Analysis of Characteristics and Main Controlling Factors of Shallow Geological Hazards in the Zhongsha Islands Region of the South China Sea

Abstract

This study utilized single-channel seismic, multi-channel seismic, and multibeam bathymetric data to examine the distribution and geomorphological background of geological hazards in the Zhongsha Islands region of the South China Sea. We elucidate the regional geological structure and its evolution while focusing on the types and characteristics of submarine hazards since the Quaternary Period. By integrating geomorphological, tectonic, and sedimentary factors, we analyzed the primary drivers of shallow geological hazards in the region. Our findings reveal that seabed topography, tectonic activity, and sedimentary processes critically influence hazard formation, particularly in geomorphic units prone to disasters, such as submarine slopes and canyons. Igneous rocks in the region display medium-acid to medium-basic compositions, with notable developmental stages during the Himalayan and Yanshan periods. From the Paleogene to the Middle Miocene, tectonic activity intensified, significantly thinning the lithosphere. By the Middle Miocene, the crust stabilized into its present configuration, marking the formation of key tectonic units in the region. Multiple phases of sedimentary evolution, influenced by the Cenozoic tectonic movements, further contribute to the region’s susceptibility to geological hazards.

1. Introduction

In the 21st century, the rise of the information age has greatly enhanced our capacity to develop and utilize marine resources [1,2]. The global surge in exploiting marine energy, laying submarine pipelines and optical cables, constructing infrastructure, and establishing monitoring systems has made shallow marine geological hazards a prominent research focus [3,4,5].

The U.S. National Research Council has identified key priority areas for marine research over the upcoming decade, emphasizing the impacts of sea-level rise and the effects of climate change on marine ecosystems [6,7,8], as well as improved methods for predicting major earthquakes and tsunamis [9]. Western nations, particularly in Europe and North America, have made significant advances in marine geological research, completing seabed studies and small-scale surveys. These efforts often focus on slope stability to mitigate risks to offshore infrastructure, such as oil platforms [10,11]. The International Centre for Geohazards (ICG) has contributed significantly to this field, integrating resources to accelerate marine geological hazard research [3].

China’s research on marine geological hazards commenced relatively recently, with the introduction of the term “coastal geological hazards” in 1996. This was followed by an in-depth examination of the five key attributes of these hazards and the principles governing their classification [12]. Subsequent studies have focused on quantifying slope stability under the combined effects of wave action and gravitational forces, as well as exploring the mechanisms and morphologies of underwater landslides in proximity to islands and reefs [13].

Research into submarine geological hazards has focused on understanding past events, assessing hazard frequencies, and monitoring active seabed processes with the potential to evolve into threats [14,15]. Common methods include geophysical surveys, geotechnical engineering investigations, and in situ observations, with geophysical surveys being the most widely used. Tools like multibeam bathymetric systems and high-resolution seismic detection systems allow detailed analysis of seabed topography and geological structures, including density, magnetism, elasticity, sediment conductivity, and radioactivity [16,17,18].

Frequent shallow geological hazards in the seabed include submarine landslides, canyons, steep ridges, tidal scour troughs, erosion trenches, shallow active faults, seismic activity, buried ancient river channels, coral reefs, hydrate deposits, seamounts, and sea mounds [19,20,21]. These hazards can damage submarine infrastructure. Each type of hazard has distinct characteristics and mechanisms. Submarine landslides involve the instability and collapse of seafloor sediments, often resulting in mass transport deposits (MTDs). Submarine canyons, generally shaped by erosion, have heads located in the outer shelf or slope break zones [22]. Active faults, which are prevalent in Chinese waters, especially shallow and middle-depth faults, pose significant threats to submarine engineering facilities [23]. The northern shelf of the South China Sea features shallow faults, many of which are exposed directly on the seabed [24]. Buried ancient river channels, formed during low sea levels in the ice age, are widespread across the Bohai, Yellow, East China, and South China Seas [25].

