volumetric expansion of the low-permeability layer, resulting in the inadequate discharge of excess water and natural gas, thereby promoting the formation of overpressured fluid. However, the reduction of effective stress in marine sediments caused by overpressure can trigger submarine landslides. Previous studies have suggested that the decomposition of gas hydrates may be linked to other triggers. For instance, global warming or alterations in ocean current patterns can result in an increase in temperatures on the seabed, which subsequently triggers gas hydrate decomposition. Furthermore, the methane generated from decomposition exacerbates the phenomenon of global warming, triggering a chain reaction that leads to more frequent undersea landslides [107]. A significant number of submarine landslides have been identified in the Pearl River Estuary Basin of the South China Sea since the 1990s [108,109]. Despite the remarkable progress made in investigating seabed surface stability within the hydrate test area of the South China Sea, further research is needed to fully understand the mechanism behind seabed instability [110,111,112].
The typical characteristics of a submarine landslide mainly include the head stretching region, body slipping region, and toe extrusion region. Herein: (1) the head stretching region of submarine landslides represents the initial stage of landslide development and exhibits typical sedimentary structures with collapse and stretching characteristics, such as a steep wall, stretched blocks, and ridges; (2) the intense movement of the sliding body slipping region exerts a strong erosive effect on the surrounding undisturbed soil, resulting in a significant deformation within the landslide and giving rise to various distinctive features such as lateral margins, basal shear planes, translation blocks, and deformational blocks; and (3) the toe extrusion region represents the furthest extent of a submarine landslide, and serves as its terminal zone. Under the influence of extrusion, the toe of the landslide exhibits characteristic compressive landforms, such as extrusion ridges, folds, and thrust faults [113,114,115].
During China’s modernization process, marine geological technology has made remarkable progress from the seventh Five-Year Plan to the fourteenth Five-Year Plan [116]. The development of coastal cities and the continuous expansion of marine resource exploitation, particularly the excessive mining of deep-sea metal mines and oil, have led to an increasing exposure to and vulnerability towards MGHs (Figure 6). The aforementioned situation presents a substantial threat to the marine ecological environment and significantly impedes the attainment of sustainable economic development.
Oil spill problems. The presence of oil and gas reserves beneath the seabed is not typically impacted by submarine landslides (Figure 6a). Nevertheless, studies have shown that submarine landslides can cause significant damage to the transportation of oil and gas in the ocean [117]. The deep-water horizon explosion in the Gulf of Mexico in 2010, which resulted in the loss of 11 lives and the release of millions of barrels of oil into the ocean, is considered one of the worst marine disasters in American history [118,119]. The South China Sea boasts abundant oil and gas reserves in its deep waters. In the deep-water area at a depth of 300 m, there are proven geological reserves of approximately 8.304 × 10 9 t of oil and geological resources and about 7.493 × 10 9 m 3 of natural gas [120]. The exploration and exploitation of deep-water oil and gas resources in the South China Sea holds significant importance for alleviating China’s reliance on imported oil and enhancing its economic security coefficient.
Gas hydrate decomposition problems. Compared to oil, NGH is shallowly buried in the seabed and its stability region is affected by submarine landslides, which primarily manifest as changes in temperature and pressure of the seafloor and cap layer [121]. The formation and decomposition of submarine gas hydrates are directly influenced by the hydrostatic pressure, submarine temperature, composition of the gas source, pore water salinity, and other factors. As a geological hazard factor, the impact of NGH on offshore oil and gas exploration is manifested through its decomposition-induced non-uniformity in formation bearing capacity. Additionally, the abrupt release of gas can cause detrimental effects on the pipeline, particularly when high-pressure shallow gas is released, which may result in welling and blowout phenomena that could lead to marine ecological damage, geological collapse, submarine landslides, seawater poisoning, and other disasters [122]. Offshore oil production facilities often sit on top of hydrate stability zones, and the destabilization of these hydrates can pose a direct threat to the submarine oil and gas production infrastructure, potentially leading to platform sinking and pipeline damage. Therefore, the prolonged exposure of deep-sea pipelines and underwater wellheads to hydrates poses a potential hazard. Due to methane’s greenhouse effect being dozens of times greater than that of carbon dioxide, the decomposition of hydrates not only impacts global climate change but also has significant environmental implications [123,124].
