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 CO₂ 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.

Carbon capture and storage technologies are an essential part of EU’s decarbonisation efforts. Combined with sustainable energy resources, they are necessary to move Europe towards a net zero carbon emissions economy. Currently, several Mt-scale CO₂ storage projects are being developed in the North Sea. Containment risk evaluation includes analysing tectonic earthquake patterns to potentially map faults, reveal their orientation and failure style, invert for stress directions and at later stages, enable the discrimination of natural and induced seismicity. In addition, seismological information may contribute to the geomechanical understanding of the reservoir and caprock response to large-scale CO₂ injection over time.

A wealth of data exists from various European seismological agencies, but much of it has not been analysed collectively. Within the framework of the ACT3 project SHARP Storage, an extensive unique earthquake bulletin was compiled using seismicity data from all relevant data centres serving as a basis for further analysis, including event relocation and magnitude homogenisation. Inverted moment tensors, shear-wave splitting measurements, and stress drop analysis will be compared to and complement a review of borehole stress measurements to better gauge the present-day stress field and provide constraints on geomechanical models. In addition, the seismological data is integrated with geomechanical data, further improving the understanding of the state of stress and how this relates to fault failure and slip tendency.

Carbon capture and storage (CCS) technology is essential to European decarbonisation efforts, and several offshore CO₂ storage projects are being developed in the North Sea. Understanding the geomechanical response to CO₂ injection is key to both the pre-characterisation and operation of a storage reservoir. A thorough assessment of seismicity gives critical insights into the stress field and faulting around reservoirs, both key controls on the geomechanical response to injection. Seismicity also illuminates potential hydraulic pathways for leakage, be it directly by revealing the extent of faults, or indirectly through fractures imaged by measurements of seismic anisotropy. High quality seismicity data is critical to underpin all of these methods of analysis. This paper presents the most complete catalogue of seismicity in the North Sea to date. The combined data are enabling revised assessments of seismic hazard and leakage risk in the North Sea, as well as a better understanding of faulting and stress. This study shows the value of unifying disparate seismicity data, allowing for more accurate seismological analyses. These lay the foundation for better management of risks for not only geologic CO₂ storage, but other offshore industries and infrastructure.

To effectively design and operate the many CO2 storage projects that are being developed in the North Sea, seismicity needs to be accurately characterised. Measuring seismicity can provide insights into the stress state in the region, fault density and faulting style, as well as fracturing in the overburden. Understanding background seismicity rates is also key to discriminating, determining the risk of, and mitigation against induced seismicity that could result from injection. Communicating earthquake risk is a key challenge in this field, and central to that is the reporting of earthquake sizes or magnitudes. These magnitudes are reported to the public, and thus their integrity needs to be ensured and their uncertainties thoroughly characterised. Magnitudes also are key input into seismic hazard analysis, a necessary step in the site characterisation of CO2 storage projects and other offshore infrastructure. This study compares and re-evaluates the magnitude estimates from a large database of North Sea seismicity compiled by the ACT3 project SHARP Storage. Magnitudes and their associated uncertainties are computed in a consistent manner and compared to the recordings reported by the regional agencies. Systematic differences are explored, and key sensitives are identified. These results will aid in the monitoring of seismicity in the North Sea, risk assessment for CO2 storage and other infrastructure, and the communication of seismic hazard.