The collapse of Mount St. Helens (United States) on 18 May 1980 is one of the only incidents of its kind that was visually witnessed and instrumentally recorded. Previous analyses determined that the northern flank of the mountain failed, which resulted in a rockslide, and disintegrated during its mobilization to form a blocky facies and hummocky terrain in the downslope region. Gary Rosenquist's iconic photographs of the initial 18 May collapse were here analyzed by using a modern photogrammetric method to track portions of the moving flank across the photographs in the sequence. Thirty years after the 1980 Mount St. Helens rockslide, the digital image correlation technique enabled a precise investigation of the associated displacement vectors and strain. A listric basal detachment and several associated faults at the lateral edge of the rockslide were identified. In addition, the heterogeneous movements and a number of shear zones were detected within the sliding block, which had previously been assumed to be cohesive. The results of my study demonstrate the value of using optical images for strain analyses of active volcanoes—even decades after recording.

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