Modelling of triggering of Orca submarine landslide, Cascadia margin, northeast Pacific

Bottom-water temperature variations and eustatic sea-level fluctuations may cause decomposition of marine gas hydrate and excess pore pressure in sediment, which leads to a subsequent decrease in effective stress of the sediment, and eventually results in submarine landslides. A numerical modeling of the mechanism of such slope failure was developed herein, and was applied to the study of Orca Slide that occurred between 14 and 9 kaBP on the Cascadia margin in the northeast Pacific. The modeling results show that with the rising sea level in the last 18 ka, the base of hydrate stability zone (BHSZ) experienced a fast upward movement whose rising rate peaked to 1.18 m/ka at 13.7 kaBP due to continuous bottom-water warming during 18～14 kaBP. Meanwhile, an excess pore pressure of 114 kPa was formed in the coarse-grained layers in the BHSZ of Orca Slide as a result of gas hydrate decomposition, which then significantly reduced the factor of safety of the strata to less than 1, thereby triggering the submarine landslides. Therefore, highly saturated hydrate decomposition caused by the bottom-water temperature rise may be the main triggering mechanism of Orca submarine landslide.