RAGAY town in Camarines Sur has been experiencing an earthquake swarm, a short-lived sequence of minor to light earthquakes, since Saturday, the Philippine Institute of Volcanology and Seismology (Phivolcs) reported on Sunday night.
Phivolcs said it has “plotted” 45 earthquakes in the area as of 3 p.m. last Sunday, of which 35 were felt.
“The highest magnitude recorded by the Philippine Seismic Network for this earthquake swarm is Mw (moment magnitude) 4.4 which was felt at Phivolcs Earthquake Intensity Scale (PEIS) V (strong) in the municipality of Ragay, Camarines Sur,” Phivolcs said.
At PEIS V, Phivolcs said, ground shaking is “generally felt by most people.”
“Indoors, strong shaking is felt through the building, hanging objects swing violently, and small, light and unstable objects may fall or overturn. Outside, standing vehicles rock noticeably and shaking of leaves and twigs of trees are observed,” said Phivolcs.
It said ground shaking may lead to landslides, rockfalls, and other types of mass movement in mountainous or hilly areas.
“Liquefaction, manifested by subsistence, ground fissures, sand boils, and/or lateral spreads may affect low-lying, water-saturated and sandy areas near water bodies,” it added.
Phivolcs cited two scenarios in the earthquake swarm. It said it might be a short-duration burst of earthquake activity that may last for a few days, similar to earthquake swarms that Camarines Sur experienced in December 1993 and October 2021.
“Another possible scenario that should not be discounted is that the earthquakes may be a precursor to a larger magnitude earthquake,” Phivolcs said.
Phivolcs said Camarines Sur is a “seismically active” area because of the presence of two active faults – the Legaspi Lineament and the offshore segments of the Philippine Fault in Ragay.
“Other local faults in close proximity might exist and some of them could potentially be concealed by recent deposits. These concealed faults have the potential to generate minor to strong earthquakes,” said Phivolcs.
“According to earthquake data, the ongoing cluster of earthquakes is caused by a strike-slip fault, a fault with dominant horizontal movement,” it added.
Phivolcs said the earthquakes do not indicate volcanic activity, and the earthquake swarm is “clearly tectonic in origin.”
It said the nearest volcano, the “potentially active” Labo Volcano, is about 15 km from Ragay while the nearest “active volcano,” Isarog Volcano, is 60 km away.
Phivolcs said the earthquake swarm in Ragay cannot generate a tsunami, noting that the epicenters of the quakes are inland.
It said tsunamis are typically produced by earthquakes that occur along active trenches, offshore faults with significant vertical movements or mass movement such as earthquake-induced landslides offshore or near bodies or water.
