The decision to raise the alert level followed a sustained increase in volcanic activity recorded at Mount Lewotobi Laki-laki, including continuous tremor and a sharp rise in deep and shallow volcanic earthquakes.
Between August 30 and September 4, instruments at the volcano registered over 240 seismic events, including harmonic and non-harmonic tremors, low-frequency events, and deep volcanic quakes.
On September 6, seismic readings showed uninterrupted tremor, interpreted as magma migration from deeper zones toward the surface.
Simultaneously, visual observations indicated continuous white emissions rising between 50 and 1 000 meters (164–3 280 feet) above the summit.
Air temperatures around the volcano ranged from 17.1°C to 40°C (62.8°F to 104°F), with winds blowing toward all quadrants.
No eruption was observed in the preceding days, suggesting a possible obstruction within the conduit, increasing the likelihood of internal pressurization and sudden explosive activity typical of open-system volcanoes.
Tiltmeter data from the volcano’s upper flanks recorded gradual inflation over the past week, indicating volumetric increase within the edifice due to magma intrusion.
This inflation was most prominent at shallow depths, suggesting imminent eruptive potential. In contrast, data from the Global Navigation Satellite System (GNSS) network indicated a deflation trend, likely reflecting gas release or shallow degassing episodes. The combined deformation signals point to active processes within the upper conduit, consistent with increased risk of sudden eruption.
In response to the elevated threat level, the Geological Agency has enforced a 6 km (3.7 miles) exclusion radius around the summit, with an extended 7 km (4.3 miles) sectoral exclusion zone from the southwest to the northeast.
These sectors correspond to the dominant vent alignment and probable pyroclastic flow paths.
Residents, climbers, and all forms of public activity are strictly prohibited within these areas until further notice.
The open-system nature of Mount Lewotobi Laki-laki allows for vertical magma ascent with minimal resistance, increasing the risk of sudden, high-energy eruptions without significant precursor signals.
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