Why Pampanga was hit hardest by earthquake when epicenter was in Zambales

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Metro Manila (CNN Philippines, April 23) — The epicenter of Monday’s magnitude 6.1 earthquake was in Castillejos, Zambales, but the biggest damage was recorded in the neighboring province of Pampanga because it sits on soft sediment, Philippine Institute of Volcanology and Seismology chief Renato Solidum Jr. said Tuesday.

Such was the case for Porac, Pampanga, which saw the collapse of a four-storey grocery store that afternoon.

“The actual location of the epicenter is in the mountains in between the provinces of Pampanga and Zambales. Unfortunately, there are several towns in both provinces that are underlain by soft sediment,” Solidum told CNN Philippines.

The PHIVOLCS director explained that this was due to the 1991 eruption of Mt. Pinatubo, which deposited lahar in parts of Pampanga and made the area prone to strong shaking and softer soil. The same can be said for the towns of Floridablanca, Guagua, and Angeles.

READ: Clark International Airport remains closed after strong quake

The cities of Subic and Olongapo in Zambales – which went through Intensity VI like Porac – also felt more powerful shaking as these are floodplains, or areas near bodies of water.

“If the foundation is softer, then the shaking will be amplified.”

Sound structures

Another factor would be the quality of the structures, the PHIVOLCS chief said.

“The design of building and how it was constructed will also matter if it will be affected or not," Solidum said. "If it’s a mall or grocery store and the span is wide, wala masyadong poste [there are not many pillars] – known naman sa Porac na malambot ang lupa [Porac is known to have soft soil] – if these factors are present, they can be affected."

“But note: not many structures were affected in that area. We have to take note what were the conditions of these buildings why they were affected, damaged when others were not?,” he added.

READ: Rescuers rush to find survivors in Pampanga supermarket rubble after quake

As of Tuesday afternoon, 16 people were reported killed in Pampanga, including five in the collapsed Chuzon Supermarket in Porac town. One man has been rescued alive from the rubble.

Porac Mayor Carling dela Cruz also said that they have recorded three children dead from other barangays after a portion of their house wall collapsed on them.

Manila's fate

The earthquake also shook parts of Metro Manila, with reports of high-rise condominiums swaying from the impact.

A skyscraper in Binondo, Manila was also seen spewing water from its rooftop swimming pool. Solidum said this was to be expected from a high-rise tower, explaining that the liquid likely swayed back and forth and got washed out of the pool.

“Every building, depending on the height, will have what we call a period or duration at which they will go back to their original position,” Solidum added.

“The shorter the building, the period is short… But if you have a very high-rise building, the period is long at 2-4 seconds. If the earthquake is far, it will affect more the mid-rise and high rise buildings compared to shorter buildings.”

Engineer Ronaldo Ison, vice president of the Association of Structural Engineers of the Philippines, added that most buildings in the capital appear to be equipped to withstand earthquakes.

Maraming buildings sa Metro Manila ang kayang mag-withstand ng ganoong earthquake… Ang talagang pinapangambahan natin ay 'yung mga buildings at bahay na hindi dumadaan sa tamang construction,” Ison said.

[Translation: “There are a lot of buildings in Metro Manila which can withstand an earthquake like that… What we’re fearing for are those buildings and homes that didn’t go through proper construction.”]

He added that building and home owners need to inspect their structures for cracks in the ceilings and walls to assess possible structural damage.