Improving earthquake ground-motion predictions for the North Sea

Estimates of the expected ground motion are essential for the design, assessment and decommissioning of offshore critical infrastructure. The North Sea is an area of moderate seismic hazard that contains many high-value offshore structures (e.g. oil, gas and wind-turbine facilities). The most recent seismic hazard assessment for the North Sea is about twenty years old, before many innovations in ground-motion modelling were developed. In this study, firstly we investigate which ground-motion model from more than a dozen recent models is the most appropriate for this area based on a residual analysis of ground-motion data from onshore seismic stations surrounding the North Sea. The limited data that are available for this area and the poor magnitude and distance coverage is an inherent weakness of this residual analysis. A recent model developed for Europe and the Middle East is the model that shows the lowest bias and minimal statistical trends with respect to magnitude and distance. Following this, we develop adjustments to this best-performing model to relax the ergodic assumption, i.e. to make the model more site- and path-specific thereby allowing a smaller aleatory variability (sigma) to be used within a probabilistic seismic hazard assessment. The use of this adjusted model within seismic hazard assessments for the North Sea should lead to better estimates of the expected ground motion for critical offshore infrastructure sites, although this would require the effects of the geotechnical properties of the seafloor to be accounted for.