The South China Sea’s geological complexity makes it one of China’s most hazardous marine regions [26]. In 2002, researchers classified the region into four main disaster zones and several subzones based on large geomorphic patterns. The Zhongsha Islands lie in the western and southern submarine slope hazard subzones [26,27]. That same year, extensive submarine landslides were identified in the major oil and gas basins in the northern South China Sea, such as the Pearl River Mouth and Qiongdongnan Basins, where their direct and secondary effects (e.g., tsunamis) were preliminarily assessed [28]. The Dongsha submarine slope in the northeastern South China Sea exhibits various geological hazards, including active faults, earthquakes, shallow gas, mud volcanoes, landslides, steep slopes, and erosion gullies (canyons). Additionally, adverse geological phenomena such as submarine volcanoes, magmatic diapirs, and shallow bedrock occur here. Earthquake and fault activity frequently trigger submarine landslides, which can decompose hydrates, causing further landslides and differential subsidence in a cascading hazard chain [29].

Research on seabed geological hazards in the Zhongsha Sea area remains limited, with relatively few findings. A study on Xuande Atoll identified three primary hazards: submarine landslides, scarps, and faults. These hazards were primarily driven by steep terrain and canyon channel erosion [30]. Researchers examining the gravity flow sedimentation of the Middle Sand Trough found that its development is controlled by sediment sources, sea-level changes, carbonate productivity, and magmatic activity. Potential gravity flows still exist, posing significant geological risks [21,22,23,24,25,26,27,28,29,30,31,32,33].

Previous analyses of shallow geological hazards in the Zhongsha Islands region have been fragmented and localized due to limited data. No comprehensive analysis has incorporated the geomorphological, tectonic, and sedimentary conditions that influence and trigger these hazards. As a result, the types, distribution patterns, and formation mechanisms of shallow geological hazards in this region remain unclear, necessitating further research. This study employed single- and multi-beam bathymetric data and multi-channel seismic data to identify the main types and distribution characteristics of geological hazards in the Zhongsha Islands. We reveal the primary controlling factors and formation mechanisms of these hazards and offer recommendations for disaster prevention and mitigation in the region.

2. Geological Setting

The Zhongsha Islands, covering approximately 70,700 square kilometers, include the eastern Xisha Islands, Zhongsha Trough, Zhongsha South Basin, Zhongsha Bank, and Zhongsha North Ridge. Bathymetric variations across the region highlight significant geomorphological units, such as the Xisha East Uplift, Zhongsha Trough, Zhongsha Seamount, Zhongsha North Ridge, and Zhongsha South Basin. Smaller features, including seamounts, sea hills, submarine valleys, shoals, and reefs, also characterize the region’s complex seabed [34]. The southern margin displays compressional tectonics with a series of imbricate structures, indicative of a collision edge related to the closure of the ancient South China Sea [35]. The western margin, adjacent to the eastern side of the Indochina block, exhibits strike-slip activity, defining it as a shear-extensional margin. The eastern margin comprises ophiolite sequences, trenches, and island arc complexes, forming a trench-arc system where oceanic crust subducts beneath continental crust, characteristic of subduction-compression margins [36] (Figure 1). The region’s stratigraphy is largely complete, with Neogene to Quaternary formations dominating the islands and reefs. On the slope, Paleogene to Quaternary formations are well developed, though Paleogene deposits prove difficult to identify due to carbonate shielding. These formations, limited to isolated depressions, suggest fault-basin sedimentation, controlled primarily by early-stage faults. These sediments represent a transitional period from continental to marine environments [37].

