As might be expected from an active volcano of such notoriety, the activity at Krakatau is heavily monitored. A key study of deformation at the volcano was performed by Agustan et al; this study covered a period of almost two years, centred around the eruption which occurred between October 2007 and August 2008. Agustan’s observations utilised PALSAR data from the Japanese ALOS satellite, and uncovered a complex pattern of deformation both prior to and immediately following the eruption.
Before the onset of explosive activity at the volcano, the edifice experienced a net inflation of up to 4cm, though there was subsidence around the crater rim up to 3 months prior to the eruption. The estimated total volume increase was approximately 1×10⁶ m³, and this period of deformation is widely believed to have been caused by magmatic activity which eventually led to the October eruptions.
After August 2008, the volcano displayed an uneven and unusual pattern of deformation; the south-western side of the edifice subsided by roughly 18cm, while the north-eastern side uplifted by almost 12cm. Following numerical simulations. Agustan et al attributed this to the tensile opening of a fault below the edifice, at a depth of approximately 400m below sea level.
The Krakatoa Monitoring System (Krakmon) has recently (2007) installed a series of three GPS stations at locations on the volcano itself, though no observations have yet been published at the time of writing.