As the aftershocks from last Friday night’s earthquake that shook Surigao City continue to rumble – more than 150 have been recorded since the 6.7-magnitude wave struck – this event has once more brought home the fact that the Philippines is prone to this unpredictable form of natural disaster. And while seismologists can fairly accurately pinpoint where movement in the Earth’s plates will occur, they cannot claim to know when. That said, they believe ‘The Big One’ could be on the way.
The Philippine Institute of Volcanology and Seismology (Phivolcs) knows from historical data that certain places are due to have a quake. And foremost among those – and by far the most worrisome – is the capital city of Metro Manila across which slashes the West Valley Fault, a 100-kilometre fault line that runs from Bulacan to Cavite.
In its path lie the Metro cities of Makati, Marikina, Muntinlupa, Pasig, Quezon and Taguig which, based on 2015 census figures, have a combined population of 3,810,705. And that figure is what troubles the quake watchers most. A major quake – and this fault line is capable of producing a magnitude 7 or higher – could run up a death bill in the tens of thousands if not more.
The West Valley Fault along with the much shorter East Valley Fault (approximately 10 km) are part of the Marikina Valley Fault System which extends throughout the densely-populated northern island of Luzon all the way from Dingalan in Aurora province to Canlubang in Laguna – a distance of around 180 miles. And towards the end of that journey it passes through the heart of Manila.
Seismic studies show that movement along the major of these two faults occurs roughly every 400 years. That’s a broad approximation but the last time a major earthquake was generated by plate movements along the West Valley was in 1658 – in other words, 357 years ago. And that’s certainly close enough to the range to start thinking about it.
Minor quakes are constantly felt in and around the Philippine capital. In the past 12 months there have been 16 such occurrences. But a major quake here presents a far more terrifying scenario than the one that took place in Surigao City, the provincial capital of Surigao del Norte that sits at the northeast tip of Mindanao – a relatively small conurbation with a population of around 160,000.
Bad enough though that was, as a location the death-and-destruction potential there is dwarfed by a shift along the West Valley Fault. For while the quake magnitude may lie within or close to the same range, the population density across the Metro cities and the sheer scale of property and structures that lie along that belt put it at the top of the danger charts for every risk metric. An earthquake anywhere here would be catastrophic.
Phivolcs Director, Renato Solidum, urges people to heed the effects of the Surigao event. “If a similar event happens in a highly urbanised area, the effects can be far more devastating,” he said. Given the stats, that’s an understatement.
Here then is a snapshot of the effects of the Surigao quake. There were eight deaths, 209 injuries and no reports of any missing. The quake zone stretched across five municipalities – Surigao, Sison, Mainit, San Francisco and Malimono – and struck 60 barangays. Of the population within the zone, 8,425 were affected of which 7,825 were displaced; in Surigao City 5,640 people were displaced while in Sison 1,800 were forced from their homes. Structural/property damage from the quake affected 1,685 houses (either totally or partially); 12 schools (partially), with further damage to six bridges, five sections of road, one airport runway and one port access road.
That was a 6.7 magnitude earthquake and will have released an energy wave equivalent to more than 15 kilotons of TNT; that equates to 63 terajoules of energy – the same as ‘Little Boy’, the atomic bomb that was dropped on Hiroshima in Japan on 6 August, 1945 which left 66,000 people dead and 69,000 injured.
A bigger quake, let’s say one with a magnitude of 7.1 – which the West Valley Fault is capable of producing – would offload around 44.7 megatons of power. Let’s put that in perspective. This is equivalent to the explosive effect of the Tsar Bomba – the most-powerful nuclear weapon ever detonated. This could give off enough energy to keep Norway in electricity for a full year.
That’s the same force of quake that turned the Caribbean island of Haiti (photo) into matchwood in January 2010. It packed enough power to fly a jumbo jet around the Earth 300 times, or burn 100 million barrels of oil. Up to 160,000 people were killed and around 3 million were affected in this disaster, with the capital, Port au Prince, close to the epicentre taking the brunt of the hit. Its population in 2009 was just 875,978. And that is less than 23% of the combined populations of the six Metro Manila cities than sit on the West Valley Fault.
In 2004, the Japan International Cooperation Agency (JICA) released the findings of study which it had carried out for Phivolcs and the Metropolitan Manila Development Authority (MMA) – a government agency under whose jurisdiction the six Metro Manila cities fall. That showed that a 7.2-magnitude quake in that region could kill up to 35,000 people and injure a further 100,000. It could also set off 500 fires simultaneously in 98,000 buildings and as many as 170,000 structures could collapse.
But that was more than 12 years ago and in that time the population of Metro Manila has swelled by around 3 million. Furthermore, there are many more buildings, high-rise hotels and office blocks, and more infrastructure – elevated highways for example like the Metro Manila Skyway – that weren’t there in 2004. That said, there are many more ‘earthquake-proof’ buildings in the capital than there were back then – and the reason for much of that is the tireless work done by Phivolcs, the MMA and JICA in creating awareness and promoting structural codes for buildings that will better withstand a quake impact. But no building can be completely secure; a magnitude-7 is equal to 1,000 Little Boys.
Of course, it’s as impossible to predict the death toll from an earthquake as it is to predict when a fault will slip to release its seismic waves. But what the quake watchers do know is the likelihood of such an event, and give the historical data and by continually monitoring seismic activity, they at least have a finger on the pulse.
Mother nature has been good to the Philippines in many ways, She’s given it good land to grow an abundance of crops and produce; clear waters teeming with fish, a wealth of metals and minerals beneath its soil and around its shores, white-sand beaches and rolling hills. But she also placed it in one of the most natural-disaster-prone places on Earth – on the Ring of Fire, the birthplace of 90% of the world’s earthquakes and 81% of the largest. Shock from the Ring