**3. Features of Earthquake-Induced Seabed Liquefaction**

Both submarine seismic liquefaction and onshore seismic liquefaction can be explained using the principle of effective stress. However, the amount of seismic damage in marine areas indicates that seabed seismic liquefaction has many characteristics that differ from those of onshore seismic liquefaction.

*3.1.* M

#### *3.1. Marine Deposits Layer 3.1.* M*arine Deposits Layer*  Based on extensive soil liquefaction cases, particle size distribution curve boundaries for the

*arine Deposits Layer* 

Based on extensive soil liquefaction cases, particle size distribution curve boundaries for the possibility of liquefaction can be drawn, as shown in Figure 1 [20]. Generally, if the curve of the seabed soil samples falls within the range defined by the two blue boundaries, it is necessary for us to consider the risks of soil liquefaction in engineering design. Well-sorted aeolian sands are widespread in offshore areas, which is inclined to liquefaction easily [46]. For example, the offshore areas of China are mainly layered soils composed of sand, silt and clay (as shown in Figure 2) [47,48], and the submarine soil layers in the North Sea of Europe are dominated by sands [49]. Based on extensive soil liquefaction cases, particle size distribution curve boundaries for the possibility of liquefaction can be drawn, as shown in Figure 1 [20]. Generally, if the curve of the seabed soil samples falls within the range defined by the two blue boundaries, it is necessary for us to consider the risks of soil liquefaction in engineering design. Well-sorted aeolian sands are widespread in offshore areas, which is inclined to liquefaction easily [46]. For example, the offshore areas of China are mainly layered soils composed of sand, silt and clay (as shown in Figure 2) [47,48], and the submarine soil layers in the North Sea of Europe are dominated by sands [49]. possibility of liquefaction can be drawn, as shown in Figure 1 [20]. Generally, if the curve of the seabed soil samples falls within the range defined by the two blue boundaries, it is necessary for us to consider the risks of soil liquefaction in engineering design. Well-sorted aeolian sands are widespread in offshore areas, which is inclined to liquefaction easily [46]. For example, the offshore areas of China are mainly layered soils composed of sand, silt and clay (as shown in Figure 2) [47,48], and the submarine soil layers in the North Sea of Europe are dominated by sands [49].

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 5 of 16

that seabed seismic liquefaction has many characteristics that differ from those of onshore seismic

*J. Mar. Sci. Eng.* **2020**, *8*, x FOR PEER REVIEW 5 of 16

**Figure 2.** Typical ground profiles in Chinese waters: (**a**) Taiwan Strait; (**b**) East China Sea; (**c**) Bohai **Figure 2.** Typical ground profiles in Chinese waters: (**a**) Taiwan Strait; (**b**) East China Sea; (**c**) Bohai Sea; (**d**) Yellow Sea (modified from [48]). **Figure 2.** Typical ground profiles in Chinese waters: (**a**) Taiwan Strait; (**b**) East China Sea; (**c**) Bohai Sea; (**d**) Yellow Sea (modified from [48]).

Sea; (**d**) Yellow Sea (modified from [48]). In addition, another important feature of the marine deposits layer is the presence of calcareous sands. It worth noting that calcareous sands are widely distributed in the South China Sea, the Gulf of Mexico, the coasts of Australia, etc. Calcareous sands may have higher resistance to liquefaction than siliceous sands [50]; however, they are also at a great risk of liquefaction [51]. The liquefaction mechanisms of calcareous sands are not very clear yet due to their unique structural characteristics, such as crushability, high content of angular particles and mineralogy surface roughness [52]. Studies on the seismic liquefaction behavior of calcareous sands are of great significance in marine engineering and need to be further carried out. than siliceous sands [50]; however, they are also at a great risk of liquefaction [51]. The liquefaction mechanisms of calcareous sands are not very clear yet due to their unique structural characteristics, such as crushability, high content of angular particles and mineralogy surface roughness [52]. Studies on the seismic liquefaction behavior of calcareous sands are of great significance in marine engineering and need to be further carried out. *3.2. Influence of* S*ea Water* 

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of Mexico, the coasts of Australia, etc. Calcareous sands may have higher resistance to liquefaction

In addition, another important feature of the marine deposits layer is the presence of calcareous

#### *3.2. Influence of Sea Water* When analyzing seismic earthquake forces in marine areas, it is necessary to consider the

