Slip-weakening friction controls coseismic displacements in a 3D fracture network: Implications for the long-term safety of nuclear waste repositories

During the long-term operational lifespan of nuclear waste repositories in crystalline rock formations, large earthquakes along nearby seismogenic fault zones may occur, coseismically triggering shear displacements of secondary fractures within the respository site. In addition, these secondary fractures that may be associated with slip-weakening friction could accommodate significant slip instabilities and large shear displacements. A cumulative shear displacement exceeding 50 mm could affect the integrity of waste canisters, potentially resulting in the escape of hazardous radionuclides into the groundwater system. To investigate this problem, we develop a novel 3D seismo-mechanical model to simulate the transient rupture of a primary seismogenic fault zone and coseismic slips in a network of secondary fractures located around the primary fault. A plausible postglacial earthquake scenario is studied, where the rupture along the seismogenic fault propagates outward from a predefined hypocenter, with the resulting static stress changes and dynamic ground vibrations captured. We explore different cases with secondary fractures having different degrees of slip-weakening friction, which is found to strongly control the spatial decay of coseismic fracture displacements in the system. The findings derived from our study have significant implications for assessing the long-term safety of nuclear waste repositories in faulted and fractured crystalline rocks.