Updated: Phreatic eruption at Pinatubo crater monitored by Phivolcs

(Eagle News) – The Philippine Institute of Volcanology and Seismology (PHIVOLCS) said a “phreatic eruption” or steam-driven explosion occured in Mt. Pinatubo’s crater on Tuesday, Nov. 30.

In its 6 p.m. update on Tuesday, PHIVOLCS said that evidence of the phreatic eruption was the visual observation of an “eruption plume” as well as data from “seismic, infrasound and satellite monitoring.”

It said that the phreatic eruption occured in between 12:09 and 12:13 p.m. on Tuesday.

This happened after an “ordnance disposal” by the military at the northern side of Pinatubo that morning that had nothing to do with the volcano’s condition, Phivolcs said.

In its 4 p.m. advisory, the agency said that “an explosion signal (was) recorded at Pinatubo Volcano.”

-No ashfall monitored so far-

The agency said that there was no ashfall that had been monitored so far in the Zambales side of the volcano where the eruption plume was seen on Tuesday.

PHIVOLCS said that based on the low number of volcanic quakes in the past few days and the “diffuse volcanic CO2 flux” measured this November in the Pinatubo Crater Lake, as well as the high energy of infrasound compared to seismic signal of the phreatic eruption that happened past Tuesday noon, the eruption could have been brought about by “hydrothermal processes” inside the volcano.

Earlier, in its 4 p.m. advisory on Tuesday, Nov. 30, PHIVOLCS said that the Pinatubo Volcano Network “recorded seismic and infrasound signals of a weak explosion at Mt. Pinatubo between 12:09 PM and 12:13PM” that day.

“The event produced a plume that was detected by the Himawari-8 Satellite and reported to DOST (Department of Science and Technology)-PHIVOLCS by the Tokyo Volcanic Ash Advisory Center,” it said.

Infrasound, also known as low-frequency sound, refer to sound waves with a frequency below the lower limit of human audibility. These are sounds beneath 20 Hz down to 0.1 Hz, the frequency range for monitoring earthquakes and volcanoes.

PHIVOLCS said that “the seismic and infrasound signals are not typical of known volcanic processes and are currently being evaluated together with other potential sources” such as aircraft activity, ordinance disposal, and others.

“In addition, there has been very low seismic activity in the past days and geochemical survey of the Pinatubo Crater Lake this November 2021 yielded low diffuse volcanic CO2 flux (257 tonnes/day), discounting magmatic activity beneath the edifice,” the PHIVOLCS advisory said.

-Phivolcs warns public not to venture into areas in the vicinity of Pinatubo-

Because of this, DOST-PHIVOLCS warned the public, saying that they should avoid venturing or entering areas in the vicinity of Pinatubo Volcano at this time.

“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,” the PHIVOLCS 4 p.m. advisory said.

“Communities and local government units 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,” it said.

DOST-PHIVOLCS said it “continues to strengthen its monitoring of the volcano with ongoing upgrades to the PVN (Pinatubo Volcano Network), periodic geochemical surveys of the Pinatubo Crater lake and GPS and satellite analysis of ground deformation.”

“DOST-PHIVOLCS is closely monitoring the volcano’s condition and any new development will be relayed to all concerned,” it added.

Pinatubo is not yet under any alert level as of now.  PHIVOLCS, in its 6 p.m. update on Tuesday, said it is analyzing results of its “GPS and inSAR data processing.”  GPS, together with satellite data, is used to monitor ground deformation in Pinatubo. InSAR or Interferometric Synthetic Aperture Radar is defined by the USGS as a technique for mapping ground deformation using radar images of the Earth’s surface that are collected from orbiting satellites.

Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits, according to the USGS.

Pinatubo’s last major eruption was in 1991. The volcano’s cataclysmic eruption on June 15, 1991 was reported as the second-largest terrestrial eruption of the 20th century after the 1912 eruption of Novarupta in Alaska.

(Eagle News Service)