SCIENTISTS from the Department of Science and Technology–Philippine Institute of Volcanology and Seismology (DOST-Phivolcs) have discovered cracks and areas of sinking ground in northern Cebu following the magnitude 6.9 earthquake that struck the province on September 30, 2025.
In an update on Saturday, October 4, Phivolcs said the damage was caused by the movement of the Bogo Bay Fault, the newly identified fault that triggered the quake.
Drone photos and field surveys showed lines of cracks and uneven ground, known as ground rupture, where the fault broke through the surface.
Phivolcs also found signs of liquefaction, a phenomenon where wet or soft soil behaves like liquid during a strong earthquake. This can cause the ground to sink or tilt, damaging houses, roads, and other structures.
Ground effects
The agency observed ground undulation, or the uneven rising and sinking of the ground, and lateral spreading, which happens when the soil moves sideways, behind the damaged Daanbantayan Municipal Hall.
It also detected lateral spreading along the coastlines of Daanbantayan and Medellin, where parts of the ground shifted horizontally toward the sea.
In Bogo City, numerous lateral spreads appeared around Polombato Port and nearby side roads, stretching 42 to 105 meters long.
A house in Barangay Maya, Daanbantayan, subsided by around 25 centimeters, meaning the ground beneath it sank. Water also seeped out of the ground in front of the main door after the earthquake.
In Barangay La Purisima Concepcion, Bogo City, watery sand surged out of the ground, a sign of liquefaction.
Phivolcs first identified the fault’s onland section on Friday, October 3, in Sitio Looc, Barangay Nailon, Bogo City. Scientists observed cracks, small mounds of soil, and a deformation zone about two meters wide.
Initial measurements showed about 200 meters of surface rupture, though drone images suggest the fault may extend up to 1.5 kilometers.
Phivolcs reminded residents to stay alert for possible strong aftershocks, avoid weakened or damaged buildings, and follow safety updates only from official government sources. (JGS/SunStar Philippines)