Earthquake swarms and clusters in stable continental regions: a case study from Northern Norway

Parts of northern Norway, located between the rifted Mid Norwegian margin and the Northern Scandinavian mountains, are seismically active despite being situated in a stable continental region. Previously, seismic swarms have been observed in different places along the coast, but detailed studies on the swarms could not yet be carried out due to sparse seismic networks. During the last decade, the number of seismic stations has increased significantly, allowing for a more detailed study of the seismicity. Here, we develop a machine-learning-based earthquake catalog from eleven years of continuous data (2010-2021) and combine it with the earthquake catalog from the Norwegian National Seismic Network. To improve accuracy, we perform relative earthquake relocation using differential times, and clustering analysis based on waveform cross-correlation. The relocation results reveal distinct clusters of possibly repeating events and several swarm sequences. A prominent seismic swarm occurred in the Jektvik area between 2014 – 2016 with the largest magnitude of M_L 3.2. We compare the spatio-temporal distribution, b-value, seismic moment rate, and seasonal variation of each sequence. The Jetkvik swarm exhibits a diffusive pattern, which together with a low VP anomaly found by a previous tomography study suggests that fluids may play a role in the source process. We find that the possibly repeating clusters are not as diffuse in space, and mostly spread along the vertical axis. These earthquake clusters may be attributed to fault intersections, and fluids may not be a major factor in their generation.