TAGBILARAN CITY, Philippines—Close
to 100 sinkholes have been discovered in nine towns and one city in Bohol after
the 7.2-magnitude earthquake that shook the province on Oct. 15.
Environment officials
said not all of the sinkholes posed dangers to the public as long as no houses
were built over them.
In Poblacion Uno village
in the capital Tagbilaran City, however, 200 families were asked to leave their
homes, as the structures were built on the roof of a sinkhole.
A sinkhole is a vacuum
or cavern beneath the ground or topsoil waiting for an occurrence (earthquake
or heavy rain) to rupture.
Bohol Gov. Edgar Chatto
said a team from the Mines and Geosciences Bureau (MGB) was assessing the
sinkholes to determine which ones posed a threat to the public.
The eight-member team is
using a ground-penetrating radar to map the island for sinkholes.
Chatto said the mapping
would be completed by the first quarter of 2014 and then the team would decide
whether there was need for engineering intervention.
“Don’t worry. Bohol is a
very safe place to stay,” Chatto told reporters on Friday.
“Makinig tayo sa mga
scientists and experts (Let us listen to the scientists and experts).”
The sinkholes were found
by the Department of Environment and Natural Resources in Central Visayas
(DENR-7) in: Tagbilaran City and the towns of Baclayon, Corella, Alburquerque,
Panglao, Dauis, Balilihan, Batuan, Loon and Carmen.
Ed Llamedo, DENR-7
information officer, said these sinkholes had been existing as long as 5.3
million years ago but only surfaced after the strong earthquake on Oct. 15 that
killed more than 200 persons and destroyed or damaged P5 billion worth of
infrastructure, churches, and public and private structures.
Llamedo urged local
officials to implement force evacuation, monitor the sinkholes, put up road
signs for sinkholes and cordon off exposed sinkholes.
Filling the sinkholes
with cement will only be advisable after the MGB team gets a complete picture
of the sinkhole below, he added.
Llamedo explained that
their radar can produce an image indicating the diameter, depth, cave pillar
and extent of the sinkholes.
The DENR issued a
Geohazard Threat Advisory on Oct. 28, recommending the preemptive evacuation of
200 families whose houses were built on the cave roof of the sinkhole found in
Poblacion Uno in Tagbilaran City. The sinkhole already ate up an interior road.
“Based on our geological
mapping, there are still more,” said MGB director Leo Jasareno. “There are
numerous sinkholes in the province based on our map interpretation.”
Most the sinkholes have
an area of four meters by five meters and a depth of two meters and are located
in the middle of barangays.
Some of the sinkholes,
Jasareno said, pose no immediate danger to residents unless the capping or the
topsoil of the sinkhole weakens.
“These sinkholes may
still widen and deepen but this will happen in geologic time, many years not in
our lifetime,” he said.
He said a particularly
large sinkhole that has not yet caved in was discovered in a school ground in
Panggangan Island through the use of ground penetrating radar.
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Radar imagery showed that the sinkhole has a length of
100 meters and a depth of five to 10 meters.
“The sinkhole is located in the school grounds but is not
directly under the school building,” said Jasareno.
The MGB, however, recommended the immediate evacuation of
residents of Barangay Poblacion 1 in Tagbilaran City after a 5-meter by 4-meter
sinkhole with a depth of two meters appeared in the barangay.
“We will recommend immediate evacuation in areas deemed dangerous,”
said Jasareno.
The MGB chief noted that 80 percent of Bohol is made up of
limestone, causing natural depressions like sinkholes to cave in as limestone
deposits underneath erode.
The agency intends to complete the survey of sinkholes in
Bohol by the end of the year.
It has already begun surveying sinkholes in Cebu City.
“We have started the survey in Cebu, but there is no
immediate danger in the province,” he said.
Compared to numerous sinkholes that appeared in Bohol after
the magitude 7.2 quake rocked the province, only one turned up in Cebu.
After the powerful earthquake struck the provinces in
mid-October, a 15-meter wide sinkhole destroyed at least five houses.
The MGB is keeping an eye on Cebu because “all of Mactan” is
limestone underneath.
