Indonesia’s Geological Agency raised the alert on Mount Sinabung, in North Sumatra, from Level II to Level III on Monday 31 August, the day the mountain produced its first eruption since 2021. An initial burst at 10:17 in the morning was followed around 12:30 by a column of ash rising roughly 3,500 metres above the summit — about 5,960 metres above sea level — drifting east-south-east in the direction of Berastagi. On the agency’s four-step scale, Level III, Siaga, is the second-highest.
The recommended danger zone runs three kilometres from the crater in every direction, widening to six kilometres in the south-south-east sector, the direction in which Sinabung has historically sent its heaviest flows. By Tuesday the national disaster agency, BNPB, counted 615 people — about 211 families — moved from the village of Kuta Tengah, in Simpang Empat district, to an evacuation post at Sukatepu. A second village inside the zone, Payung, has been prepared for evacuation but its residents remain in place. The Karo regency secretary, Gelora Kurnia Putra Ginting, described the movements as precautionary and tied them to the wider hazard zones the Geological Agency had just recommended.
What gives the eruption its weight is the mountain’s record. Sinabung, a 2,460-metre stratovolcano, has no confirmed historical eruption at all before August 2010; the Smithsonian Institution’s Global Volcanism Program lists nothing earlier in the record. It then woke properly in September 2013 and stayed awake for eight years, building and collapsing lava domes and sending pyroclastic flows down its flanks, before its last confirmed eruption in 2021. Five quiet years followed. The Geological Agency’s own bulletins through August had already noted rising seismicity, so Monday was less a bolt from a clear sky than the arrival of something the instruments had been describing for weeks.
Indonesian authorities said they were still compiling figures for casualties and material damage. The head of the Geological Agency, Lana Saria, called Monday’s event an initial eruption and said the agency does not rule out a larger one to follow.