**1. Introduction**

An increasing number of structures are being constructed in offshore areas; these include wharfs, cross-sea bridges, seabed tunnels, wind turbines and oil platforms. An important challenge of our times is to develop eco-friendly and renewable energy sources in marine areas [1,2]. Thus, offshore engineering has greatly developed in various countries and the pace of marine resource development is gradually accelerating [3]. The United Nations pointed out that the 21st century is the century of the oceans.

However, marine geohazards occur frequently because of the complex and harsh marine environment [4,5]. Under cyclic loading, such as storms, sea ices, waves and earthquakes, the strength and stiffness of marine soft clay will decrease and liquefaction may occur [6]. Seabed liquefaction can lead to catastrophic consequences, such as the creation of a submarine slope, pile foundation instability, flotation of buried pipelines, and overturning of wind turbines [7–9]. For example, Christian et al. reported a flotation accident on a 3.05-m diameter steel pipeline in Lake Ontario in 1974, which was induced by seabed liquefaction [10]. In 2010, huge waves caused liquefaction of the seabed soil in some areas of the Yellow River Delta in China, and an offshore platform capsized, causing two deaths and economic losses of 5.92 million RMB [11]. In the 2011 Tohoku Earthquake, Kamisu and Hiyama wind farms located 300 kilometers away from the epicenter survived without major damage because the wind turbine system (~3 s) is designed to have a dominant period of ~3 s, which is considerably different from

that of the seismic motions at the farm sites (0.07–1.0 s). However, one wind turbine with a monopile foundation tilted due to the seismic seabed liquefaction [12]. These liquefaction-induced accidents had huge financial impacts and severely affected the environment. Due to the serious consequences, researchers have made great efforts in the studies of seabed liquefaction induced by various types of excitation. For instance, Jia and Ye carried out systematic wave flume experiments and numerical simulation works respectively, which well explained the hydrodynamic behaviors (liquefaction and re-suspension) of marine deposits under the sea wave loads [13,14]. Sui et al. considered distribution gradient terms of soil properties and analyzed liquefaction of an inhomogeneous seabed caused by waves [15]. Huang et al. comprehensively reviewed the mechanisms of wave-induced liquefaction and relevant remedial measures [16]. Additionally, in some high-altitude areas, ice-induced vibration needs to be considered, which may also cause liquefaction around marine structures [17,18]. The duration of wave and ice loads is much longer than that of earthquakes. However, earthquakes can produce more energy in a short time compared to waves or ice sheets, so marine structures located in the earthquake zone will be at great risk due to seismic liquefaction. Unfortunately, most earthquakes occur on the seafloor, especially in offshore areas [19]. In Japan, a large earthquake occurs off the coast every three to four years on average, with potential to cause severe damage to marine structures [20]. Therefore, it is important to understand the effect of seismic seabed liquefaction on marine engineering structures.

Significant advances have been made in the study of onshore seismic liquefaction and anti-liquefaction measures [21,22]. However, the ocean environment is more complicated than the onshore one. Earthquake-induced seabed liquefaction has some unique features. For example, the dynamic response of the seawater during an earthquake event can also cause liquefaction in the seabed [23]. Furthermore, the biggest feature and most important development trend which ocean engineering faces is moving from shallow to deep sea. In marine engineering, especially in the abyssal environment, reinforcing the seabed soil skeleton or improving the pore water to prevent liquefaction are not always applicable because of the difficulty and cost. In recent years, scientists have been extensively studying earthquake-induced seabed liquefaction and damage mitigation related to the design of new marine structures. For example, Groot et al. systematically summarized the physical principles of various triggering mechanisms for liquefaction affecting ocean construction [8]. Esfeh et al. used an advanced liquefaction model with FLAC3D and successfully analyzed the liquefaction effect on floating structures [24]. Through dynamic centrifugal tests, Yu et al. studied the dynamic behaviors of different types of foundation (mono-pile and gravity) under seismic loadings that caused liquefaction [25]. Wang et al. presented a comprehensive review of research on mainstream wind turbine foundations and new suction bucket foundations based on both experimental and numerical methods [26].

However, the characteristics of marine seismic liquefaction and the latest marine structures proposed for reducing liquefaction damage have not been reviewed systematically. This article summarizes previous studies and outlines specific issues of seismic liquefaction in marine engineering. Moreover, perspectives on novel liquefaction-resistant marine structures are presented to help cope with the future trends and challenges of ocean engineering. This paper can help readers understand the problems of marine engineers in designing liquefaction-resistant marine structures, and provide useful guidelines on the subject.
