Sinkholes in Cebu after earthquake: What are they?

SINKHOLE composite image from Cebu Daily News photos

With a vast karst landscape, or areas that lie over soluble rocks such as limestones, the Philippines is no stranger to sinkholes, like what have been seen in northern Cebu in the past few weeks.

As pointed out by the United States Geological Survey (USGS), a sinkhole is a depression in the ground and is most common in karst landscapes, or regions where rocks below the surface can be dissolved by water circulating through them.

For instance, when rainwater moves down through the soil, soluble rocks such as salt beds and domes, gypsum, limestone and other carbonate rock begin to dissolve, eventually creating spaces and caverns.

The USGS said “sinkholes are dramatic because the land usually stays intact for a period of time until the underground spaces get too big.” As a result, the support below weakens, so the land, which covers the sinkhole, collapses.

As stated by the Mines and Geosciences Bureau (MGB), after the magnitude 6.9 earthquake hit northern Cebu on Sept. 30, 70 sinkholes have been discovered, with San Remigio having the highest count at 40.

Sixteen sinkholes have been confirmed in Bogo City, eight in Daanbantayan, two each in Medellin and Tuburan, and one each in Sogod and Tabuelan, said the local office of the MGB in Western Visayas.

RELATED STORY: Death toll from Cebu earthquake now at 76, says NDRRMC

The MGB, explaining how typical subsidence is in karst landscapes, stated that at least 60 percent of the island of Cebu is underlain by “highly soluble limestones, forming a unique karst topography.”

GRAPHIC: Ed Lustan/INQUIRER.net

Across the archipelago, 35,000 km2, or 11.7 percent of the total land area of the Philippines, is karst, with carbonate rocks occupying a large portion of the natural landscape, based on data from the Biodiversity Management Bureau.

This landscape stretches through 1,171 municipalities and is characterized by caves, conical hills, karst valleys and ridges, karst plains, and uplifted marine terraces, saying that the presence of “faults and fractures in limestones increases the dissolution process.”

Triggers of collapse

Sinkholes are semi-circular ground depressions or openings formed when the support of the overlying cover is weakened by the presence of underground cavities or cave systems that result from the dissolution of soluble rocks.

As to what triggered the collapse of sinkhole covers in northern Cebu, the MGB pointed to the strong earthquake that was set off by the movement of the Bogo Bay Fault. “[This] resulted in the instability of the fragile sinkholes and cave systems.”

However, aside from ground shaking, the collapse of sinkhole covers may as well be triggered by intense flooding, lowering of the water table, ponding of subterranean rivers, and heavy infrastructure development.

GRAPHIC: Ed Lustan/INQUIRER.net

Looking back, in 2013, the MGB initially saw 100 sinkholes in Bohol after the magnitude 7.2 earthquake that shook the province, but in a later study, over 1,000 more have been seen.

READ: MGB finds 1,333 sinkholes in Bohol

Now, with the Philippine Institute of Volcanology and Seismology saying that aftershocks in Cebu already reached 12,000, the MGB warned that more sinkholes could collapse or that existing ones could expand.

Sinkhole indications

As the MGB stressed, subsidence triggered by sinkhole collapse is one of the most dangerous geohazards because of its extreme unpredictability, like what happened in Danao City in Cebu in 2023, when a resident was “swallowed” by a sinkhole.

Here are the warning signs of the existence of a sinkhole and a possible collapse:

• Subsidence of a land to a level that already exposes the foundations of a building or infrastructure
• Occurrence of horizontal cracks and the chipping or breaking off of rocks on slopes
• Appearance of small holes in cemented areas
• Tilting of trees toward the sinking part of the ground
• Formation of horizontal and stair-like cracks on walls
• Appearance of circular or semi-circular cracks on the ground
• Slight ground subsidence and loss of soil moisture that causes plants to wither
• Land deformation
• Cave collapse or erosion

The International Association of Certified Home Inspectors said sinkholes, especially when covers collapse, threaten people and livelihood since they can kill or injure residents, threaten water supplies, and cause structural damage and instability.

But as the MGB pointed out, seriously concerning, as well, is the pollution of groundwater because “pollutants can easily enter and spread” in open sinkholes, making “karst systems highly vulnerable to contamination.”

READ: EXPLAINER: Why Cebu’s 6.9 quake was deadlier than Davao’s stronger quakes?

The MGP issued reminders to keep people safe from the dangers of open sinkholes, saying that it would be best to stay away from the collapse or those that have signs or indications of karst subsistence.

“Once a sinkhole or cave collapses, the area should be barricaded,” it said, pointing out that the hole should not be covered, unless advised by authorities.

Likewise, it reminded people to be on the lookout for warning signs of a possible collapse.