The North Nahanni rockslide (volume ~7.6 Mm3), Northwest Territories, Canada was triggered by the Mw 6.6 Nahanni earthquake of October 5, 1985; the earthquake was followed by a Mw 6.8 earthquake at approximately the same location in December 1985. The landslide occurred in a Palaeozoic carbonate sequence sliding along a thrust fault, which partly follows bedding and partly cuts across bedding in the source rock mass. The sliding surface thus consisted of two planes; the lower plane dipped at 20° while the upper plane dipped at 35°. Tilting table test of block samples from the sliding surface and field-measurements of large-scale sliding surface roughness indicated that the friction angle of the sliding surface was 24°. Two-dimensional (2D) slope stability analysis was performed for the slope before and during the earthquake using a discontinuum numerical technique (the distinct element UDEC code). 2D static slope stability analyses indicate that the simple-structured rock mass was marginally stable for sliding surface friction angles of 24°, and was constrained by the residual shear strength of the pre-sheared fault surface. 2D dynamic analyses of the co-seismic movements are conducted by comparing seismically-induced slope displacements computed in both UDEC and SLAMMER (a sliding-block analysis using real earthquake ground motions records of the December 1985 Nahanni earthquake). Both results are in good agreement. The dynamic analysis indicates that the slope becomes unstable for given seismic inputs that correspond closely to those estimated for the October earthquake at a friction angle of 24° for the sliding surface indicating that co-seismic rock slope movement was only possible on low-friction pre-sheared surfaces at residual strength. It also showed that the deformation behavior of the North Nahanni rock masses was dependent on the frequency of the seismic signals. Because the static slope stability analysis showed that the slope was close to instability prior to seismic shaking, the October 1985 Nahanni earthquake operated as a trigger event that accelerated the occurrence of the slide.