Assessing data requirements for stress drop and high frequency attenuation studies: insights from the North Sea Horda Platform

We assess the methodological limits of estimating earthquake source (stress drop) and high-frequency energy attenuation (κ) characteristics by analysing a dataset of seismic waveforms recorded between 2000 and 2022 from the Horda platform region of the North Sea. We observe a bias in the source spectra with strong, but artificial, magnitude dependence of stress drop. Synthetic tests demonstrate that this bias could be largely eliminated with increased station coverage and recording bandwidth. The κ results do not exhibit the expected increase with distance, which is believed to be due to pronounced lateral and vertical variations in the anelastic attenuation parameter Q across the region. These variations likely reflect contrasts in sediment thickness and the transition from the offshore sedimentary basin with extensional tectonics to the onshore Scandinavian Shield composed of crystalline bedrock. Preliminary κ values within the first 50–100 km range from 0.009–0.023 s, consistent with weathered to hard-rock site conditions and in alignment with other estimates from stable continental regions. Our analysis reveals the challenges that sparse and uneven station distribution, limited recording bandwidth, and strong geological heterogeneity pose for robust parameter estimation. Improved network coverage—particularly through the deployment of ocean-bottom sensors in offshore areas—and higher sampling rates would substantially enhance the reliability and spatial consistency of κ and stress-drop estimates. Overall, our findings underscore the importance of station coverage, bandwidth, and regional geology, and have implications for seismic hazard assessment in offshore and data-limited environments.