Crustal Stress Field Variations and Fault Reactivation Potential in the Lower Rhine Graben and its Adjacent Regions

Characterizing the crustal stress state and its spatial variability is essential for the safe and sustainable development of structurally controlled deep geothermal systems. The seismically active Lower Rhine Graben (LRG), spanning parts of Germany, Belgium, and the Netherlands, is a promising target for geothermal exploitation in fault-controlled, karstified carbonate reservoirs of Lower Carboniferous and Devonian age. However, at drillable depths, the stress field remains poorly constrained, raising concerns regarding fault reactivation and induced seismicity, as highlighted by moderate induced events at recent deep geothermal projects in Belgium and the Netherlands.

We present a quality-rated crustal stress database for the LRG and adjacent regions, integrating legacy and newly acquired stress indicators from earthquakes and recently drilled exploratory boreholes. Stress tensor inversion was performed using recent earthquake focal mechanisms, while borehole-based indicators from hydraulic fracturing tests and borehole deformation analyses provided direct constraints on stress orientations and absolute stress magnitudes at reservoir-relevant depths. These data were combined with publicly available present-day stress indicators from existing databases, and interpolated onto a regular 0.1° grid to generate a gridded stress field capturing regional-scale, long-wavelength variability.

The spatially variable stress field was integrated with mapped major faults to evaluate their reactivation potential by assigning stress orientations to individual fault segments. Our results indicate a clockwise rotation of the maximum horizontal stress from WNW–ESE in the Hohe Venn area west of the graben to NNW–SSE in the Rhenish Massif to the east. At geothermal reservoir depths, NW–SE-striking normal faults show elevated potential for shear reactivation and dilation, whereas NE–SW-striking thrust faults exhibit low potential for both mechanisms.

By integrating more than 135 stress indicators into a spatially resolved fault reactivation analysis, this study substantially increases stress data coverage in the region and provides quantitative constraints on fault stability and seismic hazard relevant for geothermal development, supporting site selection and risk-informed reservoir management in the tectonically active LRG.