Metal ore energy mining problems. From 2000 to 2020, China conducted trials of deep-sea mining systems at various water depths, demonstrating its commitment to advancing technological capabilities in the exploration and exploitation of ocean resources [125,126]. Numerous studies have demonstrated that the typical size of deep-sea polymetallic nodules ranges from 20 to 100 mm [127], while extracting polymetallic sulfides and cobalt-rich crusts necessitates cutting and stripping, resulting in mineral particles as large as tens of centimeters [128]. Deep-sea polymetallic nodules, polymetallic sulfides, and cobalt-rich crusts are found at water depths of 4000–6000 m, 500–3700 m, and 800–2400 m, respectively. As a result, the distance between the seabed and mineral transportation in deep-sea mining is often several kilometers greater than that of marine oil and gas extraction at present (Figure 6b). The development and application of mining equipment face new technological and cost-related challenges. On the other hand, frequent seismic activity in the South China Sea can lead to catastrophic submarine landslides, exacerbating the challenges and hazards of deep-sea mining. Since the fourteenth Five-Year Plan, the Chinese government has introduced a sustainable development strategy for the ocean, which emphasizes minimizing marine pollution during mineral upgrading processes [129]. To prevent mining discharges from polluting the ocean surface, the proposed scheme involves returning the dehydrated wastewater from pulp to the ocean floor. Accordingly, the sustainable development of deep-sea mining necessitates adherence to fundamental requirements such as safety, reliability, environmental protection, cost-effectiveness, and longevity [130,131,132].
Targeting submarine landslides triggered by disasters caused by oil, natural gas, and metal ore exploitation, we will scrutinize the implementation of marine policies, the diverse perspectives of researchers, the economic benefits of seabed mining, and the potential concerns regarding environmental pollution.
Currently, undersea oil exploration is being conducted worldwide. It is estimated that the world’s recoverable oil reserves amount to 300 billion tons, of which approximately 135 billion tons are submarine oil discovered across over 1600 marine oil and gas fields to date [133]. In addition, the vastest estimated mineral reserves on earth, with a potential economic value in the trillions of dollars, are located in the depths of the ocean [134]. Since 2001, the International Seabed Authority (ISA) has awarded exploration contracts to state-backed enterprises, government agencies, and private companies to explore for minerals in more than 500 square miles of seabed in the Atlantic, Indian, and Pacific Oceans. Nevertheless, the approval of mining regulations by the ISA is still pending. During the period from 2000 to 2012, the ISA sequentially promulgated regulations pertaining to the exploration and prospecting of three distinct types of deep-sea mineral resources. In 2012, the eighteenth session of the Council of the ISA initially proposed a regulatory framework for development. The ISA has engaged in extensive deliberations regarding the development of regulations for the period 2019–2023, with formal regulations anticipated to be promulgated in 2024 [128,135]. Essentially, the international seabed area has established a robust foundational management system for mineral resource exploration, investigation, and development, while also providing comprehensive guidance to other countries’ exclusive economic zones.
Nevertheless, deep-sea mining has not reached an international consensus. A vigorous debate among scholars has emerged regarding the economic environment, with two main schools of thought: those advocating for development and those advocating for environmental protection. (1) Advocating development scholars. With the increasing difficulty of land mineral resource exploitation, deep-sea mining is expected to cause less environmental pollution compared to land-based mining. For instance, the exploitation of deep-sea mineral resources is a more environmentally sustainable option as it obviates the need for constructing mines, residential areas, and other infrastructure [136,137,138]. Based on a study conducted by Deep Green, a Canadian company, deep-sea mining has the potential to significantly reduce carbon dioxide emissions (by 70%), land resource use (by 94%), forest consumption (by 92%), risky carbon storage (by 94%), and sulfur nitrogen oxide emissions (by 90%) when compared with traditional land-based mining practices [139]. (2) Advocating environmental protection scholars. Deep-sea mining activities will have detrimental impacts on the marine environment, including its biodiversity and ecosystem, as well as the various species that inhabit it. Moreover, it could lead to light pollution in the underwater environment, the discharge of waste materials, the generation of underwater noise, and unexpected oil spills during mining and transportation activities. Accordingly, a relatively cautious approach and plan should be adopted to address these uncertainties [140,141]. In addition to the mainstream views, a minority of researchers have explored alternative aspects of deep-sea mineral resource development that may yield positive environmental impacts. For example, abundant metals such as nickel, cobalt, cuprum, and manganese can be utilized to facilitate the global transition towards a green economy and mitigate climate change [142]. If deep-sea mining is conducted sustainably, it can offer significant environmental benefits compared to land-based mining.
