MANILA, Philippines – There were 18 harmonic tremors, or prolonged volcanic earthquakes linked to the movement of magma, recorded from Taal Volcano in the past 24 hours.

The Philippine Institute of Volcanology and Seismology (Phivolcs) said on Wednesday, February 5, that the Taal Volcano Network recorded a total of 156 volcanic earthquakes in the past 24 hours, including the 18 harmonic tremors which lasted 1 to 3 minutes each.

The total number of volcanic earthquakes is lower than the 223 recorded in the previous 24-hour period, but the number of harmonic tremors is much higher, as there was only 1 recorded in the previous period.

The latest harmonic tremors bring to 21 the number of that type of volcanic earthquake in a week.

According to the United States Geological Survey (USGS), harmonic tremors “often precede or accompany volcanic eruptions.”

There were also 2 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.

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

State volcanologists also observed “weak emission of steam-laden plumes 50 to 100 meters tall” coming from Taal’s main crater in the past 24 hours. The plumes in the previous 24-hour period had been 50 to 500 meters tall, described as “weak,” too.

Meanwhile, sulfur dioxide (SO2) emission was measured at an average of 55 tons per day on Tuesday, February 4, lower than the 231 tons per day recorded on Monday, February 3. Both figures are relatively low.

SO2 is a major gas component of magma.

So far, Taal remains under Alert Level 3, which has been in place since January 26. Before that, Alert Level 4 had been raised for a couple of 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)

Under Alert Level 3, these may happen:

• 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 129,171 families or 483,389 persons have been affected in Batangas, Quezon, Laguna, and Cavite as of Wednesday, said the National Disaster Risk Reduction and Management Council.

There are 5,953 families or 20,927 persons still staying in evacuation centers as of Wednesday.

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