3. Materials and Methods

3.1. Data Source

The data used in this paper are mainly single-channel seismic data and multi-channel seismic data collected by the Haikou Marine Geological Survey Center of the China Geological Survey from 2019 to 2021 in the Zhongsha Sea area, as well as multi-beam data collected from the Guangzhou Marine Geological Survey. The single-channel seismic data acquisition source system uses the G.I. gun source system (210 cu.in.), and the signal receiving system adopts GEO-SENSE48 single-channel receiving cable. The source sinking depth is 5 m, and the cable sinking depth is 0.5–1 m. Single-channel data processing adopts RadExPro (2020) processing software and the Omega processing system, and data interpretation uses Geoframe, Petrel, and Coreldraw (X4) software. Multi-channel seismic acquisition was completed in two years. The multi-channel seismic acquisition source system in the eastern part of the Zhongsha Sea area uses a G.I. gun combined source (540 cu.in.), with a source sinking depth of 5 m, a cable sinking depth of 6 m, and 360 receiving channels. The multi-channel seismic acquisition source system in the western part of the Zhongsha Sea area uses a BOLT gun combined source (deep water area 3810 cu.in./reef area 2540 cu.in.), with a source sinking depth of 5 m, a cable sinking depth of 9 m in the deep water area/6 m in the reef area, and 240 receiving channels. Multi-channel seismic data processing adopts the PARADIGM2017 seismic data processing system, and data interpretation uses Geoframe, Petrel, and Coreldraw software. Globalmapper (v17.0) software is used for multi-beam data presentation and interpretation. Acquisition of 15,000 km of multi-beam, 5130.1 km of single-channel seismic, and 3869.08 km of multi-channel seismic data was completed in 2019 and 2021 (Figure 1c).

3.2. Study Methods

The research method in this paper mainly focused on geomorphic analysis and comprehensive interpretation of seismic data for the acquired multi-beam and seismic data. Multi-beam data were primarily used to analyze the geomorphic features of the study area and the genetic processes of disaster-related landforms. Seismic data were mainly employed for stratigraphic and tectonic geological analysis below the seabed interface. Based on sequence stratigraphy and sedimentology theories, major stratigraphic interfaces were identified according to seismic reflection characteristics, and a seismic stratigraphic framework was established. After ascertaining the stratigraphic structure, distribution characteristics, and sedimentary evolution process of the study area since the Cenozoic Era, we identified structural fracture characteristics and special geological bodies; delineated geological disaster types; and systematically analyzed the characteristics, distribution patterns, and main controlling factors of shallow geological disasters by combining topographic and geomorphic features with the tectonic and sedimentary evolution background.

4. Results

4.1. Main Types of Geological Disasters

This study identified the primary geological hazards in the Zhongsha Islands region by considering regional geological conditions and previous classifications of marine hazards. Based on controlling factors, these hazards can be grouped into four categories: hazards related to seabed instability, tectonic activity, fluid dynamics, and other geological features. Additionally, in terms of potential damage, geological hazards are classified as either destructive or restrictive. Destructive geological hazards can directly impact the surrounding environment and marine engineering infrastructure. These include submarine landslides, canyons, channels, creep structures, seismic activity, steep slopes, and active faults. These hazards pose significant risks to marine construction due to their unpredictability and the difficulty in preventing their effects. Restrictive geological hazards, such as magmatic bodies, hydrates, and coral reefs, do not exhibit direct movement but can restrict engineering activities. These regions should be avoided during project planning, as neglecting them can introduce engineering risks, potentially leading to structural damage (Figure 2).

4.2. Geomorphic Characteristics

The Xisha East Ridge slopes gently towards the Zhongsha Trough, with gradients ranging from 2.5° to 5.9°. The Zhongsha Bank slopes more steeply to the northwest, with an average gradient between 11° and 25°. The Zhongsha Bank’s terrain remains relatively flat, with water depths varying from 17 to 600 m, while its surrounding slopes are steep, intersected by numerous erosion canyons and channels. The upper portions of these canyons run parallel, converging towards the trough and deep-sea basin in the lower sections.

The southeastern part of the study area features a deep-sea basin characterized by flat, open terrain dominated by abyssal plains, at depths approaching 4000 m. In contrast, the submarine slope area exhibits rugged and complex topography, with geomorphic units such as platform slopes and submarine canyons, making the region highly prone to geological hazards.

