THE magnitude 6.9 earthquake that shook Cebu Province on Tuesday night, Sept. 30, 2025, has left behind a dangerous geological legacy: an immediate and growing threat of sinkholes and land subsidence in the northern municipalities. Following the seismic event, the Mines and Geosciences Bureau (MGB) 7 confirmed the appearance of numerous sinkholes and ground cracks, issuing a high-hazard warning that demands urgent attention to a long-term risk.
What happened
In the days following the earthquake, the MGB 7 confirmed the development of sinkholes and land subsidence in several northern Cebu municipalities, including Medellin, San Remigio, Tabogon, Daanbantayan and Bogo City.
The town of Medellin was placed under a high subsidence hazard rating, particularly in all barangays where sinkholes have been reported. Sinkholes, especially those caused by a sudden cover collapse, are a serious threat to infrastructure and life. Land subsidence — the lowering of the land surface — can occur rapidly during a major earthquake or through a gradual sinkhole collapse.
The MGB 7 has advised local officials to immediately install warning signs, restrict access to affected areas, and refrain from covering or backfilling the sinkholes without expert recommendation, as this could worsen the problem. The bureau also cautioned against further development or community growth in the identified areas, noting that additional structures could trigger further collapse.
Why it matters
The confirmed presence of sinkholes and large-scale land subsidence elevates the region’s recovery challenge from a short-term cleanup to a long-term hazard mitigation problem. Sinkholes are an enduring threat that can expand over time, often triggered by heavy rainfall, changes in the water table, or subsequent seismic activity.
For residents, this means that even after homes are rebuilt and immediate infrastructure is repaired, the ground itself remains unstable and dangerous. Roads, utility lines and new structures face an ongoing risk of sudden collapse. The MGB 7’s advisory to discourage further development highlights the potential for the affected areas to be fundamentally unsafe for future habitation or major construction projects.
This situation requires a significant reallocation of local government resources toward specific geotechnical studies and hazard preparedness, diverting funds and attention from general post-disaster recovery efforts. The identification of a major geological hazard also introduces a layer of complexity to disaster risk reduction planning across the province, extending the timeline for full recovery and normal operations.
The bigger picture
Northern Cebu lies on a bedrock of limestone, a key factor in the development of karst topography, which is characterized by caves, disappearing streams and sinkholes. Sinkholes form when mildly acidic rainwater seeps into the ground and dissolves the soluble limestone, creating underground voids and cave systems.
While sinkholes are a natural feature of karst landscapes, a major seismic event like the magnitude 6.9 earthquake can dramatically accelerate their formation. The intense ground shaking can cause the roofs of existing subterranean caves to collapse immediately, resulting in a sudden, catastrophic cover collapse. The ground cracks and depressions observed by the MGB 7 suggest that the earthquake has destabilized the underground structure, making future subsidence a near certainty in the absence of mitigation.
The Philippines, sitting on the active “Ring of Fire,” is highly prone to both earthquakes and the subsequent geological hazards. The current situation in Cebu mirrors similar incidents around the world where seismic activity has caused severe ground deformation in karst regions. The MGB’s recommendation for specific geotechnical studies is crucial because the size of the surface sinkhole often belies the extent of the larger, interconnected subterranean cave system beneath.
What to watch
The most critical factor to watch in the coming weeks and months is the onset of heavy and prolonged rainfall.
The MGB 7 explicitly warned that sinkhole development may accelerate due to rainfall, which can saturate the soil, increasing its weight and lubricating the rock, thereby triggering further collapse. Monitoring the movement of existing tension cracks, particularly after rain, will be a key indicator of continued ground instability.
Authorities must prioritize the results of the geotechnical studies currently being conducted by the deployed MGB teams. These assessments will determine the true extent of the underground cave systems and will be the basis for final land-use recommendations, potentially leading to the permanent classification of some areas as “no-build” zones. The management of these areas will set a precedent for how the Philippines handles the dual threat of major seismic events and karst hazards in its densely populated regions. (Jerra Mae Librea)