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PHIVOLCS TAAL Volcano Bulletin 23 July 2021 7:30 pm

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This serves as a notice for the lowering of Taal Volcano’s status from Alert Level 3 (Magmatic Unrest) to Alert Level 2 (Decreased Unrest)

Following the phreatomagmatic eruption of the Main Crater on 1 July 2021 and nineteen (19) weak phreatomagmatic bursts until 9 July 2021, there has been a cessation of eruptive activity at Taal Volcano. Unrest since then has been characterized by renewed seismic activity, generally declining volcanic gas emission, very slight ground deformation and positive microgravity anomalies. These observations are supported by the following monitoring parameters:

Since 1 July 2021, volcanic earthquakes recorded by the Taal Volcano Network (TVN) totaled 1,195 events and ranged in strength from M1.8 to M4.6. Of these, 789 volcanic tremor, 365 low-frequency, 26 hybrid and 3 volcano-tectonic earthquakes were generated by activity in the shallow magma and hydrothermal region beneath the Taal Volcano Island or TVI edifice. Most earthquakes occurred beneath the Main Crater and the northeastern sector of TVI, indicating migration of shallow degassed magma, volcanic gas and/or hydrothermal fluids beneath these areas. Sulfur dioxide or SO2 flux based on campaign Flyspec data averaged 12,161 tonnes/day in the first week of July 2021, with the highest emission of 22,628 tonnes/day recorded on 4 July 2021. Average SO2 has declined to 4,763 tonnes/day between 8 and 22 July. The decrease in degassing activity reflects the diminishing volumes of accumulated volcanic gas beneath TVI as well as the “scrubbing” effects of rainfall-fed water recharge into Taal’s hydrothermal system. Ground deformation parameters for March to June 2021 from electronic tilt monitoring on TVI and continuous Global Positioning System (GPS) and InSAR analysis of Sentinel-1 satellite data over Taal Caldera generally indicated deflation of the caldera centered on the southeastern TVI and caldera extension at the southwest Taal Fracture Zone where fissuring occurred in 2020. GPS and InSAR data further showed that these changes were respectively caused by a shallow depressurization source beneath southeast TVI and a shallow pressurization source beneath the western Pansipit River Valley. In addition, microgravity campaigns around Taal Caldera since February 2021 yielded positive microgravity increases with ground deflation, which can be caused by magma degassing, densification and migration. These parameters are all broadly consistent with hydrothermal activity beneath the Taal Fracture Zone and magma migration from the southeast flank to other sectors beneath TVI. Activity in the Main Crater has been more often characterized by the generation of moderate steam-laden plumes and periodic but generally less vigorous lake upwelling, consistent with decreased magmatic degassing.

In view of the above observations, DOST-PHIVOLCS is lowering the alert status of Taal Volcano from Alert Level 3 to Alert Level 2 to reflect the overall decreasing trend in the level of monitoring parameters. Alert Level 2 means that there is decreased unrest but should not be interpreted that unrest has ceased or that the threat of an eruption has disappeared. Should an uptrend or pronounced change in monitored parameters forewarn a potential eruption, the Alert Level may be raised back to Alert Level 3. At such time, people residing within areas at high risk to base surges who have returned after the step-down to Alert Level 2 must therefore be prepared for a quick and organized evacuation. Conversely, should there be a persistent downtrend in monitored parameters after a sufficient observation period, the Alert Level will be further lowered to Alert Level 1.

DOST-PHIVOLCS reminds the public that at Alert Level 2, sudden steam-driven or phreatic explosions, volcanic earthquakes, ashfall and lethal accumulations or expulsions of volcanic gas can occur and threaten areas within TVI and along its coast. DOST-PHIVOLCS recommends that entry into TVI, Taal’s Permanent Danger Zone must be strictly prohibited. Local government units are advised to additionally assess previously evacuated areas within the seven-kilometer radius for damages and road accessibilities and to strengthen preparedness, contingency and communication measures in case of renewed unrest. Communities beside active river channels particularly where ash from the 2020 eruption has been thickly deposited should increase vigilance when there is heavy and prolonged rainfall since ash can be washed away and form lahars and sediment-laden streamflows along channels and low-lying areas. Civil aviation authorities must advise pilots to avoid flying close to the volcano as airborne ash and ballistic fragments from sudden explosions and wind-remobilized ash may pose hazards to aircrafts. DOST-PHIVOLCS is closely monitoring Taal Volcano’s activity and any new significant development will be immediately communicated to all stakeholders.