Low- and mid-rise buildings in Metro Manila are the most vulnerable to collapse if the long-feared “Big One” strikes, with simulation estimates showing that up to 13 percent of such structures could be heavily damaged or collapse during intensity 8 shaking along the West Valley Fault, Department of Science and Technology (DOST) Secretary Renato U. Solidum Jr said on Thursday, March 5.

Solidum said many of these buildings are non-engineered structures, making them more susceptible to the strong and rapid ground shaking expected if a magnitude 7.2 earthquake occurs along the West Valley Fault.

“Let us say that during simulation estimates, let’s say 10 percent will be heavily affected or collapsed. And most of these are the low-rise buildings and mid-rise buildings, not the very high-rise,” he said during an interview in Makati City.

“Simply because of the resilience of the structures themselves, there are many non-engineered structures in the low-rise.”

He explained that earthquake dynamics also affect how different types of buildings respond to ground shaking.

“And on earthquake science, because when the fault moves, and since this is in Metro Manila, the West Valley Fault, the shaking is kind of fast, that the tendency is for it to shake the shorter buildings because the shorter buildings are lighter. They don’t shake the taller buildings,” said Solidum.

“But if the earthquake on the other hand is far away from Manila, the taller buildings will shake more than the short buildings,” he went on.

Simulation models, according to him, indicate that around 10 to 13 percent of short- to mid-rise buildings could collapse, while only about two percent of buildings higher than 30 stories are expected to be heavily affected.

“Let’s say 10 percent to 13 percent for short to mid-rise buildings, only 2 percent for the more than 30 stories. So the real problem is the low-rise and mid-rise,” Solidum said.

Based on earlier earthquake impact studies, Solidum said a major earthquake along the West Valley Fault could result in around 33,000 deaths in Metro Manila, while fatalities could reach about 50,000 if nearby provinces such as Bulacan, Rizal, Cavite, and Laguna are included.

He warned that Metro Manila could experience Intensity 8 shaking under the magnitude 7.2 earthquake scenario.

“The shaking will be strong in the whole Metro Manila intensity 8,” he said.

Solidum emphasized that the strength of structures- - -rather than location alone- - -would largely determine whether buildings collapse during a major earthquake.

“But that is not the main factor in being affected. It is the strength of the house or the building,” he said.

“So our building code assures us that when there is an intensity 8 shaking, the buildings that follow the code will not collapse. So it’s essentially the strength of the house.”