4.3. Stratigraphic Characteristics

This study adopted stratigraphic interpretations from previous 1:1 million scale geological maps of Hainan Island, the Zhongsha Islands, Huangyan Island, and Zhongjian Island, produced by the Guangzhou Marine Geological Survey. Using seismic reflection data collected in this study, seven key seismic reflection interfaces were identified: T0, T1, T2, T3, T5, T6, and Tg (Table 1, Figure 3).

The stratigraphic interpretation references drilling data from wells such as XY 1 and XK 1 in the Xisha Sea area, alongside seismic stratigraphic analyses from decades of research in the northern South China Sea. This study adopted the existing lithostratigraphic framework, which includes the Ledong, Yinggehai, Huangliu, Meishan, and Sanya Formations from the Neogene Period, as established in the Yinggehai and Qiongdongnan basins [39,40].

Table 1. The age attribution of stratigraphic units divided by seismic sequence.

Supersequences	The Reflection Layer Groups	Tectonic Processes	Representative Period
I	T0–T3	T1 marks the base of the Quaternary system. T3 represents the boundary between the Upper and Middle Miocene, associated with the Dongsha movement, and the tectonic environment is large-scale subsidence [41,42]. T3 is in period III of the Xishan tectonic rotation.	T0–T1	Quaternary System
			T1–T2	Upper Pliocene Series
			T2–T3	Upper Miocene
II	T3–T6	T3–T6: T6 indicates the boundary between the Oligocene and Miocene epochs, corresponding to the Baiyun tectonic event [43], and is a transition from large-scale rifting to thermal subsidence [44]. T6 is in period II of the Xishan tectonic rotation.	T3–T5	Middle Miocene
			T5–T6	Lower Miocene
III	T6–Tg	Tg corresponds to the Cenozoic sedimentary basement interface of the Shenhu Movement, and the tectonic setting is a fault-trapped period of intraplate separation [45]. Tg is in period I of the Xishan tectonic rotation.	T6–Tg	Paleogene System

Table 1. The age attribution of stratigraphic units divided by seismic sequence.

Supersequences	The Reflection Layer Groups	Tectonic Processes	Representative Period
I	T0–T3	T1 marks the base of the Quaternary system. T3 represents the boundary between the Upper and Middle Miocene, associated with the Dongsha movement, and the tectonic environment is large-scale subsidence [41,42]. T3 is in period III of the Xishan tectonic rotation.	T0–T1	Quaternary System
			T1–T2	Upper Pliocene Series
			T2–T3	Upper Miocene
II	T3–T6	T3–T6: T6 indicates the boundary between the Oligocene and Miocene epochs, corresponding to the Baiyun tectonic event [43], and is a transition from large-scale rifting to thermal subsidence [44]. T6 is in period II of the Xishan tectonic rotation.	T3–T5	Middle Miocene
			T5–T6	Lower Miocene
III	T6–Tg	Tg corresponds to the Cenozoic sedimentary basement interface of the Shenhu Movement, and the tectonic setting is a fault-trapped period of intraplate separation [45]. Tg is in period I of the Xishan tectonic rotation.	T6–Tg	Paleogene System

The Cenozoic sedimentary thickness can reach up to 1600 m, with the thickest layers found in the Zhongsha Trough Basin. The Paleocene–Eocene system appears only in small, residual fault depressions in the southwest Zhongsha Bank. The Neogene sedimentary thickness reaches a maximum of 1200 m, mainly near the seabed and the Zhongsha Trough. The Quaternary system attains a maximum thickness of 380 m, with an average of 150 m. Overall, modern island and reef areas exhibit relatively thin deposits, while flatter terrains accumulate thicker sediments.

