The Mines and Geosciences Bureau (MGB) 7 of the Department of Environment and Natural Resources (DENR) 7 has strongly reminded the local government units (LGUs) to be more vigilant and to take precautionary measures where there is a sinkhole collapse brought about by the Oct. 15 magnitude 7.2 quake.

“Sinkholes are natural depressions formed as a result of the collapse of the cavern roof,” MGB 7 Regional Director Loreto B. Alburo.

Carbonic acid can gradually dissolve limestone rocks that will result to the formation of caverns, said MGB 7’s chief geologist Al Emil G. Berador.

Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by circulating ground water. As the rock dissolves, spaces and caverns develop underground.

The formation of sinkholes can be dramatic because the surface land usually stays intact until there is no longer enough support.  Then, the surface area can suddenly collapse.

In an Association of Government Information Officers (AGIO) 7 forum yesterday at the Philippine Information Agency (PIA) 7, Berador said majority of the areas in Cebu and Bohol are underlain with limestone deposits.

About 60 to 70 percent of Cebu and Bohol is composed of limestone deposits, he said.

Berador said the collapse of cavern roof could be triggered by ground vibrations such as earthquakes or by human induced activities.

Meanwhile, In Bohol, last Oct. 31, Senior Science Research Specialist Abraham Lucero Jr.  identified a sinkhole with a diameter of 20 meters and a depth of more than one meter, along the road in Sitio Basak, Barangay Taloto, Tagbilaran City .

More than 100 meters away from that sinkhole, a sinkhole was also noted within the Victoria Memorial Park with a diameter of 20 meters with water coming out.

Lucero advised the local government unit to reroute the traffic and evacuate certain residents near the affected area and place a barricade so that people will be kept away from the hazards.

In a related development, a nine-member team from the MGB central office and MGB 7 will conduct another round of assessment today (Nov. 6) to map sinkholes in Tagbilaran City and adjacent towns.

It could be recalled that close to 100 sinkholes were found in Bohol where the LGUs were issued the appropriate geohazards threat advisories.