Mount Adams is a potentially active volcano located 50 km east of Mount St. Helens, and is the second highest peak in Washington State. Although it has not had a major eruption in 1,400 years, it is not considered extinct. Thermal anomalies and gas emissions at the summit indicate that Mount Adams is still active.
Except for Mount Shasta, Mount Adams is the largest volcano in the Cascades by volume. Nearly all of its eruptions have consisted of lava flows with little to no tephra. Parts of the main cone consist of rock weakened by hydrothermal alteration. There is potential for large landslides of this altered rock to generate mud flows and lahars that flow far down valleys.
Below you will find an epicenter map and time-depth plot for a region around Mount Adams for the decade, approximately 2002 - 2012 that shows typical or background levels of earthquakes in this region. Note, that very few earthquake locate right at the volcano. Most seismicity is 20 km north of the volcano. However, there has been only one seismograph located right at the volcano and while the regional seismic network can reliably detect and locate earthquakes of magnitdue 1.5 and greater in this area very small earthquakes could occur without being detected.
- Red dots represent events occuring in the last month.
- Green dots represent events occuring in the last year, but more than a month ago.
- Circles represent older events.
- Triangles represent PNSN seismic stations. Station names are listed below each triangle.
- Black star indicates approximate position of Mt. Adams summit.
- Magnitude and distance scales appear at lower right.
Debris Avalanche at Mount Adams, Washington
Seismic recordings of the 10/20/97
Photo courtesy of Gifford Pinchot National Forest
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.
|GNW||Green Mtn., Washington||183|
|RAI||Trojan Power Plant, Oregon||110|
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.