A key element in the assessment of seismic hazard and risk due to induced earthquakes in the Groningen gas field is a model for the prediction of ground motions. Rather than using ground-motion prediction equations with generic site amplification factors conditioned on proxy parameters such as V S30 , a field-wide zonation of frequency-dependent nonlinear amplification factors has been developed. Each amplification factor is associated with a measure of site-to-site variability that captures the variation of V S profiles and hence amplification factors across each zone, as well as the influence of the uncertainty in the modulus reduction and damping functions for each soil layer. This model can be used in conjunction with the predictions of response spectral accelerations at a reference rock horizon at a depth of about 800 m to calculate fully probabilistic estimates of the hazard in terms of ground shaking at the surface for a large region potentially affected by induced earthquakes.

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In current modelling of excess pore pressures (EPPs) below marine structures, the irregular nature of cyclic loads and the real storm development are not taken into account. The effect of the irregular cyclic loading in time is investigated in this paper. The wind, wave and turbine loads on a gravity based foundation (GBF) are derived in the frequency domain. The real storm development is based on the CoastDat dataset. The load input is used in a program which takes the generation and dissipation of pore pressures under cyclic loading into account. Also, densification is included. The results show that the first storm in the lifetime of the GBF results in the highest EPPs. The EPP decreases in time, due to significant dissipation and densification during the build-up of a storm. Therefore, not the storms with the largest cyclic loads but the storms with the fastest build-up result in the highest EPPs, since this limits the process of densification. A large scatter is found in the maximum values of EPPs due to the irregular nature of the loads.