The Philippine Institute of Volcanology and Seismology (Phivolcs) recorded 98 earthquakes at Bulusan Volcano in Sorsogon from 3 a.m. to 10 p.m. on Monday, April 21.

In an advisory issued at 10 p.m., Phivolcs reported that of the volcanic earthquakes recorded, 94 were weak and shallow volcano-tectonic tremors caused by rock-fracturing, while four were low-frequency volcanic quakes due to fluid movement.

Phivolcs noted that the volcanic earthquakes occurred within 20 kilometers of depth beneath the Bulusan edifice, with nearly half being shallow events.

Meanwhile, degassing activity at the summit crater and active vents remained “very weak to moderate” during the same period.

Phivolcs explained that the slight increase in volcanic earthquake activity may be linked to hydrothermal processes, which could lead to short-term pressurization of the volcano.

While it has maintained Alert Level 0 (normal) for Bulusan Volcano, Phivolcs warned of possible steam-driven or phreatic eruptions from the crater and active vents, which could occur suddenly and without warning.

Local government units and the public are advised to stay out of the four-kilometer radius permanent danger zone, especially near the vents on the south-southeastern slopes, due to the risk of sudden volcanic hazards such as rockfall, landslide, ballistic projectiles, and small pyroclastic density currents, or “uson.”

Civil aviation authorities are also urged to caution pilots to avoid flying near the volcano’s summit, as ash from any sudden phreatic eruption may pose a threat to aircraft safety.