Abstract

This study reviews seismograms from 14 rockfalls and avalanches of moderate to large volume (104 to 107 m3) at Mount St. Helens, Mount Adams, and Mount Rainier in the Cascade Range of Washington to investigate how variations in source volume, source materials, track materials, and failure modes affect avalanche seismicity. The largest signals were generated by rockfalls at Mount St. Helens and debris avalanches at Mount Rainier that initiated as block failures. At Mount St. Helens, several rockfalls that originated as block failures near the crater rim descended slopes with similar profiles, and appear to show a linear or nearly linear relation between source volume and seismogram amplitude. Those that fell over an extended period of time were poorly recorded at the reference seismic station and did not show this correlation.

Multiple seismograms from three of the four instrumentally recorded debris avalanches at Mount Rainier indicate a tendency of unstable slopes there to fail progressively, a pattern that has important implications for the mass-movement hazard at Mount Rainier. Seismic evidence shows that the Little Tahoma rock-fall avalanches in December 1963 occurred on 6 December, over a week earlier than previously reported.

Mass movements that have been seismically detected span the spectrum of rapid flowage events, from water surges and debris flows to dry rock, debris, ice, and snow avalanches. The success in identifying and in some cases locating these events in near real time demonstrates that local and regional seismic networks can play a valuable role in reducing hazards from large, rapid mass movements.

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