Second of three parts
REPORTS such as the 2002–2004 Metro Manila Impact Reduction Study hold that certain locations are more prone to earthquake risks than others. The study, conducted by the Japan International Cooperation Agency, in partnership with the Metropolitan Manila Development Authority and the Philippine Institute of Volcanology and Seismology, estimates that about 40 percent of residential buildings in the National Capital Region would be severely damaged or collapse during the “Big One” — an Intensity 7.2 earthquake. There would be massive disruptions to water, power, telecommunications and major transportation routes. The most vulnerable territories are the ones located on or near the West Valley Fault, a 100-kilometer active geological fault line running through the Greater Manila Area.
Had the Big One happened in 2004, based on the data, Architect Jun Palafox, head of Palafox Associates, said 3 percent of the tall buildings and 10 percent of the low-rise buildings in the NCR would have collapsed. Jose Miranda, chairman of the Committee on Resilient Architects, estimated that 2 percent of 30- to 60-story buildings, and 13 percent of medium- to low-rise ones would be damaged or collapse.
Ruel Ramirez, principal of structural engineering consulting firm RBRA, said all buildings and houses in the NCR will be affected should either the West Valley Fault or the Manila Trench move. The impact on certain properties will depend on which fault line is moving. The West Valley Fault movement will place low-rise buildings, including houses, in critical condition. The Manila Trench movement will endanger high-rise buildings.
“The travel time of the earthquake on the West Valley Fault is fast. The shorter buildings also move fast because of that frequency,” he said. “The Manila Trench [travel time] is slow, the movement of the ground is slow, it is the high-rise that is affected.”
Degree of damage
The resulting damage depends on the intensity of the earthquake. Ideally, structures should not suffer any serious damage if the magnitude is 4 or 5.
When it comes to Intensity 6, Ramirez said some of the damage to the buildings can still be “reparable.” Miranda agreed, saying that given that some buildings “will just dance, sway back and forth without collapsing.” There are some reasons for this resilience. Ramirez said a building can withstand an Intensity 6 earthquake if it is well-designed.
“Non-engineered buildings are the most vulnerable to earthquakes,” said Anthony Pimentel, structural engineer and president of Pimentel & Associates Engineering Consultants. Miranda said high-rise structures can withstand Intensity 7 to 8 earthquakes if their structural design parameters are intentionally designed for that amount of resilience. He differentiates the mindset of some property developers today compared to those of earlier generations: “Most of the old, low-rise buildings may not be structurally fit or were lacking in compliance during the time they were built. Recent modern high-risers are designed by reputable structural engineers using rigorous seismic standards,” he said.
To be continued