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How earthquake-proof should Metro Manila be?

/ 04:18 AM September 21, 2019

There is no place in the Philippines that is completely safe,” said New Zealand-based scholar and map maker, David Garcia. “Rather, there are only places of varying risks and vulnerabilities.”

According to the Philippine Institute of Volcanology and Seismology (Phivolcs), the entire Philippines lies above a fault zone, which is 1,200 kilometers long and traverses from Luzon to Mindanao. Thus, online news portal Interaksyon reported that in preparing the country’s map based on its seismic activity, Garcia found that almost all regions have experienced earthquakes at varying magnitudes of six and seven, with only the Palawan and Sulu provinces appearing to be earthquake-free.

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Meanwhile, in its earthquake reduction study on the Greater Metropolitan Manila Area (GMMA), Japan International Cooperation Agency (Jica) stated that the Valley Fault System, which traverses it, may cause the largest impact should it generate a large earthquake. Moreover, active phases of the Valley Faults are approaching, with a minimum estimated magnitude of seven.

Despite the anticipated earthquake damage, Jica has found that majority of the GMMA remains vulnerable. Navotas Bay, Manila North Port, South Eastern Manila, and central Manila Bay areas are at greater risk of flammability and difficulty in facilitating evacuation. Meanwhile, North Eastern Quezon City, Western Marikina City, Eastern Pasig City, Muntinlupa Laguna Bay, and Mandaluyong-Makati City border areas may be more vulnerable to building collapse and difficulty in facilitating evacuation than other neighboring areas.

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Thus, on May 8, 2018, President Duterte issued Executive Order No. 52, mandating government agencies to take proactive steps to guarantee the resiliency of all public infrastructures in GMMA. Local government units (LGUs) in the GMMA shall ensure the resiliency of private infrastructures and the private entities’ compliance with the National Building Code.

Meanwhile, said agencies and entities are highly encouraged to go beyond the minimum standards set in the National Building Code and thus, ensure that all future infrastructures are fully earthquake-resilient and compliant with international earthquake-resistant designs and standards.

In ensuring the resiliency of their respective infrastructures, government agencies, LGUs, and private entities may be guided by the National Structural Code, which the Department of Public Works and Highways endorsed as the referral code to the National Building Code, and is based on international codes and references.

Under the National Structural Code, buildings, towers and other vertical structures shall be designed according to the applicable Strength Design, Load and Resistance Factor Design and Allowable Stress Design methods. Furthermore, said infrastructures’ structural systems shall be designed to have adequate stiffness to limit deflections, lateral drifts, vibration or any other deformations that adversely affect their intended use and performance. In this regard, the design shall consider durability, resistance to exposure to weather or aggressive environment, crack control and other conditions that affect such intended use or performance.

Unless waived by the building official, every building in Seismic Zone 4 over 50 meters in height shall be provided with at least three approved earthquake-recording instruments or accelerographs. They shall be located in the basement, mid-portion and near the top of the building. Each instrument shall be located so that access is maintained at all times and is unobstructed by room contents.

Furthermore, anchors in masonry walls of hollow units or cavity walls shall be embedded in a reinforced grouted structural element, and pursuant to the earthquake design requirements prescribed by the National Structural Code.

Meanwhile, earthquake loads, which primarily safeguard against major structural failures and loss of life, shall at least be designed and constructed to resist the effects of seismic ground motions and overturning as indicated in the National Structural Code. The procedures and limitations for the design of these structures shall be determined based on the prescribed seismic zoning, site characteristics, occupancy, configuration, structural system and height.

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