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PHIVOLCS: Series of Batangas quakes volcano-tectonic in origin

The Philippine Institute of Volcanology and Seismology (Phivolcs) has added a new classification of earthquakes following its report on a series of minor earthquakes that occurred in Batangas on Wednesday.

"Initially, the earthquakes in Mabini, Calaca, and Lemery in Batangas were reported in DOST-PHIVOLCS Earthquake Information based on the Philippine Seismic Network (PSN) data as tectonic by default due to their occurrence within the locality of the 8 April 2017 M6.0 Mabini earthquake," the agency said in a statement, "A Primer on the April 22 Series of Earthquakes in Mabini, Batangas."

"Upon review, however, of data from the Taal Volcano Network (TVN), these events were recognized as likely volcano-tectonic or VT earthquakes relocatd in the Mabini area and revised as "volcanic" in updated DOST-PHIVOLCS Earthquake Information."



The agency added that the magnitudes of the earthquakes were also revised to low magnitude, calculated by the TVN which is different from the customary surface magnitude calculated by the PSN.

The PHIVOLCS acknowledged there was a need to clarify the information regarding the series of earthquakes and the need to distinguish volcano-tectonic earthquakes from other volcanic earthquakes of direct magmatic causes.

The agency will start classifying earthquakes that are volcano-tectonic in origin not just state that the occurrence is merely "volcanic" in nature.

"Earthquakes classified as volcano-tectonic should not be interpreted by default as being related to renewal of magmatic unrest; rather, day-to-day seismic activity in Taal Volcano is explained in the Taal Volcano bulletins issued daily at 8 a.m."

Issuances of Earthquake Information reporting VT earthquakes shall continue so the affected population can be provided with information when ground shaking has been felt.

The clarification was issued after 23 earthquakes were reported to have occurred beneath Mabini, Batangas and adjacent areas of Balayan Bay ranging from Magnitude 1.2 to 3.7.

The areas mentioned are located 10 kilometers south of the magmatic dike that fed the January 2020 eruption of the Taal Main crater and could be volcano-tectonic earthquakes associated with post-eruptive crustal re-adjustment surrounding the magmatic dike. -- BAP, GMA News