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‘Quake swarm’ in Camarines Sur may indicate stronger tremor — Phivolcs

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The Philippine Institute of Volcanology and Seismology (Phivolcs) warned that an ongoing earthquake “swarm” in Ragay town, Camarines Sur may be an indication of a stronger tremor.

According to the Phivolcs, an earthquake swarm is a short-lived sequence of minor to light temblors, which have no identifiable large event or main shock.

Phivolcs said the earthquake swarm is not capable of triggering a tsunami.

In an Aug. 22 report, Phivolcs said Ragay started experiencing an earthquake swarm on June 7. Over a nearly 5-day period from Saturday until Wednesday afternoon, there were 121 earthquakes recorded in the swarm, 42 of which were felt, ranging from magnitude 1.6 to 4.4, Phivolcs reported.

The swarm could persist for a few days and disappear over time, Phivolcs said, adding that another possibility is that the quakes could “be a precursor to a larger magnitude earthquake,”

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Phivolcs said that earthquake-induced landslides, rock falls, and other types of mass movement could occur in mountainous or hilly areas of Ragay town, home to almost 60,000 people, during the swarm.

“Liquefaction, manifested by subsidence, ground fissures, sand boils, and/or lateral spreads may affect low-lying, water-saturated, and sandy areas near water bodies,” it said.

Phivolcs described Camarines Sur as a “seismically active region” due to its active faults, such as the Legaspi Lineament and offshore segments of the Philippine Fault in Ragay Gulf.

The agency said tt least 7 “significant earthquakes” had impacted Ragay town and its nearby areas since 1811, citing data.

The most damaging, it said, was the 7.4 magnitude Ragay Gulf earthquake that occurred on March 17, 1973, which was triggered by the Guinayangan Segment of the Philippine Fault. The quake was felt at a maximum intensity of VIII (very destructive) and caused “numerous” landslides, ground rupture, liquefaction, tsunami, and structural damage, Phivolcs said.

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