Graves could be anywhere— you might also have one in your backyard

When the ground slowly creeps open in your backyard, like the earth digging its own grave, it may feel like a scene straight out of a movie. But this seemingly fictional sight is a serious threat now faced by northern Cebu towns after the powerful earthquake—a looming danger of sinkholes.

Following the 6.9-magnitude earthquake that struck the Philippines, mainly Cebu City, the Central Visayas Department of Environment and Natural Resources (DENR-7), through its Mines and Geosciences Bureau (MGB), issued a Subsidence Threat Advisory or ground-collapse warning to towns, including  San Remigio, Medellin, Bogo City, Tabogon, and Daanbatayan, after confirmed occurrences of sinkholes.

Sinkholes, according to the National Museum of the Philippines, are ground cavities primarily caused by water erosion of underground soluble rocks, like limestone, dolomite, and gypsum, where natural and human-caused factors play a role in accelerating and triggering their formation.

“The recent seismic activity has destabilized ground conditions, resulting in the formation of sinkholes in certain affected areas,” the agency said in its advisory. 

The MGB added that development of sinkhole could progress through increase in rainfall and a strong earthquake could trigger its further collapse. Although the board deemed the collapse rare, its possibility leads to serious damage to infrastructure, displacement of residents, and even loss of life.

Beneath shaky ground

Seismic activities is one of the primary factors identified by the DENR-7 in the sinkhole formation in Cebu, along with the geologic assessment that 60 to 70 percent of its localities are primarily made of limestone deposits.

Limestone deposits, a type of carbonate rock, are large sedimentary beds of rock that are highly susceptible to chemical erosion and dissolution by water. These, like most other carbonate rocks, are formed from compacted shells and skeletons of marine organisms that lived millions of years ago.

With the increased rainfall and the acidity of rain and circulating groundwater, the MGB-7 chief geologist Al Emil G. Berador said, limestone then dissolves and creates underground cavities, causing the ground above to collapse when support beneath is lost. 

“Sinkholes, particularly formed by cover-collapse, pose greater risks to life and property, as continuing aftershocks and heavy rainfall may trigger subsidence,” Berador said. 

Cover-collapse, a serious type of sinkhole, occurs when the water slowly dissolves rock from the bottom and creates a hollow space underground, making the overlying ground suddenly collapse.

Sinkholes may also be solution sinkholes, where exposed surface is slowly dissolved by the rainwater, creating shallow dip or hollow, and cover-subsidence sinkholes, where overlying sediments slowly settle downward, creating noticeable depression on the surface.

“The local government [of Cebu] is still awaiting full technical guidance from national agencies to determine whether these areas must be declared no-build zones, along with safety measures needed to be implemented,” San Remigio mayor Mariano Martinez told CDN Digital. 

Filling in the gaps

Early detection of sinkhole-prone localities by Philippine Institute of Volcanology and Seismology (PHIVOLCS) remains key to minimizing, if not preventing, damage to properties, noted Martinez, as the agency identified over a hundred sinkholes in the city.

Among the tools used to predict sinkhole locations was Geographical Information Systems (GIS) mapping, where layered maps locate sinkholes based on seismic and geologic data. It then models these data, along with other geospatial information such as precise coordinates and accurate road length and building sizes, to predict sinkhole occurrences and inform disaster mitigation strategies. 

Ground-Penetrating Radar (GPR) has also been used, where radio waves penetrate the ground and detect layers and empty spaces by reflecting on different materials and boundaries underground.

These techniques were then used to validate findings of field and ground inspections that use surface clues like vegetation change and structural issues to identify sinkhole-prone areas. 

“Natural sinkholes cannot be fully prevented, but by restricting access and installing warning signs in affected areas, monitoring ground movement, and discouraging construction on identified high-risk zones, we might manage risks associated with sinkholes,” Berador recommended.

The agency added that the safety of the residents depends on proactive monitoring, information dissemination, and adherence to strict geohazard warning implemented.

Tracing back its history, Cebu has always been prone to sinkholes, according to DENR 7-MGB. Its primary limestone geology, weakened by small tremors or seismic stress, has notably led to destruction of infrastructure, partial collapse of a mountain slope, and the collapse of a road in Badian.

In response, the local government units (LGUs) in Cebu have since doubled down their efforts with mapping, warnings, and regulatory measures, but no large-scale mandated relocation has happened.

Grounded in vigilance

As aftershocks continuously rattle northern Cebu, residents fear returning indoors and have been staying in open areas and temporary tents, while the LGUs are still identifying a permanent relocation site. 

“For now, we are informing the people about the sinkholes… Ang mga duol gyud kato sila ang nag-evacuate kay nakuyawan man,” Martinez said [For now, we are informing the people about the sinkholes… Those who were near the sinkholes were the first ones to evacuate as they were afraid].

Though residents of those high-risk areas have already evacuated with some staying in open fields, others remained fearful of their surroundings, saying even the municipal halls, bridges, and churches were not spared from destruction.

While the ground beneath may shift without notice, community preparedness and mitigation remain essential; critical awareness, not fear, will keep the province steady on its feet.