Taking marine mineral resources such as nickel, cobalt, cuprum, and manganese (the grades of these polymetallic nodule resources are illustrated in Figure 7) as our research focus, we conducted an investigation and made predictions on the prices of these minerals in the C-C zone of the international seabed. The results are presented in Table 3. Therefore, the current international average price of nickel, cobalt, cuprum, and manganese is about USD 16,379.6/ton, USD 57,762/ton, USD 6,364/ton, and USD 1583/ton, respectively. In general, the cost of environmental compensation should be based on a thorough assessment of the actual environmental impact. In this study, according to the Cost-Benefit Analysis Report of Deep Sea Mining in the Pacific Island Region , we calculated the comprehensive environmental cost of USD 2.9 million/year for a polymetallic mine with an annual output of 2.5 million tons [143]. Subsequently, we present a discussion on the final production of metal ores, including the recovery rate of smelting as presented in Table 4. Considering the fundamental expenses, it is currently believed that China and Mexico are the only production sites capable of conducting large-scale deep-sea mineral resource smelting and processing worldwide [144]. Additionally, in China, for example, the operating cost of submarine mining in the collection, transportation, and smelting process are USD 40/ton, USD 60/ton, and USD 200/ton, respectively. The findings of this study have significant reference value for China in further formulating deep-sea mining policies and they can serve as strong theoretical support for Chinese policymakers to enhance the policy of maritime power. Meanwhile, compared with the operating costs of submarine mining in developed countries such as Japan, China’s deep-sea mining has more cost advantages, which may play a certain role in promoting future deep-sea mining projects [145]. However, this study is based on the calculation of operational costs in a specific sea area of China, which has certain limitations. Hence, it is necessary to conduct a study on the operational costs of submarine mining in all sea areas of China in the future.
Due to the significant impact of submarine landslides on the advancement of deep-sea energy security and the prevention and control of marine geological disasters, research in this field has experienced rapid promotion [151]. At present, the research is focused on three key areas: trigger formation mechanisms, motion evolution processes, as well as impacts on submarine infrastructure [120]. Herein, the mechanisms of trigger formation and the processes of motion evolution have been introduced and analyzed in Section 2 and Section 3. Accordingly, this section mainly discusses the influence of submarine landslide technology on submarine infrastructure. Traditional techniques such as marine geological investigations, numerical simulations, and laboratory simulation tests have become the foundation of research [132,152]. With advancements in research, the close integration of advanced techniques and conventional methods has become a necessary requirement for accurately investigating submarine landslides (Figure 8). In recent years, the research of domestic and foreign scholars has mainly focused on the impact of submarine landslides on submarine cable systems, including oil and natural gas pipelines, submarine optical cables, and other types of cables. Nevertheless, submarine pipelines, which serve as crucial “lifelines” for marine power transmission and oil and natural gas development, are prone to being severed by submarine landslides, thus impacting the smooth progress of subsea exploitation such as oil, natural gas, and metal ores [153,154,155]. Hence, subsequently, we will analyze the numerical simulation of the submarine pipeline, the physical experiment simulation, and the research progress of other deep-sea facilities.
