AVO increased the Aviation Color Code for Shishaldin to Orange and the Volcano Alert Level to Watch on 6 December following several weeks of increasing seismicity and pressure waves recorded by infrasound sensors.
Continuous infrasound waves were detected for more than 10 hours on instruments located in Sand Point, ~230 km E.
Steam emissions visible in satellite and webcam images during 5 and 8-12 December were rising hundreds of feet above the summit crater.
The steam emissions were occasionally accompanied by infrasound signals indicating episodes of short-duration energetic gas emissions and/or small explosions.
Source: US Geological Survey Alaska Volcano Observatory (AVO)