Predicting variability in the stability of slope sediments due to earthquake ground motion in the AFEN area of the western UK continental shelf

The stability of the continental slope northwest of Shetland (UK), where a mid-Holocene slide has been identified, has been assessed on the basis of existing geological data. Using a conservative infinite slope model, a methodology is presented for mapping the theoretical Factor of Safety under earthquake loading; it is based on modelling sediments on the continental shelf and slope through use of existing geological–lithological models and compatible earthquake time series (Musson, R.M.W., Long, D., Pappin, J.W., Lubkowski, Z.A., and Booth, E., 1997. UK Continental Shelf Seismic Hazard. Norwich, Health and Safety Executive—Offshore Technology Report OTH 93, 416, 101 p.; Jackson, P.D., Gunn, D.A., Stoker, M., Holmes, R., Hobbs, P.R.N., Long, D., Walker, A. and Musson, R., 1999. Impact of earthquake ground motion on the stability of slope sediments using existing ground models. British Geological Survey Technical Report, WN/99/21C.). Defining lithologies, we derive the soil and rock properties needed in simulating the propagation of earthquake ground motion from bedrock to the seabed through the sediment column. Supplemental shallow data is included via gravity, vibrocores and boreholes; seismic sections, interval velocities and well-logs underpin our mapping of changes in the thickness of Plio–Pleistocene and Miocene sediments based on two-way travel time differences. Our results show Factors of Safety are controlled primarily by sediment shear-strength and ground acceleration, the highest values of ground acceleration being seen in the depth range 1–5 mbsf. For these shallow slopes (0–2.5°), no correlation was found between Factor of Safety and slope angle. Factors of safety are largest in the area having greatest depth of mud. Discretization of lithological units was studied, 15 layers per decade, on a uniform logarithmic depth scale being shown to be suitable. Examples show the Factor of Safety to be most sensitive to changes in properties in the depth interval 2 to 5 m, i.e., most sensitive at the depths minimum Factors of Safety were calculated. Corresponding, smaller sensitivities were found adjacent to the depth where lithification begins, taken to be of the base of the Neogene (Jackson, P.D., Gunn, D.A., Stoker, M., Holmes, R., Hobbs, P.R.N., Long, D., Walker, A. and Musson, R., 1999. Impact of earthquake ground motion on the stability of slope sediments using existing ground models. British Geological Survey Technical Report, WN/99/21C.). While our results are based on earthquake time series compatible with a distant, large earthquake (10,000-year return period), a far smaller, more local earthquake occurring beneath the slope could also be significant.