The Crustal Structure in the Hälsingland Region, Central Sweden, and Implications for Seismotectonics

The Hälsingland region in central Sweden, within the Proterozoic Fennoscandian Shield, lies near a transition from relatively shallow Moho depths (<45 km) in the west to deeper Moho depths (>50 km) in the east. The region lies close to a broad east-west transition in lower-crustal seismic velocities reported in regional models. Furthermore, it is a seismically active region, host the southernmost surface scarp of a glacially triggered fault identified in Sweden, and therefore, represents a region of elevated seismic hazard. The reason for the elevated seismicity in the region is unknown, but it has been proposed that it is hosted by the postulated Hudiksvall fault crossing the region with an NNE-SSW orientation.

In this study, we employ inversion of teleseismic receiver functions (RFs) and apparent S-wave velocity (Vs_app) for 45 available permanent and temporary seismological stations between approximately 60–63°N to map key crustal discontinuities in the region, including the boundary between upper and lower crust, the thickness of high-velocity lower crust (HVLC), and the Moho depth.

We compare the crustal model to Bouguer gravity, aeromagnetic data, and seismicity, allowing us to test candidate structures that may correlate with the occurrence of earthquakes, such as the reported NNE–SSW-trending geophysical anomaly that has been interpreted as a possible crustal boundary and the postulated NW-dipping, NNE-striking Hudiksvall fault previously inferred from 3D geophysical-geological modelling. On a regional scale, our model is consistent with previous crustal models, showing thickening of the crust to the east and south. However, the increased station density due to new data from temporary stations in the area reveals finer-scale details. Comparison with the Swedish National Seismic Network earthquake catalogue shows that clusters of seismicity in the study area tend to occur preferentially near depth changes in crustal discontinuities. Most prominently, the seismicity localizes along the greatest change in crustal thickness, as well as upper crustal thickness while the relationship with lower crustal thickness is more complex. Our preliminary analysis suggests that seismicity is focused along major changes in crustal architecture. Whether these changes in crustal structure are related to the postulated Hudiksvall Fault can currently not be determined.