The 2006 eruption of Augustine Volcano, Alaska, generated more than 3,500 earthquakes in a month-long time frame bracketing the most explosive period of activity. We examine two quantitative tools that, in retrospective analysis, were excellent indicators of imminent eruption. The first tool, referred to as the frequency index (FI), is based on a simple ratio of high-and low-frequency energy in an earthquake seismogram. It is a metric that allows us to quantify the differences between the canonical high-frequency, hybrid, and low-frequency volcanic earthquakes. FI values greater than-0.4 indicate earthquakes classically referred to as highfrequency or volcano-tectonic events. FI values less than-1.3 correspond to events usually referred to as low-frequency earthquakes. Because the FI is based on a ratio and not a spectral peak, hybrid earthquakes are successfully assigned FI values intermediate to these two classes. In this eruption, we find a remarkable correlation between events with FI less than-1.8 and explosive eruptions. The second tool we examine is based on repeating seismic waveforms identified through waveform cross-correlation. Although the vast majority of earthquakes during this eruption have unique waveforms, subsets of events exhibiting a high degree of similarity occur and are closely tied to explosive eruption events. Of the 13 large explosion events, seven were preceded by clusters of highly similar earthquakes. We apply the FI and correlation tools together to identify changes in high-and low-frequency earthquake occurrences and examine their relations to the precursory, explosive, and continuous phases of the eruption. We find that earthquakes that have low FI values and earthquakes exhibiting high degrees of similarity occur almost exclusively within hours of explosive eruptions and postulate that they occur as a result of the final ascent of magma in the volcanic edifice. Because neither of these methods requires analyst-reviewed earthquake locations, we believe that they have considerable potential as automated real-time volcano monitoring tools.

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Redoubt Volcano erupted in March 2009 following 6 months of precursory seismic activity. The 4.5-monthlong eruptive sequence was accompanied by phreatic and magmatic explosions, periods of steady dome growth, lahars, seismic swarms, extended episodes of volcanic tremor and changes in the background seismicity rate. This study presents a seismic chronology of the eruption and places it in context with the variety of other geological and geophysical data that were recorded during the eruptive period. We highlight 6 notable seismic swarms, 3 of which preceded large explosions. The swarms varied from an hour to several days in duration, and contained tens to over 7000 earthquakes. Many of the swarms were dominated by low frequency type earthquakes that contained families of repeating events. Seismic tremor varied considerably in frequency, amplitude and duration during the eruption with distinct characteristics accompanying different types of volcanic activity. The explosion signals during March 23-24 were the most energetic, and the explosions on March 26-29 contained proportionally more low frequency energy (0.033-0.3 Hz). Two seismic stations were particularly well-suited to recording lahars that flowed down the Drift River valley. Data from these stations showed that lahars were generated by the majority of the explosion events, as well as during the continuous eruptive activity on March 29 when no large explosions occurred. We also examine the seismicity which occurred outside of the explosion and swarm episodes, and find several families of repeating VT earthquakes which begin shortly before the April 4 explosion and that continue through May 2009, locating between 3 and 6 km below sea level.