The Phillippine Institute of Volcanology and Seismology (PHIVOLCS) noted in their latest bulletin that seismic activity at the volcano rapidly increased as about 300 volcano-tectonic earthquakes were detected since 8 May.
Short-term electronic tilt monitoring on the upper slopes recorded a slight ground deformation that began on 6 March.
These parameters indicate elevated fluid movements of gas, water and possibly magma under the surface.
New eruptions could occur any time, most likely in the form of sudden phreatic (steam-driven) or phreatomagnatic (water-magma interaction driven) explosions from the summit crater or flank vents.
Bulusan, Luzon’s southernmost volcano, was constructed along the rim of the 11-km-diameter dacitic-to-rhyolitic Irosin caldera characterized by phreatic and phreatomagmatic explosions.
Source: Phillippine Institute of Volcanology and Seismology volcano activity update 13 May 2021