When analyzing seismic earthquake forces in marine areas, it is necessary to consider the increase in pore-water pressure on the seabed caused by earthquake-induced water waves acting in offshore areas. Thus, the external excitation that triggers the liquefaction of the soil is not only seismic action but also wave action. Waves can cause two types of seafloor liquefaction: instantaneous liquefaction and residual liquefaction [16]. increase in pore-water pressure on the seabed caused by earthquake-induced water waves acting in offshore areas. Thus, the external excitation that triggers the liquefaction of the soil is not only seismic action but also wave action. Waves can cause two types of seafloor liquefaction: instantaneous liquefaction and residual liquefaction [16]. Moreover, after the seafloor is liquefied, the soil is liable to form mud flows due to the action of

Moreover, after the seafloor is liquefied, the soil is liable to form mud flows due to the action of waves and seawater; the suspended flow can diffuse over a long distance, which results in lateral spread over a larger area compared with land liquefaction. When liquefaction occurs in soil layers below the seabed surface, the pore-water pressure dissipates much more slowly than on land, and the strength recovery is slower [53]. waves and seawater; the suspended flow can diffuse over a long distance, which results in lateral spread over a larger area compared with land liquefaction. When liquefaction occurs in soil layers below the seabed surface, the pore-water pressure dissipates much more slowly than on land, and the strength recovery is slower [53]. *3.3. Influence of Submarine Gas Composition* 

#### *3.3. Influence of Submarine Gas Composition* Gas is always present in gas-charged sediments which are widespread in marine or offshore

Gas is always present in gas-charged sediments which are widespread in marine or offshore environments [54]. Under normal conditions, methane is the dominant gas component [55]. As there are many differences between the behavior of unsaturated soils and typical saturated soils under seismic loading [56], it is necessary to clarify and summarize the differences in their liquefaction characteristics. environments [54]. Under normal conditions, methane is the dominant gas component [55]. As there are many differences between the behavior of unsaturated soils and typical saturated soils under seismic loading [56], it is necessary to clarify and summarize the differences in their liquefaction characteristics.

Firstly, seismic cyclic loading is likely to cause the discharge of shallow seabed gas, which will accelerate the increase in pore pressure and make liquefaction more likely to occur [53]. Moreover, research suggests that small amount of tiny gas bubbles suppress the accumulation of soil pore-water pressure, but may increase the instantaneous liquefaction risk under waves or vertical seismic motion [57]. Figure 3 shows the change in pore pressure with depth for saturated and unsaturated soils. If the soil contains some air or gas, the pore pressure will dissipate very rapidly with depth. In unsaturated soil, the pore pressure gradient can be extremely large, especially near the seabed surface, which means considerable lift can be generated during the passage of a wave trough [58]. Firstly, seismic cyclic loading is likely to cause the discharge of shallow seabed gas, which will accelerate the increase in pore pressure and make liquefaction more likely to occur [53].Moreover, research suggests that small amount of tiny gas bubbles suppress the accumulation of soil pore-water pressure, but may increase the instantaneous liquefaction risk under waves or vertical seismic motion [57]. Figure 3 shows the change in pore pressure with depth for saturated and unsaturated soils. If the soil contains some air or gas, the pore pressure will dissipate very rapidly with depth. In unsaturated soil, the pore pressure gradient can be extremely large, especially near the seabed surface, which means considerable lift can be generated during the passage of a wave trough [58].

**Figure 3.** Typical pore pressure distributions in saturated and unsaturated soils during the passage of a wave trough (modified from [58]). **Figure 3.** Typical pore pressure distributions in saturated and unsaturated soils during the passage of a wave trough (modified from [58]).

Additionally, natural gas hydrates are widely distributed in marine sediments. Under standard conditions, 1 unit volume of hydrate can release about 164 units of methane [59]. When a large amount of gas migrates upward, it may be trapped under the low-permeability soil layer, which can reduce the effective stress to zero and cause potential instability [60]. An earthquake can trigger dissociation of a large amount of gas hydrate. Moreover, Xu et al. studied the shear behavior of dissociated gas hydrate in undrained conditions using DEM and found that the dissociation of gas hydrate produced significant excess pore pressure and volume expansion, and occasionally static liquefaction [61].

In conclusion, under the influence of sea water and trapped gas, seabed soil layers are more prone to liquefaction than onshore soil layers, and the liquefied area may be larger.
