Bulusan Volcano in Sorsogon has recorded a significant increase in seismic activity over the past two days, prompting state volcanologists to warn of a higher likelihood of steam-driven or phreatic eruptions.

In an advisory issued on Wednesday, May 20, the Philippine Institute of Volcanology and Seismology (Phivolcs) said 442 volcano-tectonic earthquakes have been recorded since 4 p.m. on May 18, indicating rock fracturing beneath the volcano.

Of the total, 55 earthquakes were traced to locations less than five kilometers below the volcanic edifice.

“[Monitoring] parameters may indicate slow pressurization of the edifice due to shallow hydrothermal or tectonic processes beneath the volcano that could lead to steam-driven or phreatic eruptions at any of its summit vents,” Phivolcs said.

Phivolcs noted that degassing activity from the active vents have been “very weak to moderate” for the past days, while “a low average emission rate of 121 tons per day was last measured on April 22.”

The agency also observed that ground deformation data from continuous GPS monitoring indicate inflation or swelling of the northern and upper western slopes since June 2025 and January 2026, respectively.

Alert Level 1, or low-level unrest, remains in effect over the volcano, but Phivolcs warned that the current activity increases the possibility of sudden phreatic eruptions.

“Local government units are advised to strictly prohibit entry into the four-kilometer Permanent Danger Zone and exercise vigilance [within] the two-kilometer Extended Danger Zone on the southeastern sector due to possible impacts of volcanic hazards such as pyroclastic density currents or PDCs, ballistic projectiles, rockfall, avalanches, ashfall, and others on these danger areas,” Phivolcs said.

Communities west of the volcano and other areas that may be affected by prevailing winds were also advised to prepare for possible ashfall in the event of an eruption, particularly vulnerable groups including the elderly, pregnant women, infants, and individuals with existing medical conditions.

Phivolcs also advised aviation authorities to warn pilots against flying near the volcano’s summit, noting that ash from sudden eruptions could pose hazards to aircraft.

Residents living along valleys and river channels, especially on the western slopes of the volcano, were likewise urged to remain alert for possible sediment-laden flows and lahars during periods of heavy and prolonged rainfall.