Thirty years after Mt. Pinatubo's last major eruption on June 15, 1991, the volcano's overall activity has remained quiet up to this day, said Science and Technology Undersecretary Renato Solidum.
Mt. Pinatubo has shown low-level unrest early in 2021 but it is still "relatively quiet," Solidum, who is also the OIC of the Philippine Institute of Volcanology and Seismology (Phivolcs), said.
" a non-explosive eruption in 1992, when a lava dome formed inside the crater, the volcano has been on Alert Level 0 since 1996 and Alert Level was only raised this year (2021) in March," he explained.
Mt. Pinatubo has been on Alert Level 0 since Jan. 9, 1996, before it was raised to Alert Level 1 on March 4, 2021, due to "low-level" unrest in the volcano.
However, Phivolcs said that there is no imminent eruption foreseen.
"The recent earthquake events in the volcano are volcano-tectonic earthquake events, related to small movements of faults in the volcano slopes. Chemical surveys at Pinatubo crater lake have measured low concentrations of gases such as carbon dioxide indicating that the current activity is not magmatic," Solidum pointed out.
During the Phivolcs forum on the 30th anniversary of the Mt. Pinatubo eruption on Tuesday, June 15, Phivolcs-Volcano Monitoring and Eruption Prediction Division chief science research specialist Ma. Antonia Bornas said Mt. Pinatubo has started to record "imperceptible" volcano-tectonic earthquakes on Jan. 19, 2021.
A total of 826 events of volcano-tectonic earthquakes with magnitudes 1.0 to 2.8 were recorded from Jan. 19 to Jan. 27, 2021, she said.
"Ito yung mga lindol na dahilan ng pagbibitak ng bato dahil sa stress or pressure--maaaring tectonic, hydrothermal system, or deep magmatic movements (These are the earthquakes that caused the rock to crack due to stress or pressure, which is likely due to tectonic, hydrothermal system, or deep magmatic movements)," Bornas said.
Phivolcs issued an advisory on Jan. 27, as a reminder to the public that Mt. Pinatubo is active but remained under Alert Level 0.
Its status was raised to Alert Level 1 on March 4, 2021. "Dahil hindi naibsan ang mga lindol patuloy pa rin ang mga ito (because the earthquakes were not alleviated and they were still ongoing)," Bornas said.
From Jan. 27 to June 15, Phivolcs has recorded a total of 1,976 volcano-tectonic earthquakes with magnitudes 0.7 to 3.2, which are considered weak.
"Pawang mabababaw ang lindol noong 1991 kumpara sa mga lindol nitong 2021 (The earthquakes in 1991 were shallow compared to the earthquakes in 2021)," Bornas said.
She also noted that Mt. Pinatubo's caldera lake had a low carbon dioxide flux of 378 and 262 tonnes per day, which is only at the background level of less than 1,000 tonnes per day, in February and April 2021
" bahagyang ground deformation ayon sa satellite data analysis na kailangan ng mas high-resolution data para matiyak (There is a slight ground deformation according to satellite data analysis but more high-resolution data is still needed to ensure the information)," she added.
"Ang unrest ay dulot malamang ng tectonic o hydrothermal unrest sapagkat wala pang senyales ng magma batay sa katangian ng mga lindol at sa crater lake parameters (The unrest is most likely caused by tectonic or hydrothermal unrest because there are no signs of magma yet based on the nature of the earthquakes and the crater lake parameters)," she said.