The Philippine Institute of Volcanology and Seismology (Phivolcs) on Tuesday, March 17 noted a spike in seismic activity at Bulusan Volcano in Sorsogon, raising the likelihood of a steam-driven or phreatic eruption in the coming hours or days.

In an advisory, Phivolcs said a total of 90 volcanic earthquakes were recorded by the Bulusan Volcano Network between 12 a.m. and 12 p.m.

Nearly all of the recorded tremors were weak volcano-tectonic earthquakes linked to rock fracturing occurring within five kilometers beneath the volcano’s edifice.

Despite the increase in seismicity, Phivolcs said other monitoring parameters remained at background levels.

Degassing activity from active vents, when visible, has been described as very weak to moderate, consistent with low sulfur dioxide emissions measured at below-background levels of 40 tonnes per day as of March 12.

Phivolcs also noted that no significant ground deformation and no changes in monitored thermal springs have been observed.

However, the agency warned that past phreatic eruptions of Bulusan were often preceded by sporadic swarms of volcanic earthquakes, including those recorded on April 28 to 29, 2025, and June 5 and 12, 2022.

“This is the second swarm recorded this March 2026, increasing the chances of a steam-driven or phreatic eruption in the succeeding hours or days,” Phivolcs said.

Alert Level 0 remains in effect over Bulusan Volcano, indicating that no magmatic eruption is currently expected.

Nonetheless, Phivolcs said heightened seismic activity may signal ongoing hydrothermal processes that could trigger sudden explosions.

The public has been advised to avoid entry into the four-kilometer radius permanent danger zone, especially near vents on the south-southeastern slopes, due to the risk of sudden phreatic eruptions.

Such events may generate hazards including pyroclastic density currents, ballistic projectiles, rockfall, avalanches, and ashfall.

Local government units have been urged to review and prepare their emergency response plans in case the volcano’s alert status is raised to Alert Level 1.

Meanwhile, civil aviation authorities were advised to warn pilots against flying close to the volcano’s summit, as ash from sudden eruptions could pose risks to aircraft.