(Lead) Bohol prone to sink hole formation

By Catherine Teves

MANILA, (PNA) -– Mines and Geosciences Bureau (MGB) isn’t discounting the possibility sink holes dot Bohol, noting such naturally formed depressions in the ground often occur in areas of limestone which is the sedimentary rock mainly underlying this island-province.

”Almost 80 percent of Bohol is limestone,” said MGB Dir. Leo Jasareno.

He noted water dissolves limestone over time so sink holes often develop in areas underlain by such sedimentary rock.

The dissolution results in formation of underground cavities or open spaces, leading to emergence of sink holes.

After the magnitude 7.2 killer earthquake’s onslaught in Bohol on Tuesday morning (Oct. 15), reports surfaced about alleged emergence of sink holes in Calape and Loon municipalities there.

MGB supervising science research specialist Liza Manzano clarified earthquakes don’t create sink holes, however.

She noted earthquakes facilitate the ground’s rupture, possibly exposing cavities beneath the soil surface.

Manzano noted the possibility for sink holes’ formation is higher in southwestern Bohol as limestone there is still “young,” dating back to the Pleistocene period less than three million years ago.

She noted particles of such cavernous limestone aren’twell-compacted yet so this kind of rock is porous and more prone to dissolution.

Aside from Bohol, Jasareno said other areas where limestone is “widespread” are Baguio City in Luzon and Cebu island-province in the Visayas.

”Limestone also exists in parts of Negros Oriental province,” he said.

Jasareno said MGB aims to complete as soon as possible the more detailed geohazard map for Bohol so limestone areas there can be identified.

”If we know the limestone areas, we can easily see where the possible sink holes are,” he said.

MGB is already preparing to deploy a team that will look into the matter.

The team will undertake its work using the agency’sground-penetrating radar (GPR).

According to Jasareno, MGB’s GPR is like an x-ray that can probe rocks 100 meters deep.

”Unless a detailed survey is conducted using GPR, we can’t say the sink holes’ extent,” he said.

MGB’s team will also conduct post-hazard assessment and landslide mapping in Bohol, said Manzano who’ll be joining this group.

Information generated will help MGB warn communities there about possible occurrence of landslides induced by rain or low pressure areas, she said.

Earlier, Manzano raised urgency for cordoning off all suspected sink holes to protect people from harm.

”We must investigate such sites first,” she said.

She said sink holes’ diameter can reach about 600 m, putting at risk structures built over these.

MGB continues calling on people to report ground depressions they observe so this agency can investigate those.

Even depressions less than a meter in diameter must be reported as soon as possible, MGB added.