Unusual Seismic Recordings from Mount Rainier Glaciers

This is Steve Malone with the Pacific Northwest Seismic Network located at the University of Washington in Seattle. We operate seismographs throughout the Pacific Northwest and have three located high on Mount Rainier. We often record seismic events from all of our glacier-clad volcanoes that we associate with glacier motion, i.e. "ice-quakes." However, since about May 20, we have detected a strange set of these events coming from the upper Winthrop Glacier. We are calling these small events "clones" because the seismic waveforms from one event are near-duplicates of those from other events indicating a repeating source. They also seem to occur at very regular intervals.


The interval between events is often as short as every 3 minutes but changes from time to time and has been as much as 15 minutes between events. We think that their magnitude (on the Richter scale) is about M = -1 (i.e., 8 orders of magnitude smaller than the Nisqually earthquake of 2001).



So, what are these puppies? We think they represent small periodic slips at the bed of the glacier. Perhaps there is a large rock embedded in the bottom of the glacier and as the glacier moves it scrapes this rock along the bed, only a few mm in each slip. But why are they so regular in time? Maybe water pools up-hill of the rock until it slightly lifts the glacier allowing the rock to more easily slip and this then drains that small pool of water starting the process over. We think that water has an important influence on glacier sliding but don't understand the mechanism very well.


How can you help? Anyone climbing Rainier on the east side (upper Emmons or Winthrop Glacier routes) may see or hear things that would help us pin these suckers down. Please let me know of anything you think may be out of the ordinary (sounds, sights, feelings???). Particularly those of you who have been in this area before and can compare what may be different from previous climbs. Our best guess where these originate (based on stacking 4000 individual events to get the best relative seismic wave arrival times at six seismic stations and using a 1-D seismic velocity model with station elevation corrections, blah blah blah, other scientific mumbo-jumbo) puts the location at 46.85950 north 121.7610 west (i.e., 2.5 km WSW of Camp Schurman or 3.4 km NNW of Camp Muir or about 600 meters up from the top of Russell Cliffs).

To see these suckers yourself check out our "webicorders" at: http://www.pnsn.org/WEBICORDER/VOLC
and click on the date-time for one of the high Rainier stations (RCS, RCM, STAR). The small blips that have about the same size and shape are our "clones".

Send email to: steve@ess.washington.edu or give me a call (206-685-3811)


Steve Malone