Phivolcs: Event may be caused by 'shallow hydrothermal processes'

MANILA (2nd UPDATE) — A "weak explosion" was recorded on Mt. Pinatubo past noon Tuesday, sending a plume into the sky detected by Japanese scientists, 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.

In its 6 p.m. bulletin, the agency said the event might have been caused by "shallow hydrothermal processes" but confirmed that it was a "phreatic eruption."

A phreatic eruption occurs if the heated groundwater drives the eruption. When magma is erupted, it's called magmatic eruption, UP Resilience Institute's Mahar Lagmay said.

"Considering that there has been very low seismic activity in the volcano in the past days and low diffuse volcanic [carbon dioxide] flux measured at Pinatubo Crater Lake, and high infrasound over seismic energy released by the eruption, the event was likely driven by shallow hydrothermal processes beneath the edifice," the bulletin read.

Phivolcs Director Renato Solidum said they are still evaluating the development and its connection with other activities.

His agency said before the weak explosion, the Armed Forces of the Philippines conducted ordnance disposal activities on "the northern flanks of Pinatubo."

This "has no relation to the condition of the volcano," according to Phivolcs as the activity was "conducted prior noon today."

"Ang sinasabi namin 'yung signal na na-detect namin hindi typical known volcanic process... so we are also evaluating it together with other potential sources for example aircraft activity, helicopter, kasi minsan nagkakaroon ng ganoong signal or ordnance disposal, minsan nagpapasabog d'yan di ba. Hindi natin masabing galing sa Pinatubo kasi 'yung isyu nga 'yung signal," said Solidum.

(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 came from Pinatubo because the issue is the signal.)

Phivolcs also noted that seismic activity around Pinatubo has been very low, and that a survey at its crater late this month discounted magmatic activity given its "low diffuse volcanic CO2 flux of 257 tonnes per day."

Solidum said a survey by a Phivolcs team at the Pinatubo crater earlier this month showed "very low" presence of volcanic gas.

"...[A]nd wala ring masyadong earthquake activity d'yan sa Pinatubo area, very low," he said.

(And there's very low earthquake activity there in Pinatubo.)

"Sa mga parametrong aming nakikita, we are discounting magma activity beneath the volcano. 'Yun po ang pinakaimportante, wala po kaming nakikitang activity sa Pinatubo," he said.

(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.)

NO REPORTS OF ASHFALL

In Cabangan town, Zambales, a community close to Pinatubo which typically senses activity on the volcano, residents did not notice any shaking or ashfall, said a barangay officer.

Phivolcs also confirmed this in its 6 p.m. bulletin.

"So far, there has been no reports of ashfall from communities of Zambales over which the eruption plume dispersed."

When the volcano erupted in 1991, an mushroom-shaped ash 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.

"Wala naman po bagama't nagtataka nga po ako 'yung parang medyo makulimlim, parang hindi siya mainit, parang may usok nga," he said.

(There was none but I was wondering why it was overcast, it wasn't hot, it seemed there was smoke.)

As it looks into the incident, Phivolcs urged the public to avoid venturing into Pinatubo, which has become a trekking site years after its cataclysmic 1991 eruption.

"Local government units are advised to prohibit entry into Pinatubo Crater until the source of the above explosion event has been determined and to report to us any related information," it said.

ALERT LEVEL 0 'UNDER CONSIDERATION'

Phivolcs said the weak explosion could also be repeated.

"Shallow phreatic or hydrothermal explosions such as the event of today can occur without warning," state seismologists said.

The volcano's Alert Level 0 is also "currently under consideration" as the agency awaits the results of its GPS and InSAR data processing.

Local governments and communities in Central Luzon, where the volcano straddles several provinces, were reminded to be ready for quakes and volcanic hazards.

Mt. Pinatubo's eruption on June 15, 1991, the world's strongest volcanic eruption at the time, killed over 800 people and caused catastrophic damage across Central Luzon that took years of recovery.

— with reports from Gracie Rutao