The Børglum fault, Sorgenfrei-Tornquist Zone, northern Denmark: a natural laboratory to investigate a hidden active intra-plate fault

Intra-plate faults are a special challenge in seismology, because of the long intervals between individual seismic events and the fact that such faults are often hidden below young sediments. This makes such faults difficult to detect and thus they can be the source of unexpected and fatal earthquakes. The Børglum fault is located in a slowly deforming area in northern Denmark and represents one of the northern boundary faults of the Sorgenfrei-Tornquist Zone. With a length of at least 250 km, it is capable to produce significant seismic events. Previous studies indicated that the Børglum fault is seismically active and this fuelled the demand for further analysis of the fault structure and its seismic hazard potential. Due to excellent coastal outcrops and available high-resolution DEMs, the Børglum fault is a perfect natural laboratory to analyse a hidden active fault. We present a multi-method approach based on outcrop analyses, shear-wave seismic reflection surveys, DEM analysis and numerical simulations of deglaciation-induced Coulomb failure stress change. The 2D seismic surveys show that the analysed segment of the Børglum fault is a complex fault system with a strike-slip component. This interpretation is based on positive flower structures on the seismic surveys, the presence of elongated mini-basins and the geometry of the drainage pattern in the study area. On the basis of soft-sediment deformation structures and disaggregation bands developed in Late Pleniglacial to Lateglacial marine and lacustrine deposits, we derive repeated phases of fault activity with earthquake magnitudes of up to M=7. The geometry of the drainage pattern in the study area indicates a close relationship between fault activity and topography. Based on the timing of fault activity and results from numerical simulations of deglaciation-related lithospheric stress build-up, it is likely that the Børglum fault is a glacially triggered fault and that the analysed part of the Sorgenfrei-Tornquist Zone is susceptible to glacially triggered fault reactivation.