4.4. Tectonic Characteristics

4.4.1. Characteristics of Major Fractures

The influence of active faults on geological hazards has garnered significant attention in tectonic geology research [46]. Active fault zones not only control regional topography and rock mass structure but also correlate with slope stability; fault creep; and the triggering mechanisms of hazards such as landslides, collapses, and debris flows. Earthquakes generated by fault activity further exacerbate these hazards [47]. Within active fault zones, fractured rocks, structural planes, and poor-quality rock masses contribute to increased vulnerability to collapses and landslides [48]. The South China Sea also experiences highly active magmatism, with the scale of magmatic activity far exceeding previous assumptions [49,50].

Faults in the zone are well-developed, displaying a range of orientations. Six major faults were identified (Figure 4,

Table 2. Details of the main structural elements.

Structure Number	Property	Direction	Inclination	Length (km)
F1	normal fault	NE	SE	97
F2	normal fault	NE	NW	168
F3	normal fault	NW	NE	94
F4	normal fault	NE	SE	74
F5	normal fault	near EW	SSW	67
F6	normal fault	NE	SE	62

). Among these, seven NE-oriented faults (F1 (Figure 5), F2 (Figure 6), F5, F6) dominate the region, representing the primary fault direction. Additionally, one NW-oriented fault (F4) is present in the central and western regions (Figure 7). One nearly EW-oriented fault (F5) occurs along the southern rim of the Zhongsha Bank.

Faults are developed in the zone with different strikes, represented by six of these major faults

The NE-NEE trending fault system dominates the study area in both scale and number. On seismic profiles, these faults display characteristics typical of normal faults. This fault system separates into two periods: an early phase (Late Oligocene to Early Miocene) when the faults controlled basin structures and sedimentary infill, and a later phase (Middle Miocene to Quaternary), with faults generally reducing in scale. Faults located near or adjacent to igneous bodies retain activity due to ongoing magmatic processes [51]. The NW-trending fault system, concentrated in the central-western region, contains fewer and moderately scaled faults. Although smaller than the NE-NEE faults, NW-trending faults share similar orientations and magnitudes. Analysis suggests these faults formed during the same period as the NE-NEE faults and exhibit conjugate relationships. Fault activity decreased significantly by the Late Miocene. The nearly EW-trending fault system appears the least developed, mainly confined to the central and southern regions. In the central area, EW-trending faults form feather-like structures that interact with larger NE-NEE trending faults. The NE-trending fault system, the most widespread in the region, varies in scale and concentrates heavily in the northwestern part of the study area. This fault system coexists with other fault systems, often functioning as conduits for magmatic intrusions. These faults define the boundaries of igneous bodies and exert significant influence on sedimentation.

Fault activity spans an extended period, with some faults showing continued activity from the rifting phase to the present day, although fault activity generally exhibits a weakening trend. The entire regional fracture system that originated in the middle Miocene or earlier periods determined the rugged topography of the oceanic and continental basement. The following sedimentation mainly filled troughs and basins. These sediment sequences are rarely influenced by tectonic displacements.

4.4.2. Characteristics of Magmatic Activity

Magmatic rock bodies, visible on seismic profiles, display columnar structures with chaotic internal reflections and traceable boundaries. The study area experiences substantial magmatic activity, with numerous magmatic rock bodies distributed across the region, varying in size and quantity. The flat-topped rocks are basement highs overgrown by coral atolls like the Zhongsha Bank. These are typical of volcanic cones in tropical to subtropical waters, which slowly subsided below sea level. The classification of magmatic rocks includes flat-topped columnar extrusive rocks, conical extrusive igneous rocks, stocks, dykes, and sills (Figure 8, Figure 9 and Figure 10). These magmatic bodies formed during two primary periods: the Xishan and Yanshan II and III periods [52]. Although the intensity of magmatic activity diminished after the Quaternary, minor development still occurs. Magmatic intrusions elevate the slope angles of sedimentary strata, often inducing instability during active phases. In later stages, differential compaction of overlying strata may generate secondary faults and submarine instability. Magmatic activity significantly contributes to submarine geological hazards and requires thorough consideration when assessing regional risks.

