Abstract

A major rock avalanche (estimated volume, 5–7 × 106 m3) was triggered by the October 1985 North Nahanni earthquake (Ms = 6.6). The landslide is located at 62.27°N, 124.18°W, 23 km southwest of Carlson Lake, in an uninhabited part of the District of Mackenzie, N.W.T. It occurred within 10 km of the epicentre. The rock avalanche detached from the northeast flank of a ridge located along the axis of the English Chief Anticline and involved massively bedded limestones of the Middle Devonian Nahanni Formation. Whilst most of the landslide mass was deposited at the foot of the original slope, a mobile tongue of debris turned at right angles to the direction of initial sliding and travelled down valley, coming to rest on a slope of about 5°. The distal margin of the mobile tongue is approximately 1.55 km from the top of the rupture surface. The fahrböschung of the landslide is estimated to be 14.5°, and the rock avalanche exhibits exceptional mobility in relation to its volume. The rupture surface is well exposed and is a combination of planar structural elements, viz. a bedding surface and a steeply dipping fault plane. The marked streaming behaviour in the mobile tongue raises the question of a possible link between this phenomenon and intense strong ground motion during the earthquake.

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