Geological long-term evolution: Neotectonics
Nagra (2024): Geological long-term evolution: Neotectonics. Nagra Arbeitsbericht NAB 24-13.
pdf NAB 24-13 Geological long-term evolution: Neotectonics(17.17 MB)
In general, neotectonics involves recent or active crustal deformation, the forces that are responsible for the crustal state of stress and its relation to plate tectonic processes. The temporal borders are often set to the Neogene and Quaternary or, generally, since the last plate-tectonic reorganisation. Accordingly, neotectonics considers the deformation that was/is active under the prevailing stress regime.
This report provides a basis for the assessment of effects of seismicity and faulting on the deep-geological repository. It also provides argumentation for scenarios of future deformation.
During SGT Stage 3, the geoscientific database for a better assessment of neotectonic scenarios was improved by a comprehensive scientific field investigation program. For a detailed understanding of the subsurface, ~ 200 km2 of 3D-seismic reflection data have been acquired in the three siting regions. Within the analysis of the 3D-seismic surveys it was determined that faults with a minimum vertical throw of 5 m were detected. Monitoring using the seismic weak motion network, as well as the Nagra-owned GNSS network was continued. Longer operational phases of such instrumental networks allow a better signal-to-noise ratio and more complete datasets. In addition, new precise levelling measurements around the Bözberg area were integrated. The digital model of the Base Quaternary or Top bedrock surface was updated, including mapping and an uncertainty assessment of the Deckenschotter basis. This allowed a systematic evaluation of potential tectonic offsets across regional fault zones, including determination of a maximum possible rate since Deckenschotter deposition.
Present-day seismicity is sparse within all siting regions, with the exception of the Eglisau cluster of shallow, low-magnitude events. Vertical velocities derived from precise levelling show a decreasing gradient from south to north at the longitude of JO and a slight gradient in NL, but less in ZNO. Within the realm of the siting regions, the majority of vertical velocities are between 0.2 and 0.4 mm/yr with respect to a stable-Europe framework. No evidence of differential horizontal movement across regional fault zones in horizontal velocities from GNSS was observed across Northern Switzerland.
Detailed inspection of the landscape provided few indications of possible Quaternary deformation. If such deformation happened, the inferred deformation rates must be low, and deformation appears to be associated with inherited regional fault zones.
Future deformation rates are expected to be low and comparable to the rates derived from instrumental records and geological rates from the last few million years. Inherited faults act as zones of weakness and thus possible future strain will be preferentially localised along them. The rock volume in between these larger faults may also be strained, as evidenced by the small-scale faults observed in the retrieved core material, but the overall strain will be substantially lower than in the regional fault zones. Accordingly, positioning the long-term safety-relevant repository elements outside regional fault zones will minimise any neotectonic effects.