Numerical simulation technology has the advantages of precision and controllability for predicting and calculating the degree of damage caused by submarine landslide disasters, and it can even provide the technical means to prevent large-scale MGHs. In particular, the application of numerical computing technology to simulate the impact of submarine landslides on pipelines is not limited by space, and it can capture all the variable information within the calculation domain. Currently, the commonly employed numerical methods for simulating submarine landslide-induced pipeline impacts include the finite element method (FEM), particle finite element method (PFEM), computational fluid dynamics method (CFD), and material point method (MPM). In the FEM method, in simulations of pipeline impacts caused by landslides, the impact force on pipelines is typically considered as a uniform load. The safety of pipelines under the landslide impact is evaluated by taking into account various factors such as the different impact loads and widths, properties of sliding bodies, and pipeline materials [156,157]. The PFEM method is a novel approach for simulating the impact of submarine landslides on pipelines, developed as an extension of the FEM method to address the challenge of modeling large deformations in rock and soil masses. Zhang et al. employed the PFEM method to simulate the entire process of slope instability and its impact on pipelines, providing a comprehensive understanding of the phenomenon [158]. The CFD method is a conventional approach for simulating the impact of submarine landslides on pipelines, with a primary focus on resolving the impact behavior of submarine landslides on pipelines during the flow-slippage stage (Figure 9). Numerous scholars have utilized the CFD method to simulate the effects of submarine landslides on pipelines, considering various complex factors such as initial velocity, slide thickness, and pipeline burial depth. As a result, they have enriched and advanced the prediction model for pipeline stress under landslide impact [159]. Based on this, Fan et al. and Guo et al. utilized the CFD method to optimize the design of submarine pipelines and proposed a pipeline optimization scheme that effectively reduces the impact of landslides, providing assistance for the safe transmission of deep-sea energy [154,160]. Unlike the aforementioned three methods, the MPM method is a meshless approach that offers significant advantages in simulating problems involving free surfaces. Dong et al. used this method to simulate the pipe–soil interaction during the initial stage of landslide movement and subsequently refined the formula for evaluating pipeline impact by integrating principles of soil mechanics and fluid dynamics [161].
The physical simulation of submarine landslide impact on pipelines primarily relies on flume and centrifugal model testing (Figure 10). In recent years, numerous scholars have successfully achieved quantitative analysis of the impact effect of submarine landslides on pipelines by enhancing the original flume test equipment and establishing a calculation formula for predicting pipeline force. For instance, Haza et al. conducted an experimental study on the impact of landslides on pipelines in 2013, which differed from Zakeri et al.’s work. The results indicated that the content of kaolin within the mud played a significant role in determining its movement characteristics and impact strength [162,163]. The centrifugal test study of submarine landslide impact on pipelines focuses primarily on the interaction between soil and pipe during the initial stage of collapse deformation. For example, Sahdi et al. improved the pipeline stress evaluation model by indirectly obtaining the impact process of the landslide on the pipeline through actively pressing it into the stationary landslide in a horizontal direction [164]. Based on the Sahdi test, a new force measuring device was developed to conduct centrifugal tests on the pipeline slope impact at various angles in the study conducted by Wang et al. [165]. Although model testing can provide direct insight into the impact of submarine landslides on pipelines, it is often not superior to numerical simulations due to limitations in laboratory space and test costs.
In addition to impacting pipelines, submarine landslides can also cause damage to underwater structures such as anti-sinking plates and marine pile foundations. Dong et al. simplified the anti-subsidence plate into a rigid body using the MPM method and analyzed the dynamic process of a submarine landslide impacting a fixed anti-subsidence plate under different working conditions. Based on simulation results, they proposed an equation for predicting a landslide impact [166]. Li et al. conducted simulations to investigate the impact of landslides on marine pile foundations under varying viscosity, initial velocity, and slope conditions. They derived a fitting formula between the resistance coefficient of pipe piles and Reynolds number, which can serve as a valuable reference for engineering the design of marine pile foundations [167]. Subsequently, Li et al. utilized the CFD method to simulate the impact of submarine landslides on marine pile foundations and analyzed the influence mechanism of landslide thickness and movement speed on the resulting impact force [168,169,170]. With the acceleration of marine energy development, there will be an increasing number of complex submarine landslides impacting under-water facilities and other chain disaster problems in the future.
