THE 1965 HOPE SLIDE, BRITISH COLUMBIA; CATASTROPHIC FAILURE OF A SAGGING ROCK SLOPE

As David Varnes noted, sagging rock slopes are common in mountain landscapes. A challenge to landslide specialists lies in the evaluation of the catastrophic potential of such slopes. The 1965 Hope Slide involved the total movement of about 48 x 10⁶ m³ of rock from the slopes of Johnson Peak which had undergone significant gravitational deformation in postglacial time, including a large prehistoric rock avalanche which occurred at the site ca. 10,000 y. B.P. Pre-1965 aerial photographs show a series of linear trenches near the headscarp of the 1965 event and similar manifestations of slope deformation to the southeast of the landslide. Trenches recently dug into these features have allowed a reconstruction of the deformation history of the failed slope during the Holocene. Trench stratigraphy shows that deformation of the trench fill has been not been episodic but more of a gradual process. A previously published analysis of seismograph records from January 9, 1965 suggested that the Hope Slide event probably occurred as two rock avalanches separated by about 3 hours. The first, involving the failure of the headscarp of the prehistoric rock avalanche, was controlled mainly by gouge filled lithologic contacts between felsite sheets, dipping less than the slope inclination, and greenstone, which dips at or a little steeper than the slope. Debuttressing of the upper slope by the first slide event led to a complex detachment along steeply dipping joints within the greenstone three hours later. With the resolution of the seismic events recorded at the time of the rockslide, no seismic or hydrometeorological trigger is discernible for the 1965 events. It is suggested that progressive long-term deformation of the slopes of the southwest flanks of Johnson Peak caused the stability of the slope to deteriorate to a point where the 1965 events occurred. Thus we conclude that the Hope Slide was a catastrophic termination of very long term non-episodic mountain slope deformation.