Revised borehole and seismic data map stress rotation across Switzerland’s Alpine foreland

A revised observational compilation of present-day maximum horizontal stress, SHmax, orientations across northern Switzerland and neighbouring regions reveals a gradual counterclockwise rotation of about 50°, from roughly north–south in the northeast to northwest–southeast in the southwest of the Alpine foreland. The pattern follows the regional topographic gradient, suggesting that gravitational potential energy associated with the Alps is a primary influence, although plate-boundary forces cannot be ruled out.

Researchers combined newly interpreted borehole observations, earthquake focal mechanisms and records from the World Stress Map database release 2016. The resulting dataset contains 948 SHmax-orientation records, compared with 596 in the earlier release, and every record was evaluated using revised analysis guidelines and the World Stress Map quality-ranking scheme issued in 2025.

The borehole component involved re-analysis of 32 deep wells in the Swiss Molasse Basin. The team interpreted more than 30 km of image and caliper logs and picked 2,246 stress-related features, including borehole breakouts, drilling-induced tensile fractures and petal centreline fractures. These features had a combined picked length of 4,472 m and yielded 56 SHmax-orientation records after features of the same type within individual wells or comparable depth sections were aggregated.

Additional constraints came from 139 microhydraulic fracturing tests in eight of the nine wells drilled during Nagra’s comprehensive drilling campaign. Orientations of 105 induced fractures were used to derive eight SHmax records. The study also incorporated 704 earthquake focal mechanisms compiled by the Swiss Seismological Service for events from March 1977 through August 2023, deriving SHmax orientations from these mechanisms.

For the regional map, 535 records were assigned A- to C-quality and treated as reliable within ±25°. Within Switzerland, the 296 A- to C-quality records averaged one per 140 km², while individual records commonly had standard deviations of ±20–25°. These figures limit lateral resolution: small rotations across distances of a few tens of kilometres, as well as many local effects of faults or rock-property contrasts, could not be resolved reliably.

The researchers also evaluated whether mechanical decoupling across the Middle Muschelkalk had separated the orientations in Mesozoic sedimentary rocks from those in the underlying basement. In a northern Swiss subset, the mean SHmax orientation was 166° ± 12° for 43 records in sediments and 159° ± 22° for 45 records in basement. The comparison did not reveal a regional-scale rotation consistent with decoupling; the authors noted that such decoupling may either leave no detectable imprint in SHmax orientation or may not operate regionally.

Several individual boreholes showed SHmax rotations exceeding 20° over intervals of tens to hundreds of metres. The departures were associated with possible fault effects, local topography or horizontal mechanical decoupling, but they occurred in individual wells rather than as a systematic regional or depth-related pattern. The authors therefore found no general or systematic change in SHmax orientation with depth.

Stress-regime classifications require separate caution. They are based mainly on earthquake focal mechanisms and predominantly represent crust deeper than 5 km, whereas many borehole observations sample shallower levels. Map-view classifications therefore should not be treated as depth-independent, and the study indicates that stress regimes can change with depth in the Alpine foreland.

Subject of Research: Revised mapping of maximum horizontal stress orientation in Switzerland’s Molasse Basin and Alpine foreland

Article Title: The present-day crustal stress field of the Molasse Basin in Switzerland

Article References: Heidbach, O., Reinecker, J., Diehl, T., Desroches, J., Ziegler, M. O., Reiter, K., Vietor, T., & Giger, S. B. (2025). The present-day crustal stress field of the Molasse Basin in Switzerland. Swiss Journal of Geosciences, 118(1), Article 17. https://doi.org/10.1186/s00015-025-00487-6

Image Credits: AI Generated

DOI: 10.1186/s00015-025-00487-6

Keywords: Swiss Molasse Basin, crustal stress, maximum horizontal stress, borehole logging, earthquake focal mechanisms, microhydraulic fracturing, World Stress Map, Alpine foreland, tectonic stress, present-day, crustal, stress