IMS Infrasound Station Observations of the Recent Explosive Eruptions of Okmok and Kasatochi Volcanoes, Alaska J. V. Olson, K. Arnoult, C. A. L. Szuberla, C. Wilson Wilson Infrasound Observatory, Geophysical Institute, University of Alaska Fairbanks, M. Garces ISLA, University of Hawaii, M. Hedlin, L2A, IGPP, University of California, San Diego S. McNutt Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks Presented at the 2008 International Technical Workshop, Bermuda

The Pacific Basin I53US Kasatochi Okmok I56US I57US I59US Kasatochi volcano 52.169N, 175.51W Eruption times: five major explosive eruptions beginning ~22:00 UT, 7 Aug ending ~04:35 UT, 8 Aug ash emissions became continuous after these events reaching altitudes > 35,000 ft asl I53US Kasatochi Okmok I56US I57US Okmok volcano 53.397N, 168.166W Eruption time: Explosion: 12 July 08, 19:43 UT with continuing ash emissions ash cloud top 30-35,000 ft asl I59US ITW Workshop, Bermuda 2008

Eruption Details at at www.avo.alaska.edu Expl’n Date Start Duration # (UTH) (min) 1 7 Aug 22:01 69 2 8 Aug 01:50 27 3 8 Aug 04:35 35 4 8 Aug 07:12 46 5 8 Aug 12:06 26 Kasatochi Great circle distances to US IMS stations Kasatochi Expected Travel Time @ 0.3km/sec I53US: 2104km @ 241o ~1.95 hrs I56US: 4060km @ 299o ~3.76 hrs I57US: 5067km @ 313o ~4.69 hrs I59US: 4002km @ 340o ~3.71 hrs Kasatochi, 17:00 UT, 8 Aug 08 Photo by JT Ford ITW Workshop, Bermuda 2008

IMS Sites Near the Pole Seventeen IMS infrasound sites are located within about 10,000 km of the volcanic events. The sites labeled with green stars are either under construction or the data was not available to us at this time. The sites labeled with blue stars, I10, I18, I30, I53, I56, I57, I59 will be the subject of this review. I31 I37 I51 I18 I46 I34 I10 I53 I15 I56 I45 I57 I44 K O I30 I58 I60 I59 ITW Workshop, Bermuda 2008

Kasatochi – IMS Arrays (within ~6000 km) Azimuth (o) GC Dist (km) I53 241.2 2104 I30 47.6 3931 I59 339.5 4002 I56 298.7 4060 I18 299.9 4750 I57 312.6 5067 I10 304.8 5252 I18 I10 I53 I56 I57 K I30 I59 ITW Workshop, Bermuda 2008

Kasatochi Waveforms & Spectra I53US Infrasound amplitudes for the Kasatochi eruptions reached ~ 1 Pa at I53US and I56US ITW Workshop, Bermuda 2008

Signal Detection Infrasound array data, signal detection Compute MCCM on sliding window Lease-Squares azimuth/velocity estimation Pure-State filter (www.inframatics.org, Sept 04) Re-compute least-squares azimuth/velocity Pure-State filtering increases the signal-to-noise level and enables detection over longer periods of time Detection: Raw Data Detection: Pure-State filtered data ITW Workshop, Bermuda 2008

Kasatochi: I53 Detections Pure-State Data The MCCM detector along with the least-squares estimates of azimuth, velocity and the residuals (στ) are computed using a sliding window 500 sec long, updated every 50 sec I53 Azimuth: 244o Trace velocity: 358 m/sec ITW Workshop, Bermuda 2008

Kasatochi: Detections (1) Pure-State Data Azimuth: 304.8o Trace velocity: m/sec I18 Azimuth: 299.9o Trace velocity: m/sec ITW Workshop, Bermuda 2008

Kasatochi: Detections (2) Pure-State Data Azimuth: 244o Trace velocity: 358 m/sec I56 Azimuth: 300o Trace velocity: 374 m/sec ITW Workshop, Bermuda 2008

Kasatochi: Detections (3) Pure-State Data No signal found I59 Azimuth: 336o Trace velocity: 312 m/sec ITW Workshop, Bermuda 2008

Kasatochi: Detections (4) Pure-State Data Azimuth: 47.8o Trace velocity: 370 m/sec ITW Workshop, Bermuda 2008

Kasatochi Simple Back Azimuth Location Simple back-azimuth projections along the great-circle between IMS stations results in an average location that is approximately 200 km Southwest of Kasatochi and encompasses a region several hundred km wide. ITW Workshop, Bermuda 2008

Okmok – IMS Arrays (within ~6000 km) Azimuth (o) GC Dist (km) I53 241.38 1706 I30 46.6 4439 I59 347.3 3903 I56 298.8 3548 I18 294.1 4464 I57 314.8 4583 I10 303.3 4754 I34 49.7 5688 I18 I34 I10 I53 I56 I57 O I30 I59 ITW Workshop, Bermuda 2008

Okmok Waveforms (1) I53, I56 ITW Workshop, Bermuda 2008

Okmok Waveforms (2) I57, I59 ITW Workshop, Bermuda 2008

Okmok: Detections (1) Pure-State Data No signal found I53 Azimuth: 236o Trace velocity: 360 m/sec ITW Workshop, Bermuda 2008

Okmok: Detections (2) Pure-State Data No signal found ITW Workshop, Bermuda 2008

Summary Both eruptions were strong, exposive eruptions lasting several hours and producing extensive ash plumes Kasatochi: Amplitudes ~1Pa observed at distances to 4000 km Simple great-circle back-azimuths give a location ~200 km Southwest of the source Okmok: Observed only at I53US, amplitudes ~0.01 – 0.1 Pa We are collecting atmospheric data: Effects of wind Estimate signal paths through time delays ITW Workshop, Bermuda 2008

END ITW Workshop, Bermuda 2008

Eruption Details at at www.avo.alaska.edu Okmok Okmok, 20:00 UT, 3 Aug 08 Photo by Mees, Burke ITW Workshop, Bermuda 2008

Kasatochi Waveforms (2) I57, I59 ITW Workshop, Bermuda 2008

Kasatochi Waveforms (2) I10, I30 ITW Workshop, Bermuda 2008

Kasatochi Spectra ITW Workshop, Bermuda 2008

Kasatochi Spectra ITW Workshop, Bermuda 2008