The Philippine Institute of Volcanology and Seismology (Phivolcs) on Friday evening, July 23, lowered Taal Volcano's status to Alert Level 2 after it showed "a cessation of eruptive activity."
Taal Volcano had a phreatomagmatic eruption on July 1 prompting Phivolcs to place the volcano under Alert Level 3. It was followed by 19 weak phreatomagmatic bursts until July 9.
"Unrest since then has been characterized by renewed seismic activity, generally declining volcanic gas emission, very slight ground deformation, and positive microgravity anomalies," Phivolcs said in a bulletin.
Since July 1, Phivolcs recorded a total of 1,195 volcanic earthquakes with magnitudes ranging from 1.8 to 4.6.
Of the total earthquake events, 789 were volcanic tremors, 365 were low-frequency quakes, 26 were hybrid, and three were volcano-tectonic earthquakes generated by activity in the shallow magma and hydrothermal region beneath the Taal Volcano Island (TVI).
"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," Phivolcs explained.
Sulfur dioxide or SO2 flux averaged 12,161 tonnes per day on the first week of July, with the highest emission of 22,628 tonnes per day recorded on July 4.
Phivolcs observed that the average SO2 emission has declined to 4,763 tonnes per day between July 8 and 22.
"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," it pointed out.
Moreover, Phivolcs said that the 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."
Considering these observations, Phivolcs lowered 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, however, it should not be interpreted that unrest has ceased or that the threat of an eruption has disappeared, Phivolcs said.
"Should an uptrend or pronounced change in monitored parameters forewarn a potential eruption, the alert level may be raised back to Alert Level 3," it pointed out.
"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," it added.
Should there be a persistent downtrend in monitored parameters after a sufficient observation period, Phivolcs said the alert level will be further lowered to Alert Level 1.
Under Alert Level 2, Phivolcs reminded that "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."
It said that entry into the volcano island, which is a permanent danger zone, should be strictly prohibited.