Explosion Earthquakes during the 2007 Eruption of Pavlof Volcano, Alaska

Pavlof Volcano on the Alaska Peninsula began to erupt on August 15, 2007 after a 10.7 year repose. Precursor signals consisted of low-frequency earthquakes that began on August 14 and thermal anomalies that were likely coincident with the beginning of the eruption. The mainly strombolian eruptions occurred from a new vent high on the SE flank of the volcano, separate from the NNE vent that had been activeover theprevious severaldecades. Seismic activity,monitoredby a networkof6 local instruments, consists of low-frequency events, explosion earthquakes, volcanic tremor, and lahar-generated signals. Here we focus on explosion earthquakes. The first such event occurred August 14 at 17:58:16 UT and the last on September 13 at 15:14:00 UT. Events were often embedded in continuous tremor, but could be distinguished by the air waves, which appeared as a high-frequency spikes superimposed on the lower-frequency ground waves. The time differences in the airwaves at different stations are consistent with the acoustic speed of ∼330 m/s. Rates were as high as 14 explosion quakes per minute. Steam and ash plumes were generally below 15,000 ft, but rose as high as 20,000 ft on August 29 and 30. We are working to establish whether systematic changes in explosion rates, signal properties, and air-to-ground-wave amplitude ratios correlate with the height and ash content of the plumes. Measurements are corrected for changes in wind speed and direction. AVHRR remote sensing data showed an ash signal on August 29 30, consistent with pilot reports. On August 30 lightning was observed in the plume from Cold Bay, 59 km SW. Compared with recent eruptions of Pavlof in 1981, 1986, 1996, etc., the eruptive activity was of shorter duration (∼5 weeks instead of ∼3 months) and was somewhat less explosive with no episodes of ash columns to heights of 30,000 ft or more. Instead the 2007 eruption produced significant lahars that reached the Pacific Ocean on the south side of the mountain