5. Discussion

5.1. Impact of Submarine Instability on Geological Hazards

The study area exhibits rugged topography with complex geomorphic features. Located on the western side of the Zhongsha Sea area, the Xisha Uplift shows significant water depth variations and complex submarine topography. Seamounts, submarine canyons, and creep structures occur extensively (Figure 2), leading to the widespread presence of submarine landslides. Karlsrud et al. performed a statistical analysis of submarine slope instability, concluding that landslides can occur even on relatively flat slopes (<1°). Submarine landslides typically involve larger volumes and longer displacement distances compared to their terrestrial counterparts [53]. Slope angles in landslide-prone areas vary widely, from less than 0.5° to more than 60°. Large-scale landslides generally occur on flatter slopes, whereas small-scale landslides appear more frequently in steep slope areas. Hance et al. analyzed 534 submarine landslides and found that most occur on slopes between 3° and 4° [54]. The average slope angle of 399 landslides measured 4.0°, with a median of 5.8°. Slumping represents a phenomenon where soil, under its own gravity or influenced by external forces like seismic activity or wave erosion, fragments and peels off continuously, sliding or rolling down to a flatter area, forming an accumulation body. Steep ridges above slump areas typically exhibit height differences between 10 and 20 m, with gentler slope deposits found below. Slumping often occurs violently, especially in steep canyons or seamount groups. Small, localized block transport deposits mainly originate from nearby upper areas. Most block transport deposits slide along stratigraphic boundaries, driven by changes in sedimentary environments and variations in physical properties.

In planar terms, topographic uplift zones such as seamounts and oceanic hills are widespread in the Zhongsha Sea area, with steep surrounding slopes. Submarine landslides tend to concentrate near steep cliffs, submarine canyon walls, sea trench walls, and the flanks of seamounts. Landslides also occur on the outer slopes of carbonate reefs and steep reef plates, typically on terrain with slope angles ranging from 15° to 27° (Figure 11). These landslides appear as bands on flat surfaces.

Under these geomorphological conditions, combined with abundant sediment supply, sufficient slope angles, periodic sea-level fluctuations, and tectonic movements in the Zhongsha Trough area, the Zhongsha region is prone to slumping gravity flow events. Multiple sets of gravity flow deposits have developed in the shallow submarine sediments, forming widely distributed, multi-phase mass transport deposits (MTDs) (Figure 11 and Figure 12). Additionally, the high slope angles make the platform edge slopes, seamount flanks, and canyon sidewalls in Zhongsha bank high-frequency landslide occurrence sites, which require significant attention.

5.2. Impact of Tectonic Activities on Geological Hazards

The Zhongsha block lies within the transition zone between the northern lower slope of the South China Sea and the oceanic crust basin. This micro-block, formed through extensional rifting during the multi-phase expansion of the South China Sea basin, resulted from the stretching and thinning of the southern margin of China during the Cenozoic era. Sharing similar basement structures and sedimentary cover characteristics with the Xisha block, the area exhibits a transitional crust between oceanic and continental types, with stepped faults and structural belts of varying uplift and depression [36]. The highly thinned continental crust, combined with frequent magma and fault activity (Figure 4 and Figure 9), creates rugged topography and complex landforms. Platforms, troughs, and basins dominate the geomorphic types across the region, with steep slope areas—particularly platform slopes—showing a higher likelihood for submarine landslides (Figure 12).

The Zhongsha Bank, covering approximately 23,500 km², developed directly on the Zhongsha Uplift. Multiple sea-level changes, driven by global fluctuations and regional tectonic uplift, have reshaped the relative sea level over time. Currently, the Zhongsha Platform remains fully submerged, with reefs and sandbars located 10–20 m below the surface. The northwest sub-basin separates the northern part from the South China Sea’s northern slope, limiting the influence of terrestrial clastic materials. This isolation has fostered conditions conducive to carbonate rock and bioherm growth [32]. Atolls, barrier reefs, and patch reefs dominate the platform, with reef slopes steepening as they evolve, increasing the potential for submarine slope instability.

