It might be impossible to predict when the next earthquake may occur but scientists can pin down what is called a recurrence interval.
(TNS) - When a 7.8-magnitude earthquake hit Baguio on July 16, 1990, 5-year-old Klaridelle Reyes was sleeping on a couch. She woke up to a cacophony of voices and loud footsteps. She could hear people shouting, running to safety.
Kyle Yan, a 16-year-old student at Saint Louis University, was also napping on that cold afternoon when the quake struck. He awoke in the commotion and then waded through piles of books and personal belongings that had fallen to the floor during the first few seconds of the quake.
Outside, buildings were starting to crumble, landslides blocked roads and mines collapsed on hapless workers.
By the time the two of them reached the streets, they were homeless.
Strike-slip or horizontal movements of the Philippine Fault Zone and the splay Digdig Fault, which occurred at 4:26 p.m. on that day, created a 125-kilometer ground rupture from Aurora to Nueva Vizcaya. The earthquake’s epicenter was located near the town of Rizal in Nueva Ecija, but it was in the tourist destination of Baguio City where many died.
Grade 4 student Drew Calachan was waiting for a jeepney in Cubao when it hit. The road started to ripple. He was confused as he watched the ground move in waves, the Isetan Tower swaying from left to right as though it was going to break.
“The ride suddenly felt strange, like the bus couldn’t keep a straight line," Louis Alferez recalled of that afternoon he rode a bus on his way home from the University of the Philippines.
At the Duty Free mall near the Ninoy Aquino International Airport, Jose Del Mundo and his colleagues held onto bottles of expensive liquor as the ground shook. He thought of running to the tarmac but the glass windows of the mall had started to rattle, producing a deafening sound that spooked them and left them rooted to the spot.
Back in Baguio, Klaridelle, who cried the whole afternoon as the aftershocks continued, slept in the nearby park with her family. Village officials erected tents and her family stayed in one for five days until it was deemed safe to sleep indoors.
Kyle’s village was not as fortunate; many families slept on the pavement, with only blankets and raincoats protecting them from the rain. Their house, which was made of wood and concrete, was destroyed.
Around 1,600 people were said to have been killed in what is considered the second deadliest earthquake in the world that year (the deadliest struck Iran). Thousands were trapped under rubble and the city was isolated for two days.
The main earthquake lasted for 45 seconds but the damage it wrought on 15 provinces took at least a year to recover from.
It might be impossible to predict when the next earthquake may occur but scientists can pin down what is called a recurrence interval or the average time span between earthquake occurrences.
For the West Valley Fault, four major earthquakes have been determined to have taken place in the last 1,400 years. It has a recurrence interval of 400 to 500 years. The last major earthquake originating from the fault was recorded in 1658 or 357 years ago.
This means that it may be time soon — perhaps in our lifetime, or that of our children — for another major movement in the fault.
The Philippine archipelago is sandwiched in between two opposite subduction zones — that of the Eurasian Plate (South China Plate) which subducts (is forced under) Luzon along the Manila Trench and that of the Philippine Sea Plate that subducts toward the west along the East Luzon Trench. These zones explain high seismic activity in Luzon.
In a joint study with the Metropolitan Manila Development Authority (MMDA) and the Japan International Cooperation Agency (JICA), Phivolcs looked at 18 earthquake scenarios. The three organizations selected three scenarios for detailed damage analysis: a 7.2-magnitude earthquake from the West Valley Fault, an offshore 7.9-magnitude earthquake from the Manila Trench and a 6.5-magnitude earthquake hitting Manila Bay.
That of the West Valley Fault, a 100-kilometer fault that runs through six cities in Metro Manila and nearby provinces, is considered the worst-case scenario. This is “The Big One."
While the strongest earthquake recorded in the country was an offshore magnitude 8.3 earthquake in Mindanao in the 1920s, and certain seismological models predict the possibility of another offshore 8.5-magnitude earthquake, a major West Valley Fault movement could result in a disaster of never-before-seen proportions.
The longer the fault runs, Phivolcs Director Renato Solidum Jr. said, the higher the magnitude. And a higher magnitude also means a longer duration.
“The larger the earthquake, the longer the shaking," he said, adding that a major quake will probably last at least 30 seconds based on a duration magnitude formula.
A magnitude 7.2 earthquake that lasts for at least 30 seconds? If that happens in the West Valley Fault, about 34,000 people may die. That staggering estimate does not take into account the toll from other disasters that may follow, like fires.
According to the Metropolitan Manila Earthquake Impact Reduction Study (MMEIRS), a 7.2-magnitude earthquake from the West Valley Fault will result in the collapse of 170,000 residential houses and the death of 34,000 people. Another 114,000 individuals will be injured while 340,000 houses will be partly damaged.
At least seven bridges will fall off while 10 percent of public buildings will be heavily damaged.
The 4,615 kilometers of water distribution pipes will suffer 4,000 points of breakage. Thirty kilometers' worth of electric cables will be cut and 95 kilometers of communication cables will be disconnected. Cellular phone service will be congested and out of use.
Of the structures that will be damaged, those of informal settlers will have a higher damage ratio.
People living in buildings may be trapped inside elevators as electricity fails.
Of the 34,000 people projected to die within an hour of the earthquake, 90 percent will be killed from the pressure of collapsed structures. Some of them will initially survive but will die if they are not immediately rescued.
About 20,000 people will be trapped in damaged buildings and burnt to death. While many of those in informal communities will survive the main quake, a number will die from the fires, which will be triggered by short circuits and leakage from gas tanks.
Several fires will originate from factories, hospitals and kitchens. The scenario, which is assumed to occur at nighttime, predicts the fires will burn 1,710 hectares of land, claiming 18,000 lives.
Ambulances will be unable to get through the streets and bodies will be lined along the road. The scenario is reminiscent of the horrors of Supertyphoon Yolanda — although the study was released back in 2004.
The destruction, together with the losses that the economy will suffer, will constitute a national crisis, the study said.
When the Big One strikes, it will be felt this way.
Movement will start along the fault, several kilometres below the ground. It will move horizontally and will cause vibration or shaking, Solidum said.
At Intensity 8, which will be felt in Metro Manila and possibly Bulacan, Rizal, Laguna and Cavite, people will find it difficult to remain standing. Buildings and houses will suffer heavy damage, especially those not constructed to withstand earthquakes.
There will be many aftershocks and this will last for a week or so. The aftershocks may number hundreds or thousands but only structures already weakened by the main earthquake will be further damaged.
There will be landslides and, especially near the fault, there will be permanent changes in the landscape as the quake ruptures the ground.
The study paints a grim picture, of people feeling a sudden jolt and then the shaking of the ground.
“You fall to the ground, unable to keep standing. You hear a booming sound. You hear screams from people inside their homes. You hear breaking glasses. Telephone and power poles sway violently. Then the power goes off. In front of you, the village road is heaving, as if you are riding waves. The strong ground shaking goes on for 50 seconds. It is the longest 50 seconds of your life," the text says.
It tells of the panic and confusion that will follow, of being cut off from the rest of the country and living in open spaces.
But it also serves as a great reminder of the need to be prepared and for authorities to improve risk reduction and management systems. Today, Thursday, the MMDA and thousands of participating groups will hold the largest earthquake drill to be held in Metropolitan Manila.
“We are counting on your participation," the Metro Manila Shake Drill website says.
Below that announcement, a timer counts down the days and hours before the earthquake drill.
But there will be no timer for the Big One.
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