Modelling coseismic displacements of fracture systems in crystalline rock during large earthquakes: Implications for the safety of nuclear waste repositories

Crystalline rock has been tested/selected by many countries for developing underground nuclear waste repositories at similar to 500 m depth to achieve geological isolation of high-level, long-lived radioactive waste. During the assessment period of up to one million years, large earthquakes may occur around the repository and trigger coseismic displacements along secondary fractures, jeopardising the integrity of the buffer-waste canister system. It is, therefore, of great importance to understand the coseismic behaviour of the repository site during large earthquakes. In this paper, we develop a finite element method-based seismo-mechanical model to simulate the response of fractured rocks subject to both in-situ stresses and seismic activities. The model can well capture the fracture displacement behaviour under dynamic loadings involving alternating regimes of contact loss, partial slip and total sliding. We model the earthquake-induced displacement field according to the seismic source theory in combination with a generic source time function. We apply the model to the nuclear waste repository site at Forsmark, Sweden, and analyse the coseismic responses of both single fractures and fracture networks during a potential post-glacial earthquake with a moment magnitude of M-w = 5.6. The shear dislocation of a single fracture is strongly dependent on the fracture length and dip angle, while the displacement pattern of a fracture network is dominated by its backbone structures. We observe that significant coseismic shear displacement occurs if the fractured rock is close to the hypocentre and located in the dilational quadrant of the primary fault due to reduced shear resistance. However, the earthquake-induced shear displacement decreases drastically with the increased distance to the hypocentre and a distance of similar to 700 m may be needed for a fracture up to 100 m long to not displace beyond 5 cm. If two repeated earthquakes would occur, a distance of similar to 1200 m may be necessary.

Automated summary

This paper investigates the seismic behavior of a nuclear waste repository site in Forsmark, Sweden, using a seismo-mechanical model based on the finite element method. The study shows that the shear displacement of a single fracture is strongly influenced by its length and dip angle, while the displacement pattern of a fracture network is dominated by its backbone structures. The earthquake-induced shear displacement decreases significantly with distance to the hypocentre, with a distance of approximately 700 meters needed for a fracture up to 100 meters long to not displace beyond 5 centimeters.