To achieve sustainable development in the marine environment and economy, it is essential to conduct effective monitoring, accurate early warning systems, and reliable assessments of such disasters, while considering the environment, ecology, and economy. The current submarine landslide disaster monitoring, early warning, as well as management system is illustrated in Figure 11. It is primarily categorized into three layers: implementation layer, monitoring layer, and decision layer. In most cases of marine landslide disasters, effective risk assessment and prediction coupled with early warning systems for MGHs can mitigate the threat of disaster exposure and minimize the adverse effects caused by such events. Improving the submarine landslide disasters monitoring and early warning network, as well as integrating it with the disaster early warning information sharing system, are crucial components of effective prevention and control measures for submarine landslide disasters. Mathematical models based on field monitoring data have rapidly advanced in identifying MGHs, detecting marine geotechnical engineering issues, and determining geochronology. These models not only aid in understanding the formation mechanisms of submarine landslide disasters and conducting risk assessments, but also facilitate the development of disaster prediction and early warning systems. Moreover, to mitigate the escalation of the disaster risk resulting from the chain structure and amplification effect of submarine landslide disasters, it is imperative to employ a multi-hazard, integrated risk assessment and forecasting approach with early warning mechanisms. Establishing a multi-layered, multidimensional monitoring chain for submarine landslide disasters enables the realization of joint risk assessment, prediction, and early warning across multiple disaster types. However, it is imperative to not only deeply integrate the specific marine geological and dynamic conditions of different geographical regions but also shift the monitoring approach from a “disaster-centered” to an “environment-economy as sustainable development” perspective.
MGHs, particularly submarine landslides, have the potential to trigger tsunamis and cause coastal erosion, posing hidden threats to the sustainable development of marine ecosystems and economies. Herein, we conducted a comprehensive review of the temporal and spatial distribution, classification, characteristics, as well as inductions of submarine landslides. Moreover, it analyzed the safety hazards, environmental pollution, and economic losses that submarine landslides pose to the deep-sea mining industry. Finally, this research summarized the technical applications and future development directions of submarine landslides. The main conclusions and prospects are as follows:
Submarine landslide hotspots are predominantly concentrated in the Pacific, Atlantic, and Indian Oceans, with notable occurrences in northern Brazil and eastern Venezuela, as well as the eastern, western, and southern regions of the United States. The classification criteria for submarine landslides vary. The narrow definition refers to the process of weakly structured rock sliding rapidly along a slope under the influence of gravity, including translational and rotational landslides. Moreover, the broader concept encompasses various sediment transport processes, such as creeps, collapses, and gravity flows.
The primary triggering factors of submarine landslides are earthquakes and active fault activity (26.82%), rapid deposition (15.61%), and gas hydrate decomposition (7.39%). In deep-sea environments below 1000 m, seismic and active fault activity as well as natural gas hydrate decomposition are considered the two main causes. Moreover, the typical characteristics of submarine landslides typically comprise the head stretch region, body slip region, and toe extrusion region.
Based on investigations and predictions of the C-C area of the international seabed, it is believed that China and Mexico are currently the only production bases in the world capable of carrying out large-scale smelting and processing of deep-sea mineral resources, with promising prospects for development. Additionally, we contend that deep-sea mining offers superior economic and environmental advantages compared to land-based mining.
The investigation of submarine geological hazards is of great significance to the formulation and implementation of China’s marine strategy. Currently, numerical simulation and physical model testing are the primary research methods for submarine landslides. In general, the workflow of ocean engineering necessitates the investigation and assessment of geological hazards, as well as the establishment of risk plans prior to implementation. With the rapid development of China’s ocean industry, it is crucial to enhance techniques for identifying and analyzing deep-water landslide disasters, conducting in situ monitoring, and performing numerical simulations, particularly with regard to deep-sea seabed in situ monitoring. In the future, we aim to achieve sustainable development of the marine environment and economy by establishing a multi-level and multi-dimensional monitoring chain that takes into account ecological, environmental, and economic factors.
Conceptualization, Z.L. and Q.L.; methodology, Z.L. and Q.L.; software, Q.L.; writing-original draft preparation, Z.L. and Q.L.; writing-review and editing, Q.L.; formal analysis, Q.L.; investigation, Z.L.; resources, Z.L.; data curation, Z.L. and Q.L.; supervision, Q.L.; project administration, Q.L.; funding acquisition, Q.L. All authors have read and agreed to the published version of the manuscript.
The research work was funded by the Research Fund of National Natural Science Foundation of China (NSFC) (Grant No. 42277154).
