DOST-PHIVOLCS recognizes the necessity of clarifying information being released by the Institute. We hope to address the concerns of the public arising from the PHIVOLCS Earthquake Information releases today, 22 April 2020 covering the Mabini, Batangas earthquakes.

What is happening in Mabini, Batangas, Balayan Bay and vicinity?

A series of twenty-three (23) earthquakes beneath Mabini, Batangas and adjacent areas of Balayan Bay having a magnitude range of Ms1.2- Ms3.7 has been recorded since 10:31 am today by the Philippine Seismic Network (PSN). The largest of these events occurred at 11:19 am and was felt at Intensity III in Mabini and Bauan, Intensity II in Taal and San Nicolas and Intensity I in Agoncillo and Lemery, Batangas Province. 

These earthquakes occurred within 10 kilometers south of the magmatic dike that fed the January 2020 eruption of Taal Main Crater, and could thus be volcano-tectonic earthquakes associated with post-eruptive crustal re-adjustment surrounding the magmatic dike. This interpretation is supported by current GPS monitoring of ground deformation caused by the magmatic dike in the Pansipit River Valley, which shows a general subsidence of the valley since February 2020. A more detailed understanding of the processes occurring beneath Mabini, Batangas will soon be provided by ongoing analysis of Remote Sensing data.

What are tectonic, volcanic and volcano-tectonic earthquakes?

• Tectonic earthquakes are produced by sudden movement along faults and plate boundaries.
• Volcanic earthquakes produced by movement of magma or magmatic fluids or rock-fracturing beneath volcanoes.
• Volcano-tectonic (VT) earthquakes are a type of volcanic earthquake associated with failure or breakage of rock material along faults and fractures beneath active volcanoes [see references below]. The mechanism that produces tectonic earthquakes is similar, although the root causes are different. VT earthquakes are normally triggered by excess heat and pressure from, or adjustment of the crust around, a magmatic body beneath the volcano, whereas purely tectonic earthquakes are driven by regional tectonic plate motion.

Why are there changes in PHIVOLCS EQ Information?

Initially, the earthquakes in Mabini, Calaca and Lemery, 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. Upon review, however, of data from the Taal Volcano Network (TVN), these events were recognized as likely volcano-tectonic or VT earthquakes, relocated in the Mabini area and revised as “volcanic” in updated DOST-PHIVOLCS Earthquake Information. The magnitudes of these earthquakes were also revised to M_L or local magnitude calculated by the TVN, which typically differ than the customary M_S or surface magnitude calculated by the PSN, and reported as M_L2.9- M_L4.6 in the updated Earthquake Information.

DOST-PHIVOLCS acknowledges the need to clarify information regarding the Mabini earthquakes today and the necessity of distinguishing volcano-tectonic earthquakes from other volcanic earthquakes of direct magmatic causes. In view thereof, VT earthquakes shall henceforth be explicitly classified as “Volcano-tectonic” rather than just “Volcanic” in order to avoid confusion amongst the public.  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 8:00 am. Issuances of Earthquake Information reporting VT earthquakes shall continue so that populations that have been affected by these events can be provided information especially when ground shaking has been felt.

References:

[1] Zobin, V., 2003. Introduction to Volcanic Seismology First Edition. Developments in Volcanology, Volume 6. Elsevier Science.

[2] Chouet, B.A., Matoza, R.S., 2013. A multi-decadal view of seismic methods for detecting precursors of magma movement and eruption. J. Volcanol. Geotherm. Res. 252, 108–175.

[3] McNutt, S.R., 2005. Volcanic seismology. Annu. Rev. Earth Planet. Sci. 33, 461–491.