Officials said they were considering increasing the alert level after Mt. Bulusan belched ash an hour before midnight.
The Philippine Institute of Seismology and Volcanology (Phivolcs) said the ash was unlikely to cause any harm.
"This recent ash explosion is more or less typical of Bulusan’s activity during its restive period," the institute said.
More explosions are expected "in view of the volcano’s recent reactivation" following volcanic quakes last Saturday, it added.
But researcher Jojo Cordon said more earthquakes had been recorded in the area recently, a possible sign that the volcano may be about to erupt more powerfully.
He said the ash column might have been created by a reaction between water and hot materials, a "possible sign of rising magma."
A four-kilometer exclusion zone is already in place around the 1,565-meter volcano.
Phivolcs warned locals against venturing into this area and advised them to be vigilant against "further ash ejections and explosions which may generate life-threatening volcanic flows."
The institute said it may expand the permanent danger zone should more explosions occur in the coming days.
Cordon said Phivolcs is considering raising the alert level for Mt. Bulusan from level one to level three, indicating "moderate unrest." The highest alert level is five, indicating an actual eruption of lava.
He said the institute will closely monitor the volcano for further activity.
Phivolcs raised alert level one for Bulusan last Sunday after 73 quakes were recorded.
Phivolcs director Renato Solidum said the steam-driven explosion at 10:58 p.m. Tuesday lasted for about 20 minutes. Three other explosions followed at 11:30, 11:32 and 11:37 p.m., he added.
Light ashfalls were reported in Barangays Cogon, Tinampo, Gulang-gulang and Bolos in Irosin town, and Putting Sapa and Bura-buran in Juban town.
Bulusan had a series of similar explosions in 1994 and 1995. — With Sheila Crisostomo