Phivolcs: Aftershocks to weaken over time

Local

THE worst may be over.

Aftershocks rocking Northern Cebu following last week’s magnitude 6.9 earthquake are expected to weaken and become less frequent in the coming weeks, eventually fading by December, according to state seismologists.

The Philippine Institute of Volcanology and Seismology (Phivolcs) assured residents that the ongoing tremors are part of the Earth’s normal adjustment process and do not indicate another major earthquake on the horizon.

“These aftershocks are expected, and they will continue to weaken over time,” said DOST-Phivolcs Director Dr. Teresito Bacolcol in a media forum on Tuesday, October 8. “By December, most of them will be too weak to be felt.”

Since the magnitude 6.9 quake struck Northern Cebu on September 30, Phivolcs has recorded 9,108 aftershocks as of 10 a.m. Wednesday. Of these, 1,666 were plotted, and 38 were felt by residents, with magnitudes ranging from 1.0 to 5.1.

Bacolcol explained that aftershocks typically follow a declining pattern, are frequent in the first few days, and gradually taper off as the Earth’s crust stabilizes.

“If we have 2,000 aftershocks in the first 24 hours, by the 10th day, that number may drop to 200 or 300. By the 100th day…about three months after the main quake…we’d expect around 20 to 50 minor tremors,” he said. “They will become weaker and, eventually, only our instruments will detect them.”

He likened the process to snapping a ruler:

“Once the ruler breaks — that’s the main quake — the vibrations after that are the aftershocks.”

Phivolcs also clarified that the strongest possible aftershock from a magnitude 6.9 event is typically one magnitude lower, around 5.9. So far, the strongest recorded aftershock has been 5.1.

Bacolcol reassured the public that these aftershocks are not precursors to a new major earthquake.

“The likelihood of another major quake striking Northern Cebu will probably take several hundred years,” he said.

He explained that based on historical data and fault recurrence intervals, it may take hundreds of years before enough stress accumulates again to trigger another powerful earthquake in the same area.

“We haven’t had an earthquake in that part for the past 400 years, so it might take several hundred more before another major one,” he said. “The strain loading would take centuries until the rocks can no longer handle the stress.”

Phivolcs said the recurrence cycle of strong quakes typically spans a century or more, especially for dormant faults like the one that shifted in Northern Cebu.

Phivolcs field teams also documented a coastal uplift in Barangay Nailon, Bogo City, where the shoreline rose by about one meter after the quake, exposing tidal flats and raising tidal notches.

The phenomenon, Bacolcol said, confirms significant movement along the Bogo Bay Fault, which had likely been dormant for centuries.

The National Disaster Risk Reduction and Management Council (NDRRMC) reported that the September 30 earthquake left at least 72 people dead, over 550 injured, and affected around 611,000 residents across Northern Cebu.

The tremor damaged homes, schools, roads, and public facilities, prompting ongoing assessments for structural integrity and possible secondary hazards such as landslides.

Local authorities continue to monitor the affected communities while providing relief, shelter, and psychosocial support to displaced families.

While the probability of another major earthquake in Northern Cebu is extremely low, Bacolcol emphasized that preparedness remains essential, especially in other parts of the province.

“That is for northern Cebu. For central and southern Cebu, we still need more data,” he said. “Earthquakes are unpredictable.”

Phivolcs urged residents to stay alert but calm, follow official advisories, inspect homes and buildings, and prepare emergency kits.(TGP)