MANILA, Philippines — Over the past two days, Phivolcs recorded a spike in volcanic earthquakes at Taal Volcano: 56 on April 27 and 37 on April 28. This represents double to triple the previous highest daily earthquake count recorded in April.
While these figures are unusual compared to the past two months, Phivolcs told Philstar.com in an interview on Tuesday, April 29, that the quakes are of low frequency. In other words, they are not immediately alarming.
“So these are very weak na low-frequency events and tremor events that are associated with volcanic gas,” said Ma. Antonia Bornas, chief of the Phivolcs Volcano Monitoring and Eruption Prediction Division.
She explained that low-frequency volcanic quakes (LFVQ), also known as long-period events, are associated with magma or volcanic gas moving to the surface through cracks and conduits.
Volcanic tremors, meanwhile, are continuous, rhythmic signals generated by sustained fluid movement and typically last longer than a minute.
Despite the recent uptick in seismic activity, Bornas said Taal Volcano remains under Alert Level 1, indicating continued risks of a possible phreatomagmatic eruption due to ongoing “fluid motion” within the volcano.
A three-month overview
Philstar.com reviewed the volcanic earthquakes and tremors recorded at Taal Volcano since the week before its minor phreatic eruption on February 16.
In those weeks, seismic activity was minimal, with some days logging only a handful of quakes or around 12 at most. It was only in April that the volcano began showing a more consistent pattern of low-frequency earthquakes, reaching up to 20 per day and peaking at 56 and 37 on April 27 and 28, respectively.
While volcanic tremors have been more common in April, their durations have been shorter, typically under 30 minutes. This contrasts with March, when some tremors lasted up to eight hours, and February, with instances of up to four hours.
“We also take into consideration other things like how strong are these earthquakes,” Bornas said. Even when combined, the recent quakes may not match the intensity of the 2021 tremor events that affected Taal Volcano’s Main Crater Lake.
Deformation, sulfur dioxide
Other parameters beyond the number and strength of volcanic earthquakes and tremors are also considered, including volcanic deformation and sulfur dioxide emissions, which she said have not been alarming recently.
Unlike in 2021 to 2023, Taal Lake is now “quiet” and “very placid,” Bornas said, reflecting the “very low sulfur dioxide emissions” and weaker volcanic signals currently recorded by Phivolcs. On Monday, sulfur dioxide emissions were recorded at 672 tonnes per day, with other days exceeding 1,000 tonnes.
“So ibig sabihin nito, we have fluid or volcano gas moving or travelling deep within the edifice na hindi pa ito nagsusurface kaya marami tayong ganitong earthquakes,” she added.
(So this means we have fluid or volcanic gas moving or travelling deep within the edifice, but it hasn't surfaced yet, that's why we're seeing many of these earthquakes.)
Meanwhile, the Taal Caldera has been undergoing long-term deflation, with short-term inflation observed along the southeastern flank of Taal Volcano Island. Bornas explained that when inflation occurs, it signals “localized volcanic gas activity in Taal.”
“So, that’s not much concern and again, at the worst, we will have minor phreatic or phreatomagmatic events,” she added.
Bornas also noted that the primary sign of magma intrusion would be ground deformation across the entire area, including areas along Taal Lake. When this happens, it would be a concern.
For now, however, Phivolcs has only observed localized inflation at Taal Volcano, which has generally been deflating since the massive 2020 eruption, she added.
Off-limits
With the volcano still at Alert Level 1, Bornas stressed that the permanent danger zone includes the Main Crater and Daang Kastila Trail, where “very active” degassing makes these areas off-limits.
Other than the phreatic or phreatomagmatic eruptions, she warned that projectiles can be ejected at fatal speeds. She added that carbon dioxide emissions, which can displace oxygen and cause suffocation, should be a concern alongside sulfur dioxide.
Batangas residents are urged to stay out of Taal Volcano’s permanent danger zone to avoid potential hazards.