Cebu: When the magnitude 6.9 earthquake devastated Northern Cebu on Sept. 30, an invisible geological hazard violently surfaced: sinkholes that began to appear in Bogo City, San Remigio, and nearby towns, leaving residents alarmed and geologists on high alert. Immediately, the Department of Environment and Natural Resources' Mines and Geosciences Bureau (DENR-MGB) rushed to the area, highlighting a critical reality.
According to Philippines News Agency, based on the available Mines and Geosciences Bureau geologic maps, at least 60 percent 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 MGB stated that the formation of sinkholes often initiates in voids and cavities within the underlying rock. The presence of faults and fractures in limestones increases the dissolution process in the karst of Cebu.
The MGB explained that the appearance of sinkholes is a phenomenon typical in karst landscapes, or areas underlain by soluble rocks like limestone, which is the case in Cebu. A sinkhole is a hole in the ground that forms when the surface layer of land collapses, and is common in karst terrain where underground rock, like limestone, is dissolved by groundwater. Over time, this creates caverns, and when the ground above can no longer support its own weight, it collapses.
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. The MGB noted that this vast area of Cebu's vulnerable ground makes mapping this invisible threat crucial to mitigate a perpetual hazard.
The MGB reported that the ground shaking due to the movement of the Bogo Bay Fault disrupted the already fragile ground, causing several sinkholes to collapse or expand. 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.
The agency emphasized that the same limestone that gives Cebu its famous caves, rolling hills, and aquifers also hides an intricate network of voids and underground rivers. Over time, water seeps through cracks in the limestone, gradually enlarging them into cavities and cave systems. The recent earthquake further exacerbated these conditions, causing additional sinkholes to form.
Long before the recent earthquake, the MGB had already been working to understand Cebu's underground risks. Since 2015, the agency has been conducting Karst Subsidence Hazard Assessments across the country, using remote sensing, fieldwork, and geophysical surveys to identify sinkhole-prone areas. As of this month, 16 municipalities in Cebu, including Bogo City and San Remigio, have undergone detailed mapping.
These assessments have established a baseline inventory of sinkholes and karst features and karst subsidence susceptibility delineation in these areas. With the recent earthquake, a post-disaster geohazards assessment of the hardest-hit areas in northern Cebu is necessary to identify residual threats and assess the suitability of identified temporary relocation sites.
To date, the MGB has assessed 198 sinkholes before the quake, and documented 14 additional sinkhole collapses, bringing the total to 212 in northern Cebu, as well as several ground cracks in a number of areas after the Sept. 30 tremor. Moreover, additional signs of ground subsidence, such as circular tension cracks, differential settlement, subsiding ground, and progressing cracks on structures, are being monitored by the agency.
The MGB advised residents living near sinkholes and in areas of active karst subsidence to closely monitor signs of subsidence, like tilted fences and trees, horizontal cracks in houses, and exposed building foundations. It also recommended that 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.
'Mapping these hazards allows us to visualize what we can't see. It helps local governments decide where it's safe to build, where monitoring is needed, and how to plan relocation sites after disasters,' the MGB said. The agency underscored the importance of being aware and prepared, as the more people understand the geology beneath them, the safer their communities become.
