Single-station characterization of seismic events during the 2009 eruption of Redoubt Volcano, Alaska

To characterize the type and progression of seismic events throughout the 2009 eruption of Redoubt Volcano, Alaska, we use a short-term/long-term average detection algorithm to identify more than 126,000 seismic events between January 1 and June 30, 2009. This analysis was performed at station REF, a short-period seismometer, located on the Redoubt volcanic edifice. Calculated hypocenters suggest most of the detected events occurred at shallow depth within 1 to 3 km of the summit crater floor. Once events were identified we calculated the duration, inter-event time, event rate, peak amplitude, peak-to-peak amplitude, root-mean-square (RMS) amplitude, peak frequency, center frequency, frequency index, and earthquake spectral amplitude (ESAM) for each event. We also use a cross correlation technique to identify event families or multiplets that occurred within this suite of selected events. A total of eight swarms were identified with event rates exceeding 100 events per hour. Swarms between March 20 and April 4 were manually repicked, and the May 2 - 10 swarm was repicked using a correlation detection scheme. Multiplet analysis revealed a total of 149 event families throughout the study period. The first two swarms occurred in late January and were associated high amplitude, low frequency spasmodic tremor. Six more swarms occurred on February 26-27, March 20-23, March 27, March 29, April 2-4, and May 2-10. Swarms on March 20-23, March 27, and April 2-4 immediately preceded explosions. The swarm on March 20-23 was uniquely heterogeneous containing 21 separate families with a wide range of amplitudes and spectral content. This swarm took place while new magma was first observed to be forming a dome within the Redoubt crater. Swarms preceding explosions on March 27 and April 2-4, as well as swarms on March 29 and May 2-10, that did not preceded explosions, contained events that were more homogenous in waveform character and were often composed of a single event family. The April 2-4 swarm had the lowest average amplitudes of the precursory swarms, as well as the lowest spectral content. May 2-10 was the longest swarm studied and contained over 37,000 similar events occurring during growth of the final dome. A series of rockfall events on May 6 caused a change in the event rate of this multiplet suggesting a link between the growing lava dome and the source of the events. This analysis indicates that seismic event character became more homogenous throughout the course of the 2009 eruption and suggests that the source processes driving shallow seismicity became more uniform.