No earthquake swarms in Cebu, but ‘Big One’ remains possible —Phivolcs
LAPU-LAPU CITY, Cebu — The Philippine Institute of Volcanology and Seismology (Phivolcs) said no earthquake swarms have been recorded in Cebu recently, but the island remains vulnerable to a major earthquake from its active fault systems.
An earthquake swarm refers to a series of small to moderate earthquakes that occur in the same area over a short period, ranging from days to months.
Engr. Robinson Jorgio, director of the Phivolcs Visayas Cluster Monitoring Center for Earthquake and Tsunami, said earthquake swarms can sometimes occur before a stronger earthquake.
READ: Mindanao earthquake death toll climbs to 37; 479 injured – NDRRMC
He cited the case of the Bogo Bay Fault, where a series of earthquakes was recorded beginning in April before a major earthquake struck the area on September 30.
However, Jorgio emphasized that earthquake swarms do not automatically mean a destructive earthquake will follow.
Potential ‘Big One’
Even without ongoing earthquake swarms, Phivolcs said Cebu remains susceptible to a major earthquake because of the active fault systems running across the island.
Jorgio said one of the main concerns is the Central Cebu Fault System, a network of potentially active faults that passes through densely populated areas, including Cebu City, Mandaue City, Talisay City, and Danao City.
“Every fault naay ‘Big One.’ Since naa tay potentially active fault sa Cebu, naa tay ‘Big One.’ Atong estimate is 7.2 to 7.5 nga magnitude,” Jorgio said during a news forum on Tuesday.
(Every fault has its own ‘Big One.’ Since we have a potentially active fault in Cebu, we also have a possible ‘Big One.’ Our estimate is that it could reach a magnitude of 7.2 to 7.5.)
READ: EXPLAINER: Why did the magnitude 7.8 Sarangani quake happen?
An earthquake of that magnitude, said Jorgio, could generate Intensity VIII ground shaking across much of Cebu Island.
Intensity VIII is classified as “very destructive” and can cause serious damage to buildings, roads, bridges, and other structures.
Recent seismic activity
While small earthquakes continue to be recorded in parts of Cebu, Phivolcs said these events do not indicate that a major earthquake is imminent.
Jorgio said earthquakes still occur in Bogo City, but these are considered aftershocks of the magnitude 6.9 earthquake that struck northern Cebu last year.
The agency has also recorded minor seismic events in Lapu-Lapu City, Mandaue City, and Carcar City.
However, Phivolcs clarified that these tremors do not qualify as an earthquake swarm.
READ: Are Filipinos prepared for the Big One?
Jorgio explained that earthquakes occur because tectonic plates beneath the Earth constantly move and exert pressure on fault systems and ocean trenches surrounding the Philippines.
As stress builds up over time, a fault may eventually rupture and release energy, producing an earthquake.
Monitoring and preparedness
Despite the absence of unusual seismic activity, Phivolcs said it continues to monitor fault systems across the Visayas.
“We are monitoring all faults in the Visayas. Kaso di natin masabay lahat dahil sa kakulangan ng aming instruments,” Jorgio shared.
(We are monitoring all faults in the Visayas. However, we cannot monitor all of them simultaneously due to a lack of instruments.)
According to Jorgio, the agency needs additional accelerometers, seismographs, and Global Positioning System (GPS) equipment to strengthen monitoring efforts.
READ: LIST: Killer earthquakes in the Philippines
Accelerometers measure the strength of ground shaking during earthquakes, while seismographs detect and record seismic waves generated by movements beneath the Earth’s surface.
He said these instruments help scientists better understand how faults behave and identify areas where stress may be building up.
Cebu not affected by Mindanao quake
Meanwhile, Jorgio said Cebu was not significantly affected by the recent magnitude 7.8 earthquake that struck off Sarangani and nearby areas in Mindanao.
“Based sa simulation computations, pwedeng affected ang southern part in Cebu. Pero based on our verification, wala may niingon na nakafeel og Intensity I sa Cebu,” Jorgio said.
(Based on simulation computations, the southern part of Cebu could be affected. However, based on our verification, no one reported feeling Intensity I in Cebu.)
He explained that the Cotabato Trench, which generated the earthquake, is located far from the fault systems in the Visayas, including those in Cebu.
While the recent Mindanao earthquake posed little threat to Cebu, Jorgio reminded the public that preparedness remains essential because earthquakes can occur without warning.
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