Who Monitors Krakatau?
The primary authority responsible for monitoring Krakatau is PVMBG — the Pusat Vulkanologi dan Mitigasi Bencana Geologi (Center for Volcanology and Geological Hazard Mitigation), an agency under Indonesia's Ministry of Energy and Mineral Resources. PVMBG operates a network of monitoring stations in and around the Sunda Strait and maintains 24-hour watch on Anak Krakatau's activity.
International scientific bodies, including the Global Volcanism Program (Smithsonian Institution) and the Darwin Volcanic Ash Advisory Centre (VAAC), also track Krakatau's eruptions, particularly for aviation safety.
The Tools of Volcano Monitoring
Modern volcano monitoring is a multi-sensor discipline. For Krakatau, key methods include:
Seismic Monitoring
Networks of seismographs detect ground vibrations generated by magma movement, hydrothermal fluid migration, and eruption tremors. The type and frequency of seismic signals are among the most reliable indicators of changing volcanic conditions. A sudden increase in volcano-tectonic earthquakes, for example, may indicate that magma is forcing its way through new pathways.
GPS and Ground Deformation
As magma accumulates in subsurface chambers, the ground above swells. GPS receivers and tiltmeters measure these deformations — sometimes just millimetres of change — that can signal an impending eruption. After the 2018 collapse, deformation monitoring took on added importance to track the rebuilding of Anak Krakatau's structure.
Gas Monitoring
Volcanoes emit gases — primarily water vapour, carbon dioxide (CO₂), and sulphur dioxide (SO₂). An increase in SO₂ emissions often indicates that fresh magma is rising toward the surface, as SO₂ is largely released from magma at shallow depths. PVMBG uses both ground-based sensors and satellite-based instruments (such as the Sentinel-5P satellite's TROPOMI instrument) to track gas output from Krakatau.
Visual and Thermal Observation
Webcams, thermal imaging cameras, and periodic helicopter or drone surveys provide visual data on eruption plumes, lava flows, and changes to the crater area. During clear conditions, observers on the Java coast can sometimes see eruption columns from Anak Krakatau with the naked eye.
Satellite Remote Sensing
Synthetic Aperture Radar (SAR) satellites can detect ground deformation, map lava flows, and track plume dispersal even through cloud cover — essential in a tropical environment where cloud is almost always present.
Indonesia's Volcanic Alert System
PVMBG uses a four-level alert system for all Indonesian volcanoes:
| Level | Indonesian Name | Meaning | Public Guidance |
|---|---|---|---|
| Level 1 | Normal | No significant activity above baseline | No restrictions |
| Level 2 | Waspada (Advisory) | Increased activity; elevated unrest | Awareness; some access restrictions possible |
| Level 3 | Siaga (Watch) | Significant unrest; eruption likely or ongoing | Exclusion zones enforced; no public access |
| Level 4 | Awas (Warning) | Imminent major eruption | Evacuations ordered; all access prohibited |
The Challenge of Tsunami Warning
The 2018 disaster exposed a critical gap: tsunami warning systems designed around earthquake detection could not detect a tsunami triggered by a volcanic flank collapse. In response, Indonesia has worked to expand its network of coastal tide gauges and infrasound sensors that can detect the acoustic signals of large collapses and eruptions, providing earlier warning for volcanic tsunamis.
What Should Visitors Do?
Anyone planning to visit Krakatau should check the current PVMBG alert level before booking. When Anak Krakatau is at Level 2 or above, boat operators are required to keep a minimum safe distance from the island. At Level 3 or 4, trips are cancelled. Reputable operators will always comply with these restrictions — be wary of any operator who offers access during elevated alert periods.