Cebu: The Mines and Geosciences Bureau (MGB) of the Department of Environment and Natural Resources (DENR) has commenced an investigation into the sudden emergence of sinkholes and ground subsidence across multiple areas in Cebu following a magnitude 6.9 earthquake that affected the province on September 30.
According to Philippines News Agency, an eight-member technical team from the MGB, comprising geologists from both the MGB Central Office and Regional Office 7, has been deployed to conduct a post-disaster geohazards assessment in the areas most impacted by the earthquake. DENR-MGB Director Assistant Secretary Michael Cabalda stated that the geological investigation will involve ground validation and geohazards data analysis to identify areas susceptible to flooding, rain-induced landslides, and ground subsidence. The study aims to provide technical recommendations to local government units (LGUs) for zoning, building regulation, and emergency preparedness.
Cabalda highlighted that the recent earthquake has exposed vulnerabilities in Cebu's karst areas, which are landscapes formed from easily dissolved rocks like limestone, marble, or gypsum. Over time, water infiltration erodes these rocks, creating caves, sinkholes, and underground streams. Similar features can develop in areas with different rock types, known as pseudokarst, where weak or cracked rocks and soil are eroded, leading to large depressions.
Sinkholes occur when the ground collapses due to loss of support underneath, while karst subsidence involves the surface sinking or caving in, often triggered by sinkhole collapse or seismic activity. These hazards pose significant risks due to their unpredictable nature.
The agency's assessment is focused on Cebu's karst terrain, prone to underground cavity formation and collapse, especially when disturbed by strong earthquakes. Preliminary surveys in the proposed Tent City in Barangay Cogon, Bogo City, have identified several sinkhole-prone zones. Initial findings were presented during the National Incident Assessment and Coordination Center (NIACC) meeting at Camp Aguinaldo.
Cabalda assured that MGB geologists are in close coordination with the NIACC, the Department of Human Settlements and Urban Development (DHSUD), and the Office of the Civil Defense (OCD). On Friday, MGB Region 7 issued a 'Subsidence Threat Advisory' to the municipal mayors of Medellin, San Remigio, Tabogon, Daanbantayan, and Bogo City after confirming new sinkholes and widening ground cracks.
The advisory classified affected barangays as high subsidence hazard zones, urging LGUs to enforce access restrictions, safety cordons, and public warnings. The MGB cautioned against backfilling sinkholes without expert clearance and recommended monitoring ground cracks for movement, especially after heavy rains. It also advised suspending development in identified hazard zones and conducting detailed geotechnical studies to assess underground cave systems.
The agency encouraged local leaders to initiate readiness measures, noting that sinkhole development could progress over time, with increased rainfall, lowering of the water table, or strong earthquakes potentially triggering further subsidence or collapse.
