Bohol and Cebu have sinkholes just waiting for heavy rains and floods or earthquakes to collapse and swallow entire towns.
The Philippine Institute of Volcanology and Seismology (Phivolcs) said Thursday the 7.2-magnitude earthquake was possibly caused by the movement of a fault line that remained uncharted because of the limestone base of Bohol.
Environment Secretary Ramon Paje said the provinces of Bohol and Cebu, along with Baguio City, had foundations of solid limestone, which had been weathered and carved out naturally through a long period of time, making it prone to sinkholes.
“[A sinkhole] is always a danger so it is important to identify where it is in an area,” Paje told the Inquirer.
He said sinkholes already existed in most areas of Bohol, Cebu and Baguio City, but were still not apparent.
In a lay person’s view, he said, a sinkhole is a visible, deep depression on the ground. But in geology, “[i]t is a vacuum beneath the ground or top soil waiting for an occurrence (earthquake or heavy rain) to rupture.”
Paje said he had ordered the Mines and Geosciences Bureau (MGB) to expedite the geohazard mapping, specifically of sinkholes, in the provinces of Bohol and Cebu.
“It is more than 50 percent complete for the two provinces. The sinkhole mapping has already been completed in Baguio City. It was previously supposed to be due by the end of 2014 but the MGB committed to finish the geohazard mapping within the first quarter of next year,” he said.
Paje said, however, that the local governments of Bohol and Cebu are “very much aware” of the existing sinkholes.
Asked how much of Cebu and Bohol was made of limestone, Paje said the two provinces were almost entirely of solid limestone.
“Bohol’s base is limestone and coral,” he said.
Paje cited some areas of Chocolate Hills in Bohol, the topsoil of which had collapsed.
“The white that appeared is solid limestone. If you would look closely, what was shed off was the topsoil and grass,” he said.
Citing MGB records, Paje said the Chocolate Hills were formed 15 million years ago under water.
The movement of water against coral and limestone shaped Chocolate Hills, he said.
The MGB, Paje added, is verifying the report of a sinkhole somewhere in Cebu that ruptured following Tuesday’s earthquake.
According to the US Geological Survey website, “[s]inkholes 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.”
Uncharted fault line
Meanwhile, Phivolcs science research assistant Karl Vincent Soriano said the movement of the East Bohol fault may not have actually caused the powerful quake that jolted vast areas of the Visayas as well as portions of Southern Luzon and Mindanao on Tuesday.
“Based on our records of aftershocks, the movement of a different and unmapped fault system possibly triggered the 7.2-magnitude earthquake,” he said, adding that the fault system is somewhere between the towns of Sagbayan and Catigbian in Bohol.
“Our initial report that it was likely triggered by the East Bohol fault was based on our map of active faults. This fault system is not yet charted,” Soriano said, adding that limestone areas are extremely difficult for geologists to map.
He explained that limestone is easily weathered so geologists find it difficult to trace previous signs of ground movement that have been “erased.”
Soriano said one of the two Phivolcs teams deployed to Bohol had been tasked to verify the existence of the uncharted fault line, apart from making a hazard assessment in the province.
As of noon Thursday, Phivolcs has recorded 1,328 aftershocks, 27 of which were felt.
The strongest was 5.5 magnitude, which occurred at around 7:37 a.m. on Thursday.
The tremor had a depth of focus of a kilometer and had its epicenter at 18 kilometers northwest of Tagbilaran City.
An Intensity VI tremor was felt in Tagbilaran City; Intensity IV in Dumaguete City and Lapu-Lapu City as well as in Sibulan town in Negros Oriental province.
An Intensity II tremor was felt in Maasin, Southern Leyte province, and Mambajao, Camiguin province.
Phivolcs describes an Intensity VI quake to be very strong where some people lose their balance and motorists feel like driving on flat tires.
An Intensity VI quake can move heavy objects or furniture and can cause cracks on wall plaster, Phivolcs said. It can also cause slight damage to very old or poorly built houses and structures and trees are noticeably shaking.
Magnitude is the measurement of the energy released at the source of the earthquake while intensity quantifies the strength of shaking produced by the temblor.