On 4 January 2018, the Indonesian Volcanology and Geological Disasters Centre (PVMBG) published a public report on the situation of the Mount Agung volcano in Bali, which has been erupting since the end of November 2017. The volcano’s alert level remains at level IV (maximum), but the evacuation range has been reduced to 6 km, down from 8 to 10 km before. This reduction will allow part of the evacuated population to go home, and it is a strong message that the situation is improving, while maintaining vigilance and prohibiting access to the most dangerous zones.
The conclusions of this public report owe much to the results of the deformation models developed by François Beauducel, a researcher at the Paris Globe Physics Institute (IPGP), currently on secondment to IRD in Yogyakarta. At the request of the PVMBG, François Beauducel set up a WebObs system (a tool developed and used in IPGP observatories for around 15 years) in record time, despite technical difficulties (new data format, reduced network because of station breakdowns, etc.), and successfully ensured the evolution of processing and models on a data flow in real time: a rare and noteworthy occurrence for a volcano of this type.
These results submitted to the PVMBG on 29 December 2017 strongly influenced the decision to reduce the evacuation range. They indicate that the Agung seismic crisis was preceded by an inflation (swelling) of the volcano, connected to the injection of magma at a depth of around 12 km below sea level, with a volume surplus of around 35 million m³. From late November, when the magma eruption itself began, the models indicate a deflation of equivalent quantity to the period of inflation. This value also corresponds, allowing for errors, to the volume of lava present in the crater and to the materials ejected during the numerous explosions. This demonstrates that, in terms of deformations of the edifice, this first magma injection/eruption episode is over (or almost over) and that any new injection of magma could be detected, thanks to the current surveillance network and to the WebObs real-time modelling tool.
Nevertheless, vigilance is required, because the crater at the summit of Mount Agung is currently half full with around 30 million m³ of high-temperature lava, juvenile lava which is still “degassing” (expelling volcanic gases under pressure), partly hindered by a natural plug of solidified lava. Intermittent explosive episodes, with ejection of ash but at a relatively low volume, are therefore expected to continue for months, even without a new input of magma.