Steep slopes, submarine landslides, and the long-term evolution of biological reefs in the region closely correlate with tectonic activity. The study area lies within a tectonically weak zone with frequent magma activity. Multi-phase magma influences shallow seabed sediments and fluid dynamics significantly. Magma-induced slopes tend to steepen, making them prone to sediment instability. Additionally, magma activity facilitates fluid migration to shallow layers through fractures generated by differential compaction of sediments overlying magma bodies. This process contributes to the formation of features like pockmarks, shallow faults, and polygonal faults (Figure 9 and Figure 13).

Since the Miocene, sea-level fluctuations and two regional subsidence events have gradually submerged the Zhongsha Sea area and surrounding uplift zones, such as the Xisha Uplift and Zhongsha Platform. Depression zones in the west and north of the South China Sea now separate these uplift zones from terrestrial source areas, creating favorable geological conditions for carbonate rock and biological reef development [55,56]. Shallow water environments in platform areas, like the Xisha Platform, supported reef development. Sediment accumulation rates increased, leading to substantial carbonate deposits and large-scale platform growth. However, from the Late Miocene to the Pliocene, rapid sea-level rise reduced deposition rates, shrinking the platforms and steepening slopes. These conditions provided abundant sediment sources and triggered gravity flow deposition [54], which in turn further destabilized the seabed [57]. Additionally, retrogressive slope erosion at the platform’s edge promoted the gradual formation of canyon channels, shaped by upstream and downstream erosion. For instance, four sets of submarine canyons developed along the Xisha Platform’s slope, playing a pivotal role in shaping both seabed sediment deposition and landforms. These canyons are associated with numerous landslides, collapses, faults, and creep structures (Figure 14), significantly affecting seabed stability.

Seismic data from single-channel and multi-channel surveys revealed four major NE- or NW-trending faults (F1, F2, and F3) within and along the edges of the Zhongsha Trough, with maximum extensions reaching 168 km (Figure 4, Table 2). Additionally, shallow fault systems and faults related to sediment dehydration also exist (Figure 15). These fault systems, active over an extended period into the Quaternary, have weakened over time but still contribute to seabed instability, potentially triggering submarine landslides and mass transport deposit (MTD) events. During multi-phase landslides and MTD transport, coarse-grained sediment deposits, such as channel sand bodies, concentrate in the Zhongsha Trough. Due to their weak geomechanical properties, these deposits pose potential geological hazards. Additionally, while early fault systems remain inactive, they continue to act as conduits for stratigraphic fluids, facilitating the migration of deep fluids to shallow layers and inducing shallow geological hazards.

5.3. Impact of Sediment Supply on Geological Hazards

Sediment supply, topography, and the weight of deposited material influence locations such as nearshore steep cliffs, submarine canyon walls, trench walls, seamount flanks, and the outer slopes of carbonate reefs or steep reef platforms. When sediment supply remains sufficient and maintains balance with gravity flow development and basin subsidence, prograding slope deposits readily form. However, when sediment supply decreases and gravity flows outpace sediment input, the shelf edge becomes vulnerable to erosion, slippage, and gravity-driven sliding faults. Collapsed slope deposits, with their poor compaction, complex structure, and weak resistance to compression and shear forces, easily trigger large-scale landslides and collapses. These events pose significant risks to ocean engineering projects and infrastructure, qualifying as destructive geological hazards with high activity potential.

Since the Middle Miocene, new sedimentary strata have developed extensively across the Zhongsha Atoll area, overlying older layers [58]. The rapid rise in sea levels after the Late Miocene led to platform growth primarily in the vertical direction [57], resulting in steeper slopes and creating favorable conditions for gravity flow formation around the Zhongsha Platform (Figure 16). These gravity flows provided a rich source of sediment to the Zhongsha Trough. Additionally, complex circulation systems and monsoons around the platform [59] transported sediments via monsoon winds and bottom currents, further increasing the deposition rate in the Zhongsha Trough area.

