Characterization of the 2008 Kasatochi and Okmok eruptions using remote infrasound arrays

The 2008 Plinian eruptions of Kasatochi and Okmok volcanoes were recorded by six remote International Monitoring System infrasound arrays. High-amplitude infrasound at these stations, combined with remote sensing, permits insight into important volcanic source parameters, such as origin times, durations, and source characteristics. Infrasound from the 7-8 August Kasatochi eruption consists of three well-defined eruption pulses, with the first two steam-rich and the last ash-rich. Pulse 2 is the most energetic and impulsive. Okmok produced over 9 h of continuous infrasound on 12-13 July. Acoustic propagation modeling for the Okmok eruption and first Kasatochi pulse predict thermospheric ducting and origin times consistent with seismic and satellite observations. However, theoretical acoustic origin times of pulses 2-3 are predicted to occur similar to 15 min earlier than the seismic. Stratospheric ducting for these later pulses provides more consistent origin times. Although both volcanoes ejected ash into the stratosphere (>15 km), Kasatochi produced higher amplitude infrasound than Okmok. Previous studies have shown sustained infrasound with frequencies <0.5 Hz is indicative of high-altitude ash emissions. Kasatochi and Okmok recordings are consistent with this, as stratospheric emissions evident in satellite imagery are correlated with sustained 0.01-0.5 Hz infrasound. Further, the acoustic spectrum shape resembles the spectrum from man-made jets, suggesting a self-similar noise generation mechanism proposed in earlier work. Although uncertainties exist, observations and propagation modeling from Kasatochi suggest self-similarity is apparent at long distances (>2000 km) and does not seem to be appreciably affected by changes in ash content between the eruption pulses.