Understanding long-term slope deformation for stability assessment of rock slopes: the case of the oppstadhornet rockslide, norway

Abstract

The Oppstadhornet rockslide is a 10 Mm3 slide that occurs on the island of Otrøya in westernmost Norway. Terrestrial cosmogenic nuclide dating indicates that the Oppstadhornet rockslide became active ca. 16.6-14.2 kyrs ago when the retreat of the Scandinavian ice sheet exposed the island from the continental ice cover. Sliding along the main sliding surface was active during the late Pleistocene and Holocene. Our data suggest that the paleo-slip rate in the Late Pleistocene was slightly faster than in the Holocene however large uncertainty margins ask for care with this interpretation. Present day displacement rates of ca. 2 mm/year measured with differential Global Navigation Satellite Systems are similar to the paleoslip rates, however they vary over the entire rockslide body and at several locations after 10 years we could not yet measure any significant displacement. The long-term activity of this rockslide suggests that - in contrast to dynamic stability models - moderate earthquake shaking with a recurrence time of 475 years will not cause the Oppstadhornet rock slope to fail.