An upwelling of hot volcanic gases at Taal Lake's surface was observed by the Philippine Institute of Volcanology and Seismology (Phivolcs) on Monday, May 3.
Phivolcs said the upwelling was accompanied by 500-meter tall steam-laden plumes that were emitted from active fumaroles or vents.
In its volcano bulletin issued on Tuesday, May 4, Phivolcs also reported the detection of nine low-frequency volcanic earthquakes and a low-level background tremor that has persisted since April 8.
Taal Volcano's sulfur dioxide emission averaged 1,587 tonnes per day on Monday, it added.
Meanwhile, temperature highs of 71.8 degrees Celsius and pH of 1.59 were last measured from the main crater lake on March 4 and Feb. 12, 2021, respectively.
Phivolcs added that ground deformation parameters continued to indicate a "very slow and steady inflation and expansion of the Taal region since after the January 2020 eruption."
"These parameters indicate persistent magmatic activity at shallow depths beneath the edifice," it said.
Phivolcs maintained the alert level over Taal Volcano at Alert Level 2 due to continuous "increased unrest."
The restive volcano in Batangas has been under Alert Level 2 since March 9, 2021.
"Sudden steam-driven or phreatic explosions, volcanic earthquakes, minor ashfall, and lethal accumulations or expulsions of volcanic gas can occur and threaten areas within and around TVI (Taal Volcano Island)," Phivolcs pointed out.
It warned the public from entering the volcano island, which is a permanent danger zone, especially the vicinities of the main crater and Daang Kastila fissure.
Occupancy and boating on Taal Lake should also be strictly prohibited, Phivolcs added.
Local government units were advised to continuously assess and strengthen the preparedness of previously evacuated barangays around Taal Lake in case of renewed unrest.
Civil aviation authorities were also asked to advise pilots to avoid flying close to the volcano as airborne ash and ballistic fragments from explosions and wind-remobilized ash may pose hazards to aircraft.