**1. Introduction**

The Yellow River Delta is one of largest-scale deltas with the fastest progradation rate in the world. The Yellow River brings abundant high-concentration sediments from the Loess Plateau, which are rapidly deposited in the subaqueous delta of the estuary, forming a high-moisture under-consolidated silty seabed. It is estimated that the average annual sediment transport volume exceeds 800 million tons from 1950 to 2005 [1], and over 70–90% of the sediments were deposited in the sea area within 30 km from the estuary [2], gradually forming the modern subaqueous Yellow River Delta. Such newly formed seabeds under the influence of waves and storm surges are prone to instability, thereby forming unstable seabed landforms of subaqueous delta in the estuary of the Yellow River.

Previous studies reported that various ocean dynamic processes can affect the delta of the Yellow River [3–5]. The yearly winter storms coming from the northwest have the most powerful effect from

October to March. During the winter season, the prevailing northwest winter winds becomes stronger than grade 8 on average 6.4 times each year. The wave height generated by winter storms can reach 7 m [3]. In addition, the weak southeastern winds prevail from July to August. Occasionally, hurricanes or typhoons occur in a given year. For example, Typhoon Lekima meandered over the Yellow Sea and Bohai Sea in 2019. Available survey results showed that geological disasters such as liquefaction, landslide, subsidence and depression, gully, scarp, disturbed strata, and erosion are common across the subaqueous Yellow River Delta [6–9]. The Yellow River Delta is rich in oil and gas resources and is the location of Shengli Oilfield, currently the largest offshore oil field in China. Seabed instability disasters seriously threatens the safety and stability of the offshore oil platform, submarine pipeline, wharf, and embankment [10–12]. For example, on December 3, 1998, the CB6A-5 oil production platform of Chengdao Oilfield collapsed, and the casing pipes at the bottom of the oil well ruptured, causing a material oil spill lasting half a year [13]. In November 2003, a submarine landslide occurred under the action of ocean power near the oil production platform CB12B, resulting in the interruption of two submarine cables [14]. In 2010, Operation Platform No. 3 of Shengli Oilfield was overturned due to the instability of the seabed stratum during a storm surge, leading to the death of two people and a direct economic loss of RMB 5.92 million [15].

There are two main types of seabed instability under wave action. In one, the shear stress is greater than the shear strength of the soil, causing the seabed to be unstable; in the other, it is the variation in the pore water pressure in the seabed under wave action that causes the seabed to undergo liquefaction, resulting in the loss of effective stress in the soil, thereby leading to instability. With respect to the studies on the instability of the seabed under wave action, especially in the subaqueous Yellow River Delta, plenty of field surveys, numerical analyses, laboratory tests, and in situ observation studies have been carried out [16–21]. However, few wave flume experiments have comprehensively investigated landforms, failure mode, gas distribution, pore pressure response, and sediment strength. In addition, failure of the seabed has been understudied, even though some interesting phenomena have been identified. For example, previous studies have confirmed fine particle migration due to wave action. In order to deeply explore the soil failure mode, gas distribution, and pore pressure response law under wave action, in-lab wave flume experiments were designed and carried out in this study. For the first time, gas holes were identified along with their positioning and angle with respect to the sediments. The presence of gas may serve as a primer for submarine slope failure.
