This generated a plume that was detected by Japan's Himawari-8 Satellite.
"The seismic and infrasound signals are not typical of known volcanic processes and are currently being evaluated together with other potential sources," Phivolcs said in an advisory.
It added that there has been very low seismic activity in the past days, discounting magmatic activity beneath the edifice.
Phivolcs asks the public to refrain from going in the vicinity of Mount Pinatubo.
It also advised local government units (LGUs) to ban entry into the Pinatubo Crater until the source of the explosion is determined.
Likewise, communities and LGUs surrounding Pinatubo are reminded to be always prepared for both earthquake and volcanic hazards and to review, prepare and strengthen their contingency, emergency, and other disaster preparedness plans.
Mt. Pinatubo has been at alert level 0 (normal) since August 12. (PNA)