CVGHM reported that seismicity from Anak Krakatau increased during 14-21 April; the number of events per day peaked on 20 April. Ash plumes accompanied by propelled incandescent rocks were noted during field observations on 16, 17, and 18 April. The eruption affected the summit and the E and S flanks. Booming noises were reported and occasionally heard at an observation post 42 km away. The Alert Level was raised to 3 (on a scale of 1-4) on 21 April.
Source: Center of Volcanology and Geological Hazard Mitigation (CVGHM) – “Reports provided courtesy of the Smithsonian’s Global Volcanism Program and the US Geological Survey’s Volcano Hazards Program.”