![]() We estimate the mean frequency content of each event using the dominant frequency. The first arrivals are emergent rather than having a clear and sharp first motion (Figure 2c). We characterize these events as “low frequency” because their frequency content is much lower than a similarly sized tectonic earthquake occurring deeper in the volcano. Though recurrence intervals are regularly spaced and waveforms are highly similar in the short term, there can be both gradual evolutions and sudden jumps in both factors over the duration of a family. In this study, “family” refers to a set of earthquakes that share a similar waveform, and “swarm” refers to an uptick in earthquake activity that is not a main shock/aftershock sequence. They often occur at regularly spaced intervals, with some scatter around a mean (Figure 2a), and have highly similar waveforms between events in the same family (Figure 2b). Geological Survey Cascade Volcano Observatory (CVO), responsible for monitoring the volcano, observed a swarm of low-frequency (3000 m), and their waveforms and locations indicate they are within a few hundred meters of-if not at-the surface. This challenge was illustrated in May and June 2010 when analysts at the Pacific Northwest Seismic Network (PNSN) and U.S. The locations of the Paradise and Sunrise visitor center areas are indicated. Upper right inset map shows regional location of Mount Rainier. For some locations the formal errors are smaller than the dot and are not visible. Box shows extent of lower right inset map of summit area showing locations of select event families from this study maximum horizontal location errors are shown in gray. ![]() Black triangle indicates weather station PVC. LON, PANH, and OBSR are broadband stations, others are short period. White dots indicate three-component stations. Seismic stations and glaciers at Mount Rainier. The convergence of these factors makes timely and accurate identification of seismic events at Mount Rainier both difficult and an issue of great public concern. Mount Rainier is the most glacier-clad mountain, by volume, in the conterminous United States and is also an active volcano with the largest at-risk population in the country. Nowhere is this more obvious, perhaps, than at Mount Rainier. Benign events related to glacier activity, like surface crevassing, serac collapse, and basal sliding, can mimic subtle signals that are often associated with volcanic fluid or gas movement. Researchers must be able to distinguish seismicity generated by volcanic activity from ice movement, wind, rock fall, debris flows, and human activity: not always a straightforward task. Clear identification of seismic events and assessment of source mechanisms is a fundamental aspect of both volcano monitoring and understanding landscape dynamics. ![]() Mount Rainier, a 4392 m glacier-clad active stratovolcano in Washington State (Figure 1), is one of the most dynamic landscapes in the world and thus also one of the seismically noisiest. Identification of the source of these frequent signals offers a view of basal glacier processes, discriminates against alarming volcanic noises, documents short-term effects of weather on the cryosphere, and has implications for repeating earthquakes, in general. This behavior is specific to winter months because it requires the inefficient drainage of a distributed subglacial drainage system. We propose a hypothesis that this increase is caused by the redistribution of basal fluids rather than direct loading because of a 1–2 day lag between snow loading and earthquake activity. Coda wave interferometry shows that source locations migrate over time at glacial speeds, starting out fast and slowing down over time, indicating a sudden increase in sliding velocity triggers the transition to stick-slip sliding. The sudden added weight of snow during winter storms triggers a temporary change from smooth aseismic sliding to seismic stick-slip sliding in locations where basal conditions are favorable to frictional instability. Mixed polarity first motions, a linear relationship between recurrence interval and event size, and strong correlation between swarm activity and snowfall suggest the source is stick-slip basal sliding of glaciers. Each family contains up to thousands of earthquakes repeating at regular intervals as often as every few minutes. We have detected over 150,000 small ( M 3000 m) on the glacier-covered edifice and occur primarily in weeklong to monthlong swarms composed of simultaneous distinct families of events.
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