Seismic activity and changes in Taal Volcano's main crater lake have been observed on Monday despite the volcano under alert level 1 (abnormal), the Philippine Institute of Volcanology and Seismology (Phivolcs) said.

In an advisory, the Phivolcs said 50 weak tremor episodes were recorded from 5 a.m. to 3 p.m., occurring at shallow depths. Seismic activity describes earthquake occurrences in the volcano.

This signals increased hydrothermal activity beneath Taal Volcano island.

"Tremors are typically not recorded," Phivolcs director Renato Solidum told the Philippine News Agency (PNA) on Monday night.

The Phivolcs recorded 68 shallow tremor episodes in the volcano island since Feb. 13.

Solidum said the main crater lake is becoming more acidic and hotter.

Local government units are advised to prepare in case of the volcano's renewed unrest and to assess previously evacuated barangays around Taal Lake for damages.

The public is urged to observe precautions due to possible ashfall, ground displacement across fissures, and minor earthquakes in the event of a phreatic eruption.

According to the advisory, geochemical data on the main crater lake indicate continuous acidification of lakewater from a pH 2.79 to pH 1.59 between January 2020 and February 2021, an unseasonal temperature high of 77 degrees Celsius and CO2/H2S gas flux ratios consistent with shallow magma degassing.

Phivolcs said there are increased possibilities of sudden steam-driven or phreatic explosions, lethal accumulations, or expulsions of volcanic gas and minor ashfall from the main crater that can occur and threaten areas within the Taal Volcano island.

Entry into the island, Taal’s Permanent Danger Zone, especially the vicinities of the main crater and the Daang Kastila fissure, must remain strictly prohibited, Phivolcs added.

Pilots are advised to avoid flying close to the volcano as airborne ash and ballistic fragments from sudden explosions and wind-remobilized ash may pose hazards to aircraft.