The Philippine Institute of Volcanology and Seismology (Phivolcs) said Taal Volcano in Batangas and Mt. Kanlaon in Negros remain at an "abnormal" status due to increased possibilities of steam-driven or phreatic explosions.
Phreatic eruption happens when groundwater comes into contact with magma.
In its volcano bulletin issued on Wednesday, Feb. 17, Phivolcs' Taal monitoring network has recorded 69 tremor episodes having durations of one to five minutes.
Phivolcs said the activity in the main crater consisted of weak emission of white steam-laden plumes from fumaroles or volcano openings reaching 20 meters high.
Temperature highs of 77.1 degrees Celsius and pH of 1.59 were last measured from the main crater lake on Feb. 12, it added.
"Ground deformation parameters from continuous electronic tilt on Volcano Island record a slight deflation around the main crater since October 2020 but overall, very slow and steady inflation of the Taal region has been recorded by continuous GPS data after the eruption," Phivolcs said.
The agency pointed out that Taal Volcano remains under Alert Level 1, as sudden steam-driven or phreatic explosions, volcanic earthquakes, minor ashfall, and lethal accumulations or expulsions of volcanic gas can occur and threaten areas within the Taal Volcano Island, a permanent danger zone (PDZ).
Phivolcs advised the public to refrain from entering Taal's PDZ, especially the main crater and Daang Kastila fissure.
The local government units were urged to continuously assess previously evacuated barangays around Taal Lake for damages and road accessibilities and to strengthen preparedness, contingency, and communication measures in case of renewed unrest.
People were also advised to observe precautions due to ground displacement across fissures, possible ashfall, and minor earthquakes.
Meanwhile, Phivolcs' Kanlaon Volcano monitoring network recorded 20 volcanic earthquakes in the past 24 hours.
Moderate emission of white steam-laden plumes that rose 500 meters was also observed.
Phivolcs measured sulfur dioxide emission at an average of 1,130 tonnes/day on Feb. 13, 2021, while ground deformation data from continuous GPS and electronic tilt measurements have been recording a slight inflation of the lower and middle slopes since June 2020.
"These parameters may indicate hydrothermal, tectonic, or deep-seated magmatic processes occurring deep beneath the edifice," the agency said in the bulletin. Phivolcs said Kanlaon Volcano is still at Alert Level 1, or under a period of unrest.
The local government units and the public were urged not to enter the four-kilometer radius PDZ due to increased possibilities of sudden and hazardous steam-driven or phreatic eruptions.
Civil aviation authorities were asked to advise pilots to avoid flying close to the summit of Taal and Kanlaon volcanoes as volcanic materials from any sudden phreatic eruption can be hazardous to aircraft.