IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3P1_2A-O15

Room A1

Date/Time: July 23 15:00-15:15

Sources of long-period swarms and very-long-period signals during the explosive anddome-building phases of the 2009 eruption of Redoubt Volcano
Matthew M Haney1 , Bernard A Chouet2 , John J Lyons1
1Alaska Volcano Observatory, U.S. Geological Survey, Anchorage, Alaska, USA, 2 U.S. GeologicalSurvey, Menlo Park, California, USAE-mail: mhaney@usgs.gov

The 2009 eruption of Redoubt Volcano, Alaska, produced as many as 4 lava domes and over 19 explosions
between March 23 and April 4. Following the explosive phase, the volcano began a dome-building phase that
continued until mid-summer. We present the analysis of swarms of small, repetitive, long-period (LP, 0.5-2 speriod) events that occurred during both the explosive and dome-building phases of the eruption. The resultscomplement previous research conducted on the source of very-long-period (VLP, 2-30 s period) signals observedto accompany several of the explosions. We also observe ultra-long-period (ULP, >100 s period) signals following
the onset of certain explosions in the local seismic and infrasound that vary from 120 s to more than 400 s, which
may indicate ultra-long-period oscillations of both the explosion ash plumes and the surrounding atmosphere, thelatter producing acoustic-gravity waves.

We apply waveform inversion to locate the LPs and find their source mechanism. We focus on two prolific
LP swarms from the eruption: a swarm of 5500+ similar events that occurred from April 2-4 (the April swarm) anda swarm of 30,000+ similar events that took place in early May (the May swarm). The April swarm ended with thefinal explosion of the eruption, a dome-collapse on April 4. The May swarm, in contrast, was not associated withan explosion, although a small dome-failure and steam and ash emission occurred approximately midway throughthe swarm.

A challenge in the analysis of the swarms is that the individual LPs are relatively small in amplitude.

Weexploit the similarity in waveforms among the individual LPs and improve the signal-to-noise ratio substantially bystacking over all events on each station. An additional challenge is that the LPs, being higher in frequency thanthe VLPs, suffer more from path effects. This limits the maximum range for stations to be included in the inversion.
From waveform inversion using the 5 broadband seismometers on the Redoubt edifice within 4.5 km of the summit
crater, we find a source depth of 1.5 km ASL, slightly shallower than obtained from a standard earthquake location.
This places the LPs between the VLP source at Redoubt (0.8 km ASL) and the summit crater (2.3 km ASL).
Based on our previous study of VLPs at Redoubt, we consider a moment-only inversion since the local network is
not dense enough to uniquely resolve both moments and forces. The obtained moment tensor indicates the LPs
are dominantly volumetric with consistently shaped source-time functions. Typical moments are on the order of1010 Nm. We plan to further refine the LP source model by conducting reconstructions of the moment tensor withcombinations of cracks and pipes. In addition, waveform inversion of the May swarm should provide clues to thevariation in the dynamics of LP generation during different phases of the eruption.

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