Spectrograms - Glacier Noise

 Because some of our volcanoes have ice cover (Mount Baker, Glacier Peak, Mount Rainier, Mount Adams and Mount Hood), we need to be aware of the seismic signals generated by glacier motions.  Seismic events generated by glaciers may occur for a number of reasons: crevassing, slip at the base of a glacier, thermal fracture, serac collapse, etc..  Ice-quakes can look very similar to long-period (LP) volcanic earthquakes, with significant energy at frequencies less than 4 Hz.  In some cases, ice-quakes show up only on the closest stations (i.e., VALT at Mount St. Helens) while in other cases (such as at Mount Rainier), they may be visible across the whole geographic group.  Recently, repeating series of very small ice-quakes at Mount Rainier have been recognized as being triggered by snowstorms. 


Series of medium large glacier events at Mount Rainier.  While the signals look the strongest on OBSR, we know that this station has a very strong response to glacier events which means the sources are not necessarily closer to this station than others.


A series of very small glacier events at Mount Rainier.  Note that most of these show only on the three stations (STAR, RCM and RCS) that are high on the volcano near active glaciers. Station RCS located near the largest and very active glacier (Emmons) usually shows the most such signals.