The Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG) reported increasing unrest at Krakatau. Sulfur dioxide emissions began to be detected in satellite data on 1 June and thermal anomalies were identified starting on 10 June. Emissions at the active crater intensified. The seismic network recorded an increase in shallow earthquake activity, specifically signals indicating gas-and-steam emissions, hybrid/multiphase events, and low-frequency events; the number of these signals notably increased during 18-19 June, averaging more than 50 events per day. Crater incandescence was visible in a webcam image at 2313 on 19 June. Though there was an increase in shallow activity, there were no deep volcanic earthquakes nor ground deformation detected. The Alert Level remained at 2 (on a scale of 1-4), and the public was warned to stay 2 km away from the active crater.

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

Geological Summary. The renowned Krakatau (frequently misnamed 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.