Volcano collapse promoted by progressive strength reduction: new data from Mount St. Helens |
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| Authors: | Mark E Reid Terry E C Keith Robert E Kayen Neal R Iverson Richard M Iverson Dianne L Brien |
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| Institution: | (1) U.S. Geological Survey, 345 Middlefield Rd., Menlo Park, CA, 94025, USA;(2) Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011, USA;(3) U.S. Geological Survey, 1300 SE Cardinal Ct. #100, Vancouver, WA 98683, USA |
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| Abstract: | Rock shear strength plays a fundamental role in volcano flank collapse, yet pertinent data from modern collapse surfaces are
rare. Using samples collected from the inferred failure surface of the massive 1980 collapse of Mount St. Helens (MSH), we
determined rock shear strength via laboratory tests designed to mimic conditions in the pre-collapse edifice. We observed
that the 1980 failure shear surfaces formed primarily in pervasively shattered older dome rocks; failure was not localized
in sloping volcanic strata or in weak, hydrothermally altered rocks. Our test results show that rock shear strength under
large confining stresses is reduced ∼20% as a result of large quasi-static shear strain, as preceded the 1980 collapse of
MSH. Using quasi-3D slope-stability modeling, we demonstrate that this mechanical weakening could have provoked edifice collapse,
even in the absence of transiently elevated pore-fluid pressures or earthquake ground shaking. Progressive strength reduction
could promote collapses at other volcanic edifices. |
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