By Ellalyn De Vera-Ruiz
The soil condition in Pampanga contributed to the serious damage caused by a powerful 6.1-magnitude earthquake in the province last Monday, according to Science and Technology Undersecretary for Disaster Risk Reduction and Climate Change Renato Solidum.
The 15-second ground shaking last Monday caused the Chuzon Supermarket in Porac, Pampanga to collapse, and several other structures damaged in Metro Manila and other parts of Central Luzon.
(PNP Carlo Lagunay / MANILA BULLETIN)
As to why there is lesser damage in Zambales, where the tremor originated, than its neighboring province Pampanga, Solidum explained that the earthquake’s epicenter was “actually in the mountainous area in between Zambales and Pampanga.”
Areas in both provinces felt the earthquake at Intensity 6 as a “very strong” tremor.
Solidum also attributed the serious structural damage in Pampanga to the “soft lahar deposits” where structures are built, unlike in Zambales where structures are built on “flood plain.”
“Damage is a function not only of distance to epicenter but also foundation of structure, design, workmanship and materials of the building or structure, and its age,” he said.
“Essentially, foundation character and structural integrity when undesirable can lead to damage,” he pointed out.
According to Phivolcs, at least 18 onshore moderate earthquakes occurred in the vicinity of Zambales and Pampanga in the past.
Earthquakes ranging from magnitudes 5.1 to 5.8 and maximum Intensity of 6 were generated in the region from 1928 to 1993.
Tremors occur in Central Luzon, which includes Zambales and Pampanga, because the region is one of the seismically active areas in the country due to the presence of a number of faults, namely Philippine Fault, Iba Fault, East Zambales Fault, and Manila Trench.
These are the main earthquake generators that can affect the area. In addition, there are other nearby local faults, which may be covered by recent deposits, and may be sources of small- to moderate-magnitude earthquakes.
Phivolcs also pointed out that after the eruption of Mt. Pinatubo in 1991, moderate-sized earthquakes occurred as a result of the activity of the volcano.
But the 6.1-magnitude earthquake may not be a result of a volcanic activity.
Although the nearest active volcano in Central Luzon is the Mt. Pinatubo, the 6.1-magnitude earthquake was determined to be tectonic in origin, it explained.
But as part of Phivolcs’ monitoring procedures for moderate to large earthquakes occurring near active volcanoes, the agency said it will closely monitor earthquake events in relation to any activity that may be associated to Mt. Pinatubo.
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