Cause
The landslide was caused by the presence of pre-existing tectonic structures (faults and shear zones) within the southwestern slope of Johnson Ridge. The lower parts of the slide scar are underlain by felsite sheets (which may have failed first) while the upper parts of the slide scar are underlain by highly jointed Paleozoic greenstone beds. Ongoing weathering and tectonic activity weakened the slide mass to the point where it had reached limiting equilibrium. Johnson Peak was the site of a previous smaller prehistoric rock-slide.
Just what triggered the 1965 landslide remains unclear; the two so-called earthquakes were likely too small to trigger the slide and thus the seismic events were more likely caused by the impact of the landslide masses on the opposite valley wall. Changes in groundwater condition, often a trigger for landslides, is not thought to have played a role in the Hope Slide as the slide occurred during a protracted period of sub-zero temperatures in the winter, though some have suggested that freezing of seepage exit points may have caused an increase in water pressure at the toe of the slide.
Read more about this topic: Hope Slide