The present paper describes a method to calculate fracture shear displacements occurring as a result of the stress waves and the static stress redistribution following a seismic slip on a nearby fault. Such secondary fracture shear displacements can theoretically, if large enough, damage intersected canisters containing spent nuclear fuel. The method is applied to a type of seismic event that is, or may be, of concern for the long term safety of a KBS-3 nuclear fuel repository: namely, endglacial earthquakes of magnitude 6 and larger. The numerical scheme used to simulate rupture initiation and propagation is described and illustrated. Result examples are given that show that secondary shear displacements on 300 m diameter fractures will be smaller than the damage criterion now applied by the Swedish Nuclear Fuel and Waste Management Co (SKB), provided that the fracture center is at a distance of 200 m or larger from the fault plane. The relevance of the rupture representation is discussed based on comparison with slip velocity records from a recent, large and well-documented crustal earthquake and with stress drop estimates made for large endglacial faults observed in Lapland, northern Sweden.