In this context, the study area features high sediment supply rates and prone overpressure in the strata. Steep platform edges along the trough heighten the risk of shallow geological disasters, such as sliding, collapses, and associated gravity flows, especially when triggered by magmatic activity, stratigraphic fluid movement, and fault activity (Figure 2 and Figure 12). The Zhongsha Trough, characterized by a low, narrow, elongated shape, shows multi-phase landslides distributed in a NE-SW pattern, often overlapping. This region contains the highest concentration of submarine landslides surveyed.

Figure 16. Two seismic profiles and bathymetric maps from the Zhongsha Scarp on the west side of the Zhongsha Bank.

Figure 16. Two seismic profiles and bathymetric maps from the Zhongsha Scarp on the west side of the Zhongsha Bank.

5.4. Impact of Fluids on Geological Hazards

In some seismic profiles from the Zhongshabei Trough and the southern part of the Zhongshabei Ridge, features resembling bottom simulating reflectors (BSR) have been observed. These features exhibit polarity reversals and cross-layered characteristics, typically associated with the presence of gas hydrates. These observations suggest that certain areas may host natural gas hydrates, although further verification is required due to a lack of supporting drilling data. Continued research is necessary to confirm this interpretation (Figure 17).

In summary, this study identified the main factors affecting geological hazards in the region: submarine topography, tectonic activity, and sedimentary environment, including sea-level changes and global and regional climatic shifts. The distribution of these disaster factors varies by region. Hazards like landslides, canyon channels, creep structures, seamounts/volcanoes, scarps, and earthquakes mainly occur around the Xisha Platform and relate to topography, tectonic movement, and sediment supply. In contrast, canyon channels and coral reefs mainly develop around the Zhongsha Platform, linked to sediment supply and sea-level changes. Gravity flow deposits, buried channels, hydrates, and shallow faults predominantly occur in the Zhongsha Trough, influenced by submarine topography, significant sediment input, and tectonic changes. These factors play a crucial role in shaping submarine topography stability and influencing submarine engineering projects.

6. Conclusions

(1)

Seven active geological hazards have been identified in the Zhongsha Islands region, namely, submarine landslides, submarine canyons, submarine channels, submarine creep structures, active faults, seismic activity, and scarps. Additionally, magmatic rock bodies, hydrates, and coral reefs, although currently inactive, remain significant factors for potential future hazards. Among these, submarine landslides exhibit the widest distribution and pose the greatest disaster potential due to their multi-phase nature. Hazardous areas concentrate around slope and canyon walls, seamount flanks, and outer slopes of carbonate reefs, with fewer risks identified in the southeastern deepwater basins.

(2)

Submarine topography, tectonic activity and the sedimentary environment, including changes in sea level and climate, are the main influences on geological hazards in the survey area. Submarine instability represents the key driver for the formation of mass transport deposits (MTDs). Sufficient sediment supply, steep slopes, and periodic tectonic movements in the Zhongsha Trough region create favorable conditions for sliding gravity flow events. These processes result in the formation of multi-phase gravity flow deposits, while tectonic activity intensifies slope steepening and platform atrophy, triggering geological hazards such as active faults, seismic events, and magmatic rock activity.

(3)

Given the presence of destructive geological hazards, such as submarine landslides, submarine canyons, and active faults, careful attention is required in future marine infrastructure projects. Detailed disaster risk assessments must guide the selection of construction sites to avoid high-risk areas. Establishing a long-term in situ geological monitoring and warning platform will enhance the capacity to track surface deformation, temperature, pressure, and current changes in real time, improving risk management and enabling timely interventions to optimize ocean engineering construction and maintenance efforts.