The Philippine Institute of Volcanology and Seismology (Phivolcs) on Wednesday said it has once again recorded increased seismic activity at Bulusan Volcano in Sorsogon.
In an advisory issued on May 20, the Phivolcs recorded 442 volcano-tectonic earthquakes from 4 p.m. on Monday, May 18, until 12 p.m. on Wednesday, May 20.
Of the total number of earthquakes, 55 were located less than five kilometers beneath the volcano’s edifice and were associated with rock fracturing beneath the volcano.
Phivolcs said the Bulusan Volcano remains under Alert Level 1, indicating low-level unrest, but warned of a higher possibility of sudden phreatic or steam-driven eruptions.
Monitoring data also showed inflation or swelling of the volcano’s northern and upper western slopes since June 2025 and January 2026, respectively, indicating possible pressure buildup beneath the edifice due to shallow hydrothermal or tectonic processes.
Phivolcs said degassing activity from the volcano’s active vents has remained weak to moderate in recent days. The agency last measured an average sulfur dioxide emission rate of 121 tonnes per day on April 22, 2026.
Despite the increased seismic activity, Alert Level 1 remains in effect over the volcano.
However, Phivolcs warned that the heightened unrest could trigger sudden phreatic eruptions from any of the summit vents.
The agency reiterated that entry into the four-kilometer radius Permanent Danger Zone (PDZ) remains strictly prohibited. Residents and local government units were also advised to remain vigilant within the two-kilometer Extended Danger Zone (EDZ) on the volcano’s southeastern sector due to possible hazards, including pyroclastic density currents, ballistic projectiles, rockfall, avalanches, ashfall, and lahars.
PHIVOLCS further advised residents to wear face masks or use wet cloths in the event of ashfall, especially children, senior citizens, pregnant women, and individuals with respiratory illnesses.
“Local government units are advised to strictly prohibit entry into the 4-kilometer radius Permanent Danger Zone and exercise vigilance in the 2-kilometer Extended Danger Zone on the southeast sector due to the possible impacts of volcanic hazards,” the agency said in its advisory.
