Repeating glacier quakes at Mount Rainier, October 18 - November 3, 2014

Since October 18, a sequence of repeating low-frequency seismic events has been occurring at Mount Rainier (see the webicorder plot below from station RCS). These events are small repeating glacierquakes and do not appear on the Pacific Northwest Seismic Network (http://pnsn.org) earthquake maps, as they are not traditional earthquakes and are not locatable by standard methods. Such sequences are often seen during the winter months at Mount Rainier, and researchers have shown that their occurrence correlates strongly with storms that drop heavy snow on the mountain. As of this writing, over 10,000 events have occurred in this ongoing sequence and the waveform of each event is virtually identical to its predecessor. At times the interval between "repeating events" has been remarkably consistent, averaging one event every 2-3 minutes, which creates spectacular patterns on webicorder plots such as the one included in this post.

Most of these repeating sequences occur high on the heavily glaciated north side of the mountain. The source locations have been found to migrate by a few feet per day, suggesting they move with the glacier as it slides and flows down the mountain. Usually glacier sliding is a relatively smooth and seismically silent process, but the sudden addition of a load of snow seems to temporarily change the sliding behavior of certain parts of the glaciers on Rainier from smooth to stick-slip sliding. Stick-slip sliding is the same process that generates regular earthquakes. Here it means that the sliding glacier gets stuck on a sticky spot, but as the ice around this spot continues to move, the sticky spot eventually breaks suddenly, generating a tiny earthquake. After this sudden slip, the glacier gets stuck again, and the process repeats. The regular intervals between events occur because the glacier slides down the mountain at a relatively stable velocity, which loads this sticky spot at a constant rate. In this model the repeating glacier quakes stop either when the glacier has adjusted to the new snow load and returns to smooth sliding, or when the sticky spot slides past where conditions were favorable for this type of behavior. Although different in detail, the broad similarity of these glacially generated earthquakes to those that can indicate volcanic unrest highlights the importance of understanding and characterizing the full range of seismic sources, which can be unique to each volcano being monitored.

You can also listen to an audio file of the 6 hours of data shown on the webicorder, which is sped up to be audible to the human ear.