The Magnitude 6.9 earthquake in Northern Cebu last September 30, 2025 triggered several sinkhole collapse incidents in Bogo City, San Remigio, and other nearby areas. This phenomenon is typical in karst landscapes or areas underlain by soluble rocks like limestone. Sinkholes are semi-circular to circular ground depressions or openings formed when support of the overlying surface weakens due to the presence of underground cavities or cave systems.
Why did sinkholes appear in Cebu after the Magnitude 6.9 Bogo Earthquake?
Based on the available Mines and Geosciences Bureau (MGB) geologic maps, at least 60% of Cebu Island is underlain by highly soluble limestones, forming a unique karst topography. Common landforms in such topography include caves, conical hills, karst valleys and ridges, karst plains, and uplifted marine terraces. The formation of sinkholes often initiate in voids and cavities within the underlying rock. The presence of faults and fractures in limestones increases the dissolution process in karst of Cebu.
The ground shaking due to the movement of Bogo Bay Fault resulted in the instability of the fragile sinkholes and cave systems. The manifestations of circular cracks, and differential settlement in identified sinkholes are evidence of unstable ground. Hence, the possible collapse of these depressions may be extremely unpredictable. Aside from ground shaking, the collapse of sinkhole covers may be triggered by intense flooding, lowering of the water table, ponding of subterranean rivers, and heavy infrastructure development on top of sinkholes.
MGB Initiatives: Karst Subsidence Hazard and Post-Disaster Geohazards Assessment
Since 2015, the Mines and Geosciences Bureau (MGB) has been conducting the Karst Subsidence Hazard Assessment in all identified karstic terrains in the country using a combination of remote sensing, field validation, and geophysical surveys. As of October 2025, 16 municipalities of Cebu Province were already assessed, including Bogo City and San Remigio in Northern Cebu. These assessments have established a baseline inventory of sinkholes and karst features and karst subsidence susceptibility delineation in these areas.
With the recent earthquake event, post-disaster geohazards assessment of hardest hit areas in Northern Cebu is necessary to identify residual threats and to assess the suitability of identified temporary relocation sites. To date, the MGB Post-Disaster Assessment Team has documented 14 additional sinkhole collapse incidents in Northern Cebu from the 198 sinkholes that were previously assessed in Bogo City and Municipality of San Remigio before the earthquake. In addition, signs of ground subsidence such as circular tension cracks, differential settlement, subsiding ground, and progressing cracks on structures are being monitored. Residents living near sinkholes and in areas of active karst subsidence should also monitor the signs of subsidence (e.g. tilted fences and trees, horizontal cracks in houses, exposed building foundation) proactively. The presence of sinkholes and signs of subsidence should be reported to the MGB Regional Office through their Local Disaster Risk Reduction and Management Officer (LDRRMO).
Geologic Map of Cebu Island
Sinkhole in San Remigio
Sinkhole collapse in Daangbantayan (Photo courtesy of MDRRMO and MGB)