The seismicity of the 2009 Redoubt eruption

Redoubt Volcano erupted in March 2009 following 6 months of precursory seismic activity. The 4.5-month-long 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. (C) 2012 Elsevier B.V. All rights reserved.