Probabilistic and deterministic seismic hazard results and influence of the sedimentary More Basin, NE Atlantic [An article from: Marine and Petroleum Geology]

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Description:
Seismic hazard assessments have been conducted for the Ormen Lange Gas field in the Norwegian Sea on the flank of the More Basin. The study used logic-tree based probabilistic methods combined with semi-deterministic methods for hazard contributions from mapped faults, and the hazard was computed in terms of Peak Ground Acceleration (PGA) and spectral acceleration. The probabilistic seismic hazard results were complemented by deterministic modelling in 1D (wave number integration) and 2D (finite difference) crustal models. The deterministic modelling first of all elucidated the shaking duration problem, indicating that thick offshore sediment structures like the ones in the More Basin may significantly extend the shaking duration of an earthquake. The presence of sedimentary layers with large velocity contrasts are responsible for this observation, and a conventional soil amplification analysis would not have revealed this phenomenon. A modelling along similar lines as indicated in the present paper may be recommended in certain vulnerable situations, and in geologic provinces with thick sedimentary layers with significant internal velocity contrasts. Even though large earthquakes most likely have occurred in the region during the postglacial period, the study could not resolve if the huge Storegga slide, which occurred about 8100 yb, was triggered by an earthquake.