THE powerful earthquake that shook Cebu in late September left not only cracked buildings and frayed nerves but also a lingering question: Could this be the start of something bigger?
The magnitude tremor reignited a common fear in seismically active regions — that a major quake on one fault line could trigger a catastrophic “chain reaction” along others.
Here’s a look at the science behind the quake, its aftermath and what experts say is next for the region.
What happened
On Sept. 30, 2025, a magnitude 6.9 earthquake struck northern Cebu, with its epicenter traced to a newly identified fault line: the Bogo Bay Fault. The quake caused significant damage to homes, schools and churches in the area. Since the main shock, the Philippine Institute of Volcanology and Seismology (Phivolcs) has recorded over 9,000 aftershocks, raising public anxiety about further seismic activity.
One quake does not trigger another
Despite widespread speculation, the head of the country’s seismology agency says the recent earthquake is an isolated event. Phivolcs Director Dr. Teresito Bacolcol said there is no scientific evidence to suggest the movement of the Bogo Bay Fault will directly activate other fault systems in Cebu or neighboring islands.
“Each fault behaves independently. Energy release is localized within the fault that ruptured,” Bacolcol said.
He acknowledged that minor stress can be transferred between nearby faults, but said that current data shows no signs of induced activity elsewhere. Cebu Province is home to several fault systems, including the Central Cebu Fault and the South Cebu Fault, which run under at least 18 local government units from Boljoon in the south to Daanbantayan in the north.
Permanent changes to the coastline
The earthquake’s impact was not limited to ground shaking. Residents in coastal areas have observed lasting changes to their shorelines.
In Barangay Nailon, Bogo City, Councilor Nodjie Mandawe reported that the sea level near the local fish port has noticeably dropped. “The tide no longer reaches its usual height since the quake,” Mandawe said, adding that about 15 sinkholes have also appeared in the sea.
Bacolcol confirmed these observations, noting a slight sinking, or subsidence, along parts of San Remigio’s coastline as well. He explained this is a normal geological effect when a strong earthquake causes vertical movement along the fault. Some coastal areas may sink, making the sea appear to advance, while others may rise, exposing formerly submerged land.
“Such changes are permanent,” Bacolcol said, citing a similar phenomenon after the magnitude 7.2 Bohol earthquake in 2013, which caused some coastlines to rise and others to sink.
Thousands of aftershocks explained
While the number of aftershocks — more than 9,000 — may sound alarming, Phivolcs clarifies that this is a reflection of modern, highly sensitive seismic instruments that detect even the smallest tremors. The vast majority of these aftershocks were too weak to be felt by humans.
Bacolcol said that an aftershock, by definition, cannot be stronger than the main quake. The strongest aftershock recorded so far was a magnitude 5.1.
What’s next
The ground in Cebu is gradually stabilizing. According to Bacolcol, the aftershocks will continue but their frequency is expected to diminish significantly over the next few months.
“The frequency of aftershocks will gradually decrease over time,” he said. “After about three months, only around 20 to 30 small ones will remain per day, most of them unfelt.”
As for the Bogo Bay Fault itself, residents can be reassured that a repeat event is not imminent. Bacolcol said that earthquakes of this magnitude on the same fault typically recur only after “several hundred years.” / EHP, DPC