A dynamic environment of moderate tectonism, sediment instability, and active erosional and depositional processes on the shallow sea floor of northern Bering Sea creates several potential geologic hazards. Active faulting, thermogenic gas seeps, sea-floor gas cratering, sediment liquefaction, ice gouging, scour-depression formation, storm-sand deposition, and large-scale bedform movement all occur in the study area. Geologic processes may interact to cause such potential hazards as nearsurface faulting south of Nome, Alaska that provides pathways for seeps of thermogenic gas leaking from a large subsurface gas accumulation. Numerous faults also cut the sea floor in the area west of Port Clarencef fault activity is difficult to determine, however, because current scour may be preserving or exhuming old fault scarps.
Interaction between the processes of liquefaction and the formation of gas craters, scour depressions, storm-sand deposits, and slumps results in potential sediment instability problems. Liquefaction of the upper 1-2 m of sediment can be caused by cyclic storm-wave loading of the Holocene coarse-grained silt and very fine-grained sand covering Norton Sound. The widespread occurrence of gas-charged sediment with small surficial craters (3-8 m in diameter and less than 1 m deep) in central Norton Sound indicates that the sea-floor sediment is periodically disrupted by sudden venting of biogenic gas from the underlying peaty mud. During major storms, liquefaction not only may help trigger crater formation, but also may enhance erosional and depositional processes that create large-scale scour areas and transport sand in the Yukon prodelta area. Surficial, small-scale slumps on the west flank of the Yukon prodelta may also be triggered by liquefaction.
Interaction of erosional and depositional processes may result in hazards in the shallower parts of northern Bering sea. Ice gouges are numerous and ubiquitous in the area of the Yukon prodelta, and the sediment is scoured to depths of 1 m. Although much less common, ice gouges are present throughout the remainder of northern Bering sea where water depths are less than 20 m. In the Yukon prodelta area and in central Norton Sound, where currents are constricted by shoal areas and flow is made turbulent by local topographic irregularities (such as ice gouges), storm-induced currents have scoured large (50–150 m diameter), shallow (<1 m deep) depressions. Abundant storm-sand layers in Yukon mud show that storm-surge activity has a significant effect on the bottom, particularly around the Yukon prodelta, where storm surge and waves have generated bottom transport currents that deposit layers of sand as far as 75 km from land. Large sand waves (10–200 m wavelength and 0.5–5 m wave height) west of Port Clarence do not continually migrate, but may move only intermittently when storm surge run-off reinforces the strong geostrophic currents that continually flow north over the Bering Shelf.
