Sequences of small shallow repeating earthquakes occur often at Mount Rainier but escaped our detection until recently. Both glaciers and volcanoes can generate repeating earthquakes, and the seismic signals of the two can be virtually indistinguishable, but Mount Rainier has the highest at-risk population of any US volcano so a correct interpretation of the seismic source is critical. Our analysis of data from permanent seismic and weather stations on the mountain suggests that these sequences are generated by glaciers reacting to snow-loading during intense storm events and are not signs of a reawakening volcano.
To test this hypothesis, in 2012 we used a ground-based portable radar interferometer (GPRI) to monitor glacier velocities before, during, and after a snowstorm (and associated repeating earthquake swarms appeared) to test whether a small additional added load of snow could change the sliding behavior of a glacier. An additional benefit of this work is we have collected spatially continuous velocity measurements of glacier motion for the Nisqually, Wilson, Emmons, and Winthrop glaciers that can lend insight to the dynamics of alpine glaciers.
Status and Trend
Though we now know that the repeating earthquakes we are investigating have occurred every year for as long as we have continuous seismic data, this type of seismic activity has not previously been documented in detail at Mount Rainier because the earthquakes are too small to be detected automatically by the seismic network. The sources of the repeating earthquakes are primarily located at high glacier-covered elevations (>3 km above sea level). The waveforms of these earthquakes are lower frequency than regular earthquakes of comparable size and closely resemble volcanic earthquakes from a low-frequency source such as resonance of fluid (e.g. gases, magma, or geothermal fluids) in cracks. The strongest indication that the source of these repeating earthquakes is glacial and not volcanic is that the onset of repeating earthquake activity is strongly correlated with intense snowfall at high elevations. A comparison between weather and repeating earthquake occurrence is shown on Figure 1.