MANILA, Philippines – The number of volcanic earthquakes recorded from Taal Volcano rose in the past 24 hours, with one of them being a harmonic tremor, or prolonged shaking.

The Philippine Institute of Volcanology and Seismology (Phivolcs) said on Tuesday, February 4, that the Taal Volcano Network recorded 223 volcanic earthquakes in the past 24 hours, compared to 134 in the previous 24-hour period.

Of the 223, there was a harmonic tremor that lasted nearly 3 minutes long.

Harmonic tremors are associated with the movement of magma. They also “often precede or accompany volcanic eruptions,” according to the United States Geological Survey (USGS).

This is the 3rd harmonic tremor recorded in less than a week. There was a 97-second harmonic tremor last Thursday, January 30, and another one that was 3 minutes long last Saturday, February 1.

There were also 8 low-frequency events among the volcanic earthquakes in the past 24 hours. These are “caused by cracks resonating as magma and gases move toward the surface,” said USGS on its website.

Phivolcs reiterated that the volcanic earthquakes “signify magmatic activity beneath the Taal edifice that could lead to eruptive activity at the main crater.”

In the past 24 hours, Phivolcs also observed “weak emission of steam-laden plumes 50 to 500 meters tall” coming from the main crater. These are weaker compared to plumes which rose 800 meters high in the previous 24-hour period.

But the sulfur dioxide (SO2) emission, another indicator of the presence of magma, has gone up.

Phivolcs said SO2 emission was measured at an average of 231 tons per day on Monday, February 3. Last Saturday, it was measured at an average of only 97 tons per day.

Phivolcs said on Tuesday that it is maintaining Taal under Alert Level 3, which has been in place since January 26. Before that, the restive volcano was under Alert Level 4 for two weeks, starting January 12.

The highest category is Alert Level 5, which would be raised if a hazardous eruption occurs. (READ: FAST FACTS: Taal Volcano alert levels)

Phivolcs warned that the following dangers remain possible:

• sudden steam-driven and even weak phreatomagmatic explosions
• volcanic earthquakes
• ashfall
• lethal volcanic gas expulsions

Areas in Batangas within a 7-kilometer radius from Taal’s main crater are still on lockdown. These include:

• Taal Volcano Island
• barangays of Bilibinwang, Subic Ilaya, and Banyaga in Agoncillo
• barangays of Gulod, Buso-Buso, and Bugaan East in Laurel

At least 129,010 families or 482,925 persons have been affected in Batangas, Quezon, Laguna, and Cavite as of Tuesday, said the National Disaster Risk Reduction and Management Council.

There are 6,830 families or 23,915 persons still staying in evacuation centers as of Tuesday.

When a 14-kilometer-radius danger zone was enforced during Alert Level 4, there were as many as over 38,000 families in evacuation centers. – Rappler.com