Mines bureau maps sinkholes, says they are greater hazards

MANILA, Philippines—The Mines and Geosciences Bureau (MGB) is racing to map out “concealed sinkholes” in Bohol and Cebu, saying that they pose a “greater hazard” with the continuing aftershocks jolting the Central Visayas.

MGB Director Leo Jasareno said that the agency had confirmed the existence of a ruptured sinkhole in Lapu-Lapu City, and was looking into reports of six others in Calape town, Bohol.

Jasareno said that MGB teams had been dispatched to Bohol and Cebu to locate both “ruptured” and “concealed” sinkholes in the provinces.

“We have to find the (sinkholes). The concealed sinkholes pose a greater hazard now, and Bohol is a priority,” he said.

According to Jasareno, a ruptured sinkhole is apparent and seen as a hole in the ground caused by the collapse of the top soil. A concealed sinkhole, as its classification indicates, has the top soil intact but is hollow underneath.

The MGB official said that the only confirmed ruptured sinkhole in the aftermath of the magnitude 7.2 earthquake last week was found in Lapu-Lapu City. The mouth of the sinkhole measured 40 meters (about 131 feet, or equivalent to almost three buses in a line) and spanned 20 meters across (about 66 feet). “It is about one to two meters (about 6.6 feet) deep,” Jasereno said.

He added that MGB personnel were in the process of verifying reports of ruptured sinkholes in six different sites in Calape.

Jasareno admitted that before the powerful earthquake last week, the sinkholes, both ruptured and concealed, were not considered to be much of a concern although they were included in the agency’s geohazard mapping activities.

The geohazard maps are focused primarily on areas prone to flooding and landslides.

“But now (sinkholes are) a priority, particularly the concealed ones in Bohol. Bohol is 80 percent limestone, while 50 percent of Cebu is limestone,” he said, explaining that sinkholes usually occurred in limestone.

According to the US Geological Survey (USGS) website, sinkholes are “common where the rock below the land surface is limestone, carbonate rock, salt beds or rocks that can naturally be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground.”

The USGS further said that the land on top of sinkholes usually stayed intact until the void underground got too big and there was no longer sufficient support for the topsoil, which then collapsed.

Meanwhile the Philippine Institute of Volcanology (Phivolcs) has, as of noon Monday, recorded a total of 2,278 aftershocks from the Oct. 15 earthquake. Of the number, 49 were felt, including a magnitude-5.4 tremor that jolted Bohol and Cebu early Monday morning.