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Paper No. 16-6, Geol. Society of |
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Presentation Time: 9:15 AM-9:30 AM |
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THE 1965 HOPE SLIDE, |
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EVANS,
Stephen G. and COUTURE, Réjean,
Geol Survey of 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 |