Excess Pressure Of Fluid And Triggering Mechanism Of Submarine Landslide In Marine Gas Hydrate Area

Natural gas hydrates are ice-like compounds that form and remain stable under low temperature and high pressure conditions and are mainly found in seafloor sediments at depths greater than 300 m on continental margins.Sea level fluctuations,seafloor warming,sedimentation,tectonic and human engineering activities can all lead to changes in seafloor temperature and pressure conditions,which can cause hydrate decomposition.The large amounts of methane gas released by hydrate decomposition contribute to global warming,and the methane entering seawater can cause seawater acidification and marine ecological hazards,while widespread hydrate decomposition can also cause slow slip and seafloor landslides within the strata.One of the most significant subsea geological hazards is submarine landslides,which can cause the destruction of offshore engineering facilities and trigger tsunamis,making them a huge hazard.Therefore,hydrate decomposition and the triggering mechanism of submarine landslides caused by hydrate decomposition have become a hot topic of research.In order to better study the dynamic process of hydrate decomposition,this paper proposes a coupled model of the bottom boundary and overpressure in the hydrate stability zone based on existing studies,and also combines the dynamic changes of sea level and seafloor temperature to obtain the dynamic evolution of the bottom boundary and overpressure in the hydrate stability zone.The stability zone bottom boundary model considers both heat transfer and the effect of overpressure generated by prehydrate decomposition on the bottom boundary depth,and applies the global relative sea level record and the palaeo-seabed temperature record to the actual temperature and pressure calculations.The overpressure model uses the rate of movement of the bottom boundary of the hydrate stability zone and the degree of saturation to participate in the calculations.When the rate of upward movement of the bottom boundary of the stability zone is small,the influence of hydrate saturation on the overpressure is small,and the rate of upward movement of the bottom boundary of the stability zone plays a major role in controlling the overpressure.The model was applied to three typical hydrate zones in the South China Sea,the Hikurangi margin of New Zealand and Storegga to obtain the dynamic history and overpressure of the bottom boundary of the stability zone for the last 26 ka.The bottom boundary of the stability zone in the South China Sea has become shallower by about20 m in the last 25 ka,and the upward shift of the bottom boundary can reach a high value of 2.4-3.2 m/ka,corresponding to a d overpressure of 200-270 k Pa;the Hikurangi margin in New Zealand is mainly in the hydrate generation stage,with the bottom boundary of the stability zone continuously shifting downwards.The Storegga area experienced a short period of rapid warming,with temperature significantly controlling the hydrate development process,and the maximum rate of upward shift of the bottom boundary could reach 21 m/ka,with an overpressure of 150 k Pa,and the bottom boundary of the stability zone became shallower by about 100 m in the last 12 ka.The modelling results show that the bottom boundary of the stable zone at 10 ka BP was synchronously shallowed in all three areas and reached high overpressure values in each area.It is clear that the delayed warming of the bottom boundary at the end of the Neosemian was an important factor in the simultaneous decomposition of hydrates in the three areas.The main factors influencing overpressure in the three hydrate systems differ due to differences in geological conditions,with high overpressure in the South China Sea likely due to very high hydrate saturation and relatively high hydrate decomposition rates,and high overpressure in Storegga significantly controlled by high hydrate decomposition rates due to rapid bottom water warming,with the New Zealand conditions averaging out to a smaller overpressure than the other two areas.Extensive geological and seismic data data exist for all three typical hydrate zones indicating widespread hydrate decomposition events during the modelled periods of high overpressure.A coupled hydrate stability zone bottom boundary and overpressure model was combined with a stratigraphic stability assessment for the Orca landslide formation process that occurred during 14-9 ka BP on the Cascadia shelf in the Northeast Pacific.The study shows that the depth of the stability zone bottom boundary calculated from the most recent 0.55 ka BP simulation is 272.3 mbsf,which is in good agreement with the depth of the present-day stability zone bottom boundary determined from drilling at station U1326.The difference between the bottom boundary depths due to hydrate decomposition at saturations of 0.056 and 0.4 does not exceed 0.9 m,indicating that the heterogeneous hydrate development in the sediment layer has little influence on the bottom boundary of the stability zone.A numerical model related to the mechanism of such slope failure was therefore developed herein,and was applied to the study of Orca slide that occurred between 14-9 ka BP,on the Cascadia margin in the northeast Pacific.The modelling results indicated that,with the rising sea level over the last 18 ka,the base of hydrate stability zone(BHSZ)shows a fast upward movement,reaching a maximum rising rate of 1.18 m/ka at 13.7 ka BP,due to the continuous warming of the bottom water during 18-14 ka BP.Meanwhile,an excess pore pressure of 114 k Pa has been built up in the coarse-grained layers in the BHSZ of Orca slide as a result of decomposition of gas hydrates,which significantly reduced the safety factor of the strata to less than 1,thereby initiating the submarine landslide.Therefore,the decomposition of highly saturated hydrates caused by the rising bottom-water temperature may be the main triggering mechanism of Orca submarine landslide.In this paper,a coupled model of the bottom boundary of the stability zone and the overpressure is developed in conjunction with sea level and seafloor temperature changes,and the dynamic decomposition of hydrate and the development of overpressure are comprehensively portrayed,providing a model basis for the study of the dynamic changes of hydrate decomposition.This paper further analyses the dynamic evolution of submarine stratigraphic stability in the context of stratigraphic stability assessment,and enriches the triggering mechanism of submarine landslides related to hydrate decomposition,providing theoretical support for more studies on submarine landslides.