Manila: Scientists have begun tracking down sinkholes in central Philippines after the discovery of a more than five-kilometre long and three-metre high rock formation in Bohol, following a 7.2 magnitude earthquake that occurred in October, sources said.
Three geologists of the Mines and Geosciences Bureau (MGB) have been conducting a geo-hazard mapping in Bohol and Cebu. Efforts include identification of areas with sinkholes, which could swallow buildings or entire towns during heavy rains and floods, Mines and Geosciences Bureau (MGB) Director Leo Jasareno said.
They have inspected a 15-metre suspected sinkhole in Cebu, which appeared after a 7.2 magnitude earthquake occurred between the towns of Sagbayan and Catigbian in Bohol on October 15. But the team is looking for wider, hidden sinkholes that could collapse mountains and towns, said Jasareno.
Part of the team’s initial report was given to local government leaders of Bohol and Cebu to guide them in the reconstruction and rebuilding efforts being undertaken in the earthquake affected areas where 198 people were killed, said Jasareno.
The base of Bohol and Cebu is made of limestone. Bohol’s base has additional coral. Erosion of limestone and corals often result in sinkholes that open up and swallow houses and towns during heavy rains and earthquakes, explained Jasareno.
Now, it is easier for geologists and residents to look for sinkholes at Sagbayan and Catigbian towns in Bohol because a rocky wall, more than five-kilometres long and three-metres tall, was discovered there. Underneath the wall is a deep fissure, said Jasareno.
The impact and importance of this geological occurrence is being studied, but it is not yet known if there is a wider sinkhole underneath the jutting fence-like rock formation, said Jasareno.
Scientists are wary because a comprehensive mapping of sinkholes could take time. For example, the sinkhole mapping of Baguio City could end by the first quarter of 2014, said Environment Secretary Ramon Paje.
Baguio City was one of the hardest hit areas when a 7.9 magnitude earthquake occurred near Nueva Ecija in 1990. At the time, it killed more than 1,600. Baguio City’s base is also made of limestone.
Geologists are now exploring Chocolate Hills, a famous tourist spot in Bohol, where some part of its topsoil collapsed after the earthquake. The tourist spot’s limestone turns brown during summer.
Movement of water on coral and limestone had formed the hills 15 million years ago, geologists said, hinting that the earthquake could, in time, impact the hills’ future structure.
At the same time, geologists are busy making a map of the uncharted fault line in central Philippines that was discovered after the earthquake’s epicentre was located days after its occurrence in a town in Bohol in mid October, said Renato Solidum, head of the Philippine Institute of Volcanology and Seismology (Phivolcs).
Earlier, Phivolcs said the earthquakes’s epicentre was near Bohol’s Carmen town, adding it was due to the movement of the East Bohol fault line.
“The new fault line in central Philippines is not yet charted. It is really difficult to track down earth movement in areas with a limestone base,” explained Phivolcs scientist Karl Vincent Soriano.
The Philippines is part of Asia Pacific’s Ring of Fire.
A 7.9 magnitude earthquake triggered a tsunami that killed 6,000 people in the Moro Gulf, southern Philippines in 1976. A 7.3 magnitude earthquake killed 270 in Casiguran, Quezon, southern Luzon in 1968.