CVGHM reported that seismicity from Anak Krakatau in 2011, as late as 10 July, consisted of 20-30 volcanic-earthquake events per day and shallow events ranged from 120 to 135 events per day. Hundreds of events per day were detected during swarms. On 10 July, the seismic equipment was damaged by Krakatau's activity but was again operational in mid-September. During 18-30 September seismic events reached 4-5 events per minute. Visual observations in 2011 until 13 September indicated occasional explosive eruptions that would eject material and produce ash plumes. During 14-30 September fumarolic activity from the crater and in the fumarolic fields was visible. The Alert Level was increased to 3 (on a scale of 1-4) on 30 September based on an increase in seismic activity and widespread fumarolic activity.

Geological Summary. The renowned Krakatau (frequently mis-named as Krakatoa) volcano lies in the Sunda Strait between Java and Sumatra. Collapse of an older edifice, perhaps in 416 or 535 CE, formed a 7-km-wide caldera. Remnants of that volcano are preserved in Verlaten and Lang Islands; subsequently the Rakata, Danan, and Perbuwatan cones were formed, coalescing to create the pre-1883 Krakatau Island. Caldera collapse during the catastrophic 1883 eruption destroyed Danan and Perbuwatan, and left only a remnant of Rakata. This eruption caused more than 36,000 fatalities, most as a result of tsunamis that swept the adjacent coastlines of Sumatra and Java. Pyroclastic surges traveled 40 km across the Sunda Strait and reached the Sumatra coast. After a quiescence of less than a half century, the post-collapse cone of Anak Krakatau (Child of Krakatau) was constructed within the 1883 caldera at a point between the former Danan and Perbuwatan cones. Anak Krakatau has been the site of frequent eruptions since 1927.

Source: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as CVGHM)