A "weak explosion" was recorded on Mount Pinatubo past noon Tuesday, sending a plume detected as far as Tokyo, or 2,981 kilometers from Zambales, Phivolcs said.
The state seismology agency said the Pinatubo Volcano Network "recorded seismic and infrasound signals of a weak explosion at Mt. Pinatubo between 12:09 p.m. and 12:13 p.m."
The explosion produced a plume "detected by the Himawari-8 Satellite and reported to DOST-PHIVOLCS by the Tokyo Volcanic Ash Advisory Center."
Phivolcs said seismic and infrasound signals detected at the volcano are "not typical of known volcanic processes," and so the agency is looking at other possible sources.
Phivolcs Director Renato Solidum said. “What we're saying is the signal that was detected is not a typical known volcanic process… so we are also evaluating it together with other potential sources for example aircraft activity, helicopter, because sometimes there are signals like that or ordnance disposal, sometimes they detonate there.
We can't say that it came from Pinatubo because the issue is the signal.”
Phivolcs noted that seismic activity around Pinatubo had been very low, and that a survey at its crater earlier in November discounted magmatic activity given its "low diffuse volcanic CO2 flux of 257 metric tons per day."
Solidum said a survey by a Phivolcs team at the Pinatubo crater earlier this month showed "very low" presence of volcanic gas.
He added: “Based on the parameters we observed, we are discounting magma activity beneath the volcano. That's the most important, we don't see activity on Pinatubo.”
In Cabangan town, only 32 kilometers from Pinatubo, which typically senses activity on the volcano, residents claim they did not notice any shaking or ashfall, said a barangay officer.
When the volcano erupted in 1991, the mushroom plume was within sight from the village, said Genaro Ofunda Jr., chief of Barangay Felmida-Diaz.
But at the time the activity was recorded on the volcano Tuesday, he said there was no ashfall. Neither did they feel any volcanic quake.
Meanwhile, Phivolcs urged the public to avoid venturing into Pinatubo, which has become a trekking site years after its cataclysmic June 15, 1991 eruption.
More than 350 people died during the eruption, most of them from collapsing roofs.
Disease that broke out in evacuation camps and the continuing mud flows in the area caused additional deaths, bringing the total death toll to 722 people. The event left more than 200,000 people homeless.