The Philippine Institute of Volcanology and Seismology (Phivolcs) said Bulusan Volcano's seismic activity increased on Tuesday, July 28, after 109 volcanic earthquakes were recorded since midnight.

In an advisory issued at 3 p.m. Tuesday, Phivolcs said almost all of the earthquakes were volcano-tectonic events associated with rock-fracturing at shallow depths beneath the volcano.

It also observed that the degassing activity from the active vents remained weak to moderate, while low sulfur dioxide emissions of 50 tons per day were measured on July 21.

“These parameters likely indicate active shallow hydrothermal or tectonic processes beneath the volcano which could lead to steam-driven or phreatic eruptions at any of its summit vents,” Phivolcs said.

Although the volcano remains under Alert Level 1 (low-level unrest), the agency warned that the elevated seismicity may increase chances of phreatic eruptions from any of its summit vents.

Phivolcs advised local government units to strictly prohibit entry into the four-kilometer permanent danger zone (PDZ) and exercise vigilance in the two-kilometer extended danger zone (EDZ) on the southeast sector “due to the possible impacts of volcanic hazards such as pyroclastic density currents or PDCs, ballistic projectiles, rockfall, avalanches, ashfall and others on these danger areas.”

“Communities south and downwind of the volcano are also advised to prepare for ashfall in case phreatic eruption occurs, paying special attention to vulnerable members of the community including the elderly, expecting mothers, infants, and those with medical conditions,” Phivolcs said.

Civil aviation authorities were advised to caution pilots against flying close to the volcano's summit because ash from any sudden phreatic eruption could be hazardous to aircraft.

“Furthermore, people living within valleys and along river/stream channels especially on the western sectors of the edifice should be vigilant against sediment-laden stream flows and lahars in the event of heavy and prolonged rainfall should phreatic eruption occur,” Phivolcs said.