Crispolo Deolata Jr., officer-in-charge of Phivolcs’ monitoring station in Sorsogon, said earlier harmonic tremors they had recorded were almost negligible since they only lasted for a few seconds or a minute.
"The tremor episode, although brief, is significant because it represented an internal process which could lead to more explosions," Phivolcs said.
"The outward quiet activity of Bulusan Volcano belies its general explosive potential," it added.
Apart from the harmonic tremor, sulfur dioxide emission and volcanic quakes remained low during the past 24-hour observation period.
Bulusan has shown signs of unrest since March, but steam-driven or phreatic explosions have become frequent in the past weeks, sending ashfalls over a number of villages, particularly in Casiguran, Irosin and Juban towns which are now under a state of calamity.
Some 45,000 residents face evacuation if Bulusan erupts.
Meanwhile, Phivolcs expects more ash explosions at Kanlaon Volcano in Negros Occidental.
This, after the volcano emitted "voluminous cloud steams" about 500 meters above the crater at 6 a.m. yesterday, preceded by minor ash explosions that started Tuesday night.
Phivolcs also recorded two low-frequency volcanic quakes.
Alert level one remains hoisted at Kanlaon. Phivolcs warned residents against entering the four-kilometer radius permanent danger zone because sudden steam and ash explosions may occur. — With Sheila Crisostomo and Celso Amo