Phivolcs Director Renato Solidum Jr explained in a hearing of the Senate committee on urban planning, housing, and resettlement that they again recorded what is called a harmonic tremor, or rhythmic shaking associated with magma movement, early on Thursday.
"The harmonic tremor is an indication na may umaakyat pang magma.... Actually, mas mataas po ang bilang ng earthquakes for the past 24 hours than the previous [24-hour period]," Solidum said.
(The harmonic tremor is an indication that there's still rising magma.... Actually, we had more earthquakes in the past 24 hours than the previous [24-hour period].)
In its 8 am bulletin on Thursday, Phivolcs said the Taal Volcano Network recorded 137 volcanic earthquakes in the past 24 hours, compared to the previous figure of 123. These include the harmonic tremor, which lasted for 97 seconds, and two low-frequency events.
Low-frequency quakes are "caused by cracks resonating as magma and gases move toward the surface," according to the United States Geological Survey.
Meanwhile, the Philippine Seismic Network has plotted 763 volcanic earthquakes since 1 pm of January 12. Of these, 177 were magnitudes 1.2 to 4.1 and were felt at Intensities I to V.
That same network also plotted 7 volcanic earthquakes from 5 am on Wednesday, January 29, until 5 am on Thursday, at magnitudes 1.7 to 2.5 with no felt event.
Phivolcs had lowered Taal's status to Alert Level 3 from Alert Level 4 last Sunday, January 26. When it did so, it warned that volcanic activity could either ease further – leading to Alert Level 2 – or escalate again – meaning a possible return to Alert Level 4.
"When we lowered the alert level, we based it on what the volcano is showing us. We were hoping that there will be [a] continuous [downward] trend of all the parameters that we are seeing to lower it to 2," Solidum said.
"Usually our observation period, dalawang linggo po 'yan. Kaya lang po...meron tayong nakitang harmonic tremor (Usually our observation period is two weeks. But we saw a harmonic tremor earlier)."
Solidum then went on to say that out of around 80 million cubic meters of magma at Taal, only around 30 million cubic meters had been released from January 12 to 13, the first two days of the volcanic unrest.
"At this moment, 'yung magma ay nando'n sa around 5 kilometers [below the crater], parang nag-park muna siya. 'Yun po 'yung binabantayan natin, kung ano pang ikikilos nito.... Tumigil na lang sana siya," Solidum said.
(At this moment, the magma is around 5 kilometers below the crater, virtually parked there. That's what we're monitoring, whether it will rise further or not.... We hope it'll just stop.)
In the past 24 hours, Phivolcs also observed "weak emission of white to dirty white steam-laden plumes 300 to 500 meters tall" coming from Taal's main crater.
The level of sulfur dioxide (SO2) emitted was "below instrumental detection" or too low to be detected. SO2 is a major gas component of magma.
Phivolcs reiterated that under Alert Level 3, the following might still occur:
These areas are within a 7-kilometer radius from the main crater, which is Phivolcs' recommended danger zone while Taal is under Alert Level 3. (WATCH: Broken hearts as Agoncillo residents return to ruined homes)