At 12:31 AM PDT on October 20, a large mass of rock fell from the Castle, a prominent rock formation east of the summit of Mount Adams. This avalanche was recorded on many seismograph stations of the Pacific Northwest Seismic Network in southern Washington and northern Oregon, and some examples of the seismic signal from five broad-band stations are shown below. The table below the signals indicates where each of the stations are located, and their distance from the avalanche source area on Mount Adams.

Station	Location	Distance, km
COR	Corvallis, Oregon	230
GNW	Green Mtn., Washington	183
LON	Longmire, Washington	66
RAI	Trojan Power Plant, Oregon	110
RWW	Satsop, Washington	180

The total time window shown is 195 seconds; the signals shown have durations between 80 and 110 seconds. The total time duration of motion for this avalanche is estimated to be about 6 minutes, based on recordings from the closest seismograph station (not shown). The elongated, spindle-like shape of the signals is typical of large avalanches; this characteristic shape helps seismologists to differentiate them from earthquakes, which typically display one or more sharp peaks in amplitude, after which the signal decreases smoothly back to the level of background "noise" typically seen at each station.

The debris avalanche on October 20 followed an earlier sequence of debris avalanches from the Avalanche Glacier on the southwest side of Mount Adams. These avalanches occurred between August 29 and 31, and consisted mostly of ice from the Avalanche Glacier.

For more images and detailed information on the Mount Adams avalanches, see the web pages produced by the Cascades Volcano Observatory.

For more information on the seismic signals generated by large avalanches, see Seismic detection and location of debris avalanches.