U.S. Geological Survey
By Robert D. Norris1
Local networks of short-period seismograph stations have been successfully used to detect and locate debris and ice avalanches at Mount Rainier and other volcanoes in the Cascade Range. The best detected debris avalanches had source volumes greater than 106 m3, originated as block failures on steep slopes, and had runout distances of 1 km or greater. Seismic signals from such avalanches have characteristic teardrop-shaped signal envelopes, and four debris avalanches of this type that occurred at Mount St. Helens in 1981 showed a linear relation between the amplitude of the signal and the source volume of the avalanche. A similar linear relation between signal amplitude and volume was reported for pyroclastic flows from the compound dacite dome extruded at the Mount Unzen volcano in Japan between 1991 and 1993, which originated as block failures from the dome. Seismic records were used to estimate a volume of 10,000 m3 for a small rockfall at Lassen Peak in August 1993; this was in good agreement with the estimate of 12,000 m3 made from field observations. The largest seismically recorded debris avalanches at Mount Rainier have been more complex than the simple block failures discussed above, and their seismic signals do not correlate as well with volume. However, debris avalanches at Mount Rainier in 1989 and 1992 were seismically detected, located, and reported to officials at Mount Rainier National Park. The seismically determined locations of the source areas were within 1 km of the actual locations. The ability to locate source areas of large debris avalanches in near real-time can reduce the hazards posed by these events. Such locations can be used to direct rescue operations to the runout zones, and in the case of large avalanches, identify the valleys that may be affected by subsequent debris flows.
1 U.S. Geological Survey, Box 351650, Geophysics Program, University of Washington, Seattle, Washington, 98195-1650