A study of the intraplate Hälsingland earthquake cluster in central Sweden

The seismic activity of the Hälsingland region on the northeast coast of Sweden, about 270 km north of Stockholm, presents an intriguing case study of intraplate seismicity in a low-seismicity setting. The activity consists of an elongated earthquake cluster, about 100 km in length, stretching from inland in the southwest to off-shore in the northeast, at approximately 45 degree angle to the coast line. In contrast to most of the main earthquake clusters in Sweden, this Hälsingland earthquake cluster has not yet been associated with any distinct geological feature, such as a postglacial fault or a major deformation zone. In 2021, a network consisting of thirteen temporary broadband seismic stations was installed in the Hälsingland area, as part of a joint study between Uppsala University and the Geological Survey of Sweden, with the aim of better understanding the drivers of the Hälsingland seismicity. The addition of the temporary network has increased detectability by almost a factor six compared with the permanent seismic network operated by the Swedish National Seismic Network. With the aid of an automatic processing system and an in-house, machine learning based classification system, we have extracted and manually analyzed a catalog of about 900 earthquakes, and 50 industrial blasts in the area, with origin times between September, 2021 and December, 2024. The analyzed earthquakes have local magnitudes ranging from -0.2 to 2.9. This presentation will focus on the preliminary seismic results from the study. We derive a new, improved, one-dimensional seismic velocity model for the Hälsingland area, using data from analyzed blasts, and re-locate the analyzed earthquakes in this improved velocity model. We identify the presence of a swarm of highly similar earthquakes, closely located in space, using waveform cross-correlation of analyzed earthquakes and a subsequent cross-correlation search through the continuous waveform data. Finally, we perform relative re-locations of the analyzed earthquakes, based on differential travel times, and calculate focal mechanisms to study potentially activated fault planes.