Comparison of earthquake clusters in a stable continental region; a case study from Nordland, Northern Norway

Nordland, Northern Norway, is a seismically active stable continental region. Along its coast, clusters of small earthquakes controlled by local extensional stresses are observed. In this study, we present a comparison between two adjacent seismically active areas along the Nordland coast: Jektvik and Rana, which have distinct spatiotemporal patterns. The seismicity in Jektvik, which presents a swarm-like behavior, shows outward activity progression from its center hinting at triggering between earthquake clusters. In contrast, the seismicity in Rana, where swarms are also observed, does not exhibit such pattern. Earthquakes in the Rana cluster occur within isolated spots and show repeating earthquake behavior. Singular spectrum analysis shows that seismicity in Jektvik has a dominant annual periodicity and is modulated by hydrological load, which is also observable on Global Navigation Satellite Systems stations. Although hydrological load changes also affect Rana, its seismicity does not exhibit an annual periodicity. We hypothesize that the Jektvik seismicity occurs within a fluid-rich fracture system that is affected by hydrological modulation, whereas the Rana seismicity occurs within fault irregularities, which accumulate stress and rupture repeatedly. This study presents a case where adjacent areas within an intraplate setting can have significantly different seismogenic behaviors.