Relocating seismic events in the North Sea: challenges and insights for earthquake analysis

SUMMARY In this paper, we present a catalogue of relocated seismic events in the North Sea spanning 1961 to 2022. Data from all relevant agencies were combined, incorporating all available seismic phase readings, thereby enhancing station coverage. As a result, our updated locations reveal a more clustered and aligned seismicity pattern compared with the original catalogue. Even with our combined data set, only 157 of the 7089 relocated events have azimuthal gaps of less than 90 deg. Additionally, the distances between onshore stations and offshore events are considerable. Both of these factors lead to relatively poorly constrained hypocentres for most events. We therefore evaluate the performance of 1-D velocity models routinely used by different North Sea adjacent monitoring agencies for earthquake location estimations in the North Sea. The variations in assessments due to the seismic velocity model used are significantly larger than the uncertainty ellipses calculated in the relocation, demonstrating that arithmetic uncertainties systematically underestimate location uncertainties in this setting. Obtaining a realistic estimate of location uncertainty is however crucial, particularly for distinguishing between natural and induced seismicity. This is fundamental to safe monitoring of the North Sea offshore industries, including geological $\mathrm{ CO}_2$ storage. To overcome these discrepancies between the uncertainty ellipses and our multiple relocations, we introduce an alternative method that accounts for variability in the 1-D velocity models. This approach enhances the reliability of the earthquake catalogue, and provides a more robust assessment of seismic activity in the North Sea.