Debris avalanches are common occurrences in mountainous areas. In the Pacific Northwest, debris avalanches have been most common at the large volcanoes, particularly Mount St. Helens and Mount Rainier. Small avalanches can endanger people on climbing and hiking routes, and those large enough to reach valley bottoms can incorporate enough wet soil and stream water to become debris flows (mudflows). Large debris flows can cause considerable damage to communities in the stream and river valleys down which they travel.
As they break away from a slope and fall, debris avalanches often generate seismic signals that can be detected by seismographs. Many such seismic signals have been detected by the Pacific Northwest Seismic Network, which includes about 120 seismograph stations that are used to monitor earthquake activity in the Pacific Northwest. Seismologists can use the seismic signal generated by a debris avalanche to locate its source area within minutes, just as they do with an earthquake.
The capability of rapidly detecting and locating debris avalanches can be useful in directing response teams to the site of an avalanche, particularly at night and during periods of poor visibility. Seismologists at the University of Washington notified the National Park Service of large debris avalanches at Mount Rainier in August, 1989, and October, 1992; and the USGS office in Menlo Park, California reported a debris avalanche at Lassen Peak to the Park Service in August 1993.
The seismic signal can also be used to determine the sequence of events in a debris avalanche. Seismologists at the University of California-Berkeley and the USGS analyzed the seismic signal from a large debris avalanche at Yosemite National Park in July 1996, and determined that two separate sections of a cliff above the Yosemite Valley fell over a period of 14 seconds, and the second slab was about four times as large of the first. This analysis agrees with reports from eyewitnesses.
The Mount Rainier Seismicity page
The Yosemite Rockfall, compiled by Bob Uhrhammer - UC Berkeley
The Northern California Earthquake Data Center
1995, Seismic Detection of Debris Avalanches at
Mount Rainier and Other Cascade Volcanoes: Successes and Limits:
EOS, Transactions, American Geophysical Union,
v. 76, p.F651.
1996, The Release and Descent of the
July 1996 Happy Isles Rockfall, Yosemite National Park,
California, As Interpreted From NCSN Seismic Data:
EOS, Transactions, American Geophysical Union, v. 77, p.F651.