Workreport 2018-20



Brittle Fault Systems of the ONKALO Underground Research Facility


Nordbäck, N., Mattila, J.



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This study aims to improve the characterisation of the brittle structures intersecting the ONKALO underground research facility, located at Olkiluoto. The objective is to gain a better understanding of the structural evolution of the site, with the ultimate goal of enhancing the modelling and understanding of the reactivation potential of different fault systems and associated seismic hazard.

The following brittle fault zones intersecting ONKALO were selected for this study: BFZ019c, BFZ020a, BFZ020b, BFZ045, BFZ084, BFZ100, BFZ297, BFZ300 and BFZ346a. The previously available descriptions of these fault zone intersections were enhanced by the compilation of all the existing mapping data, analysis of fault trace data, systematically taken tunnel photographs and by additional detailed fault zone mappings in the tunnel. Based on fault zone orientation, properties, kinematics and crosscutting relationships the fault zones were classified into four different fault systems: BFZ100, BFZ300, E˗W and Low-angle fault systems. The properties and effects of ductile precursors and the properties of core and damage zone were described for each characterised fault zone and fault system. The BFZ100 fault system is post-dated by the E-W and Low-angle fault systems while the BFZ300 fault system probably formed simultaneously or shortly after the BFZ100 fault system. Due to the observed mineralogical characteristics and hydrothermal alterations, it is likely that the E-W fault system predates the Low-angle fault system which appears be the youngest of the four fault systems.

As an additional component of this report, the diameters for different single fracture types were defined through modelling. Based on extensive mapping dataset on ONKALO and the analysis of high-confidence cases, single fractures represent structures less than 130 m in diameter. So far, all the observed or modelled brittle structures which are over 200 m in diameter, are associated with damage zones and can therefore be classified as deformation zones.


Brittle deformation, fault, fault zone, fault system, structural geology, ONKALO, Olkiluoto


WR 2018-20_web (pdf) (13.7 MB)


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