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West Valley Fault ‘ripe’; DPWH inspects bridges, buildings

By TJ Burgonio
Philippine Daily Inquirer
First Posted 04:39:00 03/17/2011

Filed Under: Earthquake, Infrastructure, Emergency Planning, tsunami, Housing & Urban Planning, Disasters (general)

MANILA, Philippines?Many deaths are expected from low-cost homes and low-rise buildings if a powerful temblor hits Metro Manila, officials said Wednesday.

They said the West Valley Fault, which runs from Sierra Madre through eastern Metro Manila to Southern Tagalog, could move anytime and generate a 7.2-magnitude earthquake.

Unlike ?well-constructed? high-rises, low-cost homes and low-rise buildings that skew the National Building Code could not withstand a powerful earthquake, said Public Works Secretary Rogelio Singson.

The Department of Public Works and Highways (DPWH) has also made a quick inspection of 1,100 buildings in the metropolis based on whether they comply with the National Building Code, or on what earthquake intensity they can withstand.

These included hospitals, schools and housing projects.

?These only needed minor repairs,? Public Works Undersecretary Raul Asis later told reporters of the 1,100 buildings.

?The bigger problem that was coming out was the low-cost buildings that tend to do shortcuts or do not follow strictly to the letter the National Building Code,? Singson said at a Senate hearing on disaster preparedness.

These structures include the homes of informal settlers that are often expanded to accommodate more people beyond their structural capacity, he said.

?And then how safe are the two-story structures? If a big earthquake happens, we should expect a bigger damage there because they don?t follow the safety code,? he said.

Movement anytime

These low-cost homes lie within the band of the West Valley Fault, and hence, vulnerable to a powerful earthquake, another official said.

The Senate committees on climate change and environment opened the inquiry after Friday?s cataclysmic 9-magnitude earthquake hit northeastern Japan and triggered a devastating tsunami and a nuclear crisis.

The Philippine Institute of Volcanology and Seismology (Phivolcs) said the West Valley Fault was ?ripe? for movement anytime.

Phivolcs Deputy Director Bartolome Bautista said that its last recorded movement was 200 years ago.

?It?s already 200 years after the last movement. So there?s a very high probability that this will move in the future but we can?t say the exact time. Based on our assessment, the fault is ripe for movement,? he said.

Sen. Loren Legarda, chair of the climate change committee, had to clarify what he meant by ?ripe? so Bautista would not sound too alarmist.

Bautista, however, maintained that the country had entered the cycle of 200-400 years of recurrence of the fault?s movement.

?It can move anytime. It can strike on the 200th year. Better if it hits on the 400th year. But we should be prepared if it strikes on the 200th year,? he said.

Citing a 2002 study, ?Metro Manila Earthquake Impact Reduction,? Bautista said the West Valley Fault could deal the ?highest intensity? of earthquake in the metropolis if it moved.

?It?s capable of producing a 7.2-magnitude earthquake,? he said.

Not easy to detect

While Phivolcs has installed a network of seismic stations around active faults, it?s not easy to detect an upcoming earthquake because some faults trigger fore-shocks while others don?t, Bautista said.

Citing another 2004 study, Singson said that the West Valley Fault could cause a 7- to 7.2-magnitude earthquake, which he said was a ?potentially damaging? earthquake.

Malacañang

But the DPWH has yet to inspect Malacañang, the Government Service Insurance System building now being rented by the Senate, and the structures of the mass transit systems, Light Rail Transit and Metro Rail Transit.

?We have identified which buildings need a more intrusive inspection because visually you may notice cracks, but you need to do an in-depth assessment,? Singson said.

The committees asked Singson to submit the DPWH assessment of the 1,100 buildings.

Schools retrofitted

Buildings that had been retrofitted were Quirino Grandstand and Araullo High School, in Manila, among others, according to Undersecretary Asis.

Tenement housing projects in Tondo, Sta. Ana and Taguig were ?condemned? by the city governments and ordered demolished, he added.

In contrast, newer buildings that adopted the latest structural code would withstand an 8- or 8.9-magnitude earthquake. Next to be assessed are buildings that are at least 15 years old, Singson said.

?The information that we?re getting is that high rise buildings, because they can afford top-notch structural engineers, civil engineers, they are well constructed,? he said.

Bridges

To ensure continuity of business activity, the DPWH has also checked bridges in the metropolis, and identified nine that need reconstruction and 11 that require retrofitting, Singson said.

?As far as public facilities, we?re practically done,? he said. ?Now we?re moving to the provinces. That?s why as the national building official, we?ve directed all building officials to do the same in their areas of jurisdiction.?

The bridges that had to be retrofitted because they were not at par with the updated National Building Code included the Nagtahan Bridge, Ayala Bridge, a bridge on Osmeña Highway (formerly South Superhighway), and Jesus Bridge, Asis said.

Vulnerable to tsunami

Bautista said coasts facing the Pacific Ocean in the east and the South China Sea in the west were most vulnerable to tsunamis because of the active trenches running under these bodies of water.

?It?s possible it could reach up to 4 meters,? he said when asked by Senate President Juan Ponce Enrile if a tsunami would hit Manila Bay.

Bautista said a tsunami in Manila Bay could be caused by the Manila Trench which lies 50 kilometers off Corregidor, and which could generate an 8.5-magnitude earthquake. Its last recorded movement was during the Spanish times.

He said Phivolcs was developing low-cost tsunami sensors for tsumani-prone communities.

?We are targeting to develop demonstration models in chosen sites. In Japan, these could emit a warning in three minutes. We?re targeting that in our project,? he said.

A tsunami occurred in the Moro Gulf in 1976 and off Calapan, Mindoro province in 1994. The tsunami in 1976 killed some 8,000 people in Mindanao.



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