The 72 earthquakes that shook Kanlaon Volcano on Monday, Sept. 15, were caused by underground rock fracturing as pressure pushed upward beneath the volcano, the Philippine Institute of Volcanology and Seismology (Phivolcs) said.

“Ang ibig lamang sabihin ng pagdami ng volcano-tectonic earthquakes ay may pagbitak o paggalaw po ng bato sa ilalim habang may tumutulak na pressure or gas na pataas (The increase in volcano-tectonic earthquakes means rocks beneath the volcano are fracturing as pressure or gas pushes upward),” Phivolcs Director Dr. Teresito Bacolcol said during an interview with state-controlled PTV-4.

He said that the quakes, recorded between midnight and 8 a.m., were shallow at depths of zero to 12 kilometers on the northwestern flank of the volcano.

Some were felt at Intensity I in La Carlota City and Canlaon City, the agency noted.

Bacolcol explained that unlike tectonic earthquakes triggered by fault movement, volcano-tectonic quakes happen inside or beneath a volcano due to internal unrest.

“Ang volcano-tectonic earthquake po ay lindol sa loob o sa ilalim ng bulkan na dulot ng pagbitak ng bato habang may pressure, init o fluid that could either be gas or magma na gumagalaw (A volcano-tectonic earthquake happens inside or beneath a volcano when rocks crack due to pressure, heat, or fluids that could either be gas or magma),” he said.

Phivolcs noted that Kanlaon remains under Alert Level 2, or “moderate volcanic unrest,” with the four-kilometer permanent danger zone strictly off-limits.

“Even at alert level zero dapat wala po talaga iyong tao doon (Even at Alert Level 0, people should not be inside the danger zone),” Bacolcol stressed.

The Phivolcs director warned that continued rock fracturing, combined with gas buildup underground, increases the likelihood of sudden steam-driven or phreatic eruptions that could eject ash, rocks, and gas without warning.

“So, kapag patuloy ang ganito, kasabay ang pagdami ng ating volcano tectonic earthquakes at pamamaga ng bulkan tumataas ang tsansang maglabas ang bulkan ng singaw o abo o di kaya’y this may lead to a weak or even moderate eruption (So, if this continues, along with the increase in volcano-tectonic earthquakes and swelling of the volcano, the chances of the volcano releasing steam or ash rise, or this may even lead to a weak or moderate eruption),” he said.