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The hidden, fractured earth under Metro Manila's streets

April 21, 2012 6:29pm

Now and then, an earthquake occurs somewhere in the country and we read in the newspapers the following day that the tremor was due to the movement along a certain fault. 
 
Everybody knows what an earthquake is. But if I went out into the street and asked a passerby to show me a fault, I’m sure he would just stare at me and simply move on. Nobody knows what a fault looks like. 
 
So let me be your tour guide for the day and let me show you one famous fault.



The blue line above approximates the Marikina Fault, based on Phivolcs data. Click on the + and - symbols on the upper left to zoom in. Press and hold the left mouse button to move around. Or click here to view a larger map of the Philippines' major fault lines.


Let’s start our field trip at Katipunan Street in Quezon City. Imagine entering La Vista and moving on eastward to Loyola Grand Villas. You will discover that the two subdivisions are not at the same elevation. La Vista is high up on a plateau while Loyola Grand Villas is down in the valley. As you leave La Vista and approach Loyola Grand Villas, you will go down a very steep road (Soliven Avenue). At the bottom of that inclined road, right where it becomes level again, is a segment of the Marikina Fault. 
 
Unfortunately, the fault can’t be seen directly at this location. If you want to see the fault minus the pavement, you have to go to Ateneo. Enter the campus through the gate near the Blue Eagle Gym and follow the road toward the east. You will soon come to the edge, literally, of the school grounds. You can’t go any farther. There’s a cliff-like slope in the way. At the base of that slope is another portion of the Marikina Fault. It doesn’t look much. But if it suddenly moved, you could get a nasty earthquake. (From your perch, you can see Loyola Memorial Park down below.)
 
Now, let’s continue our tour. Let us go southward along Katipunan, up the flyover and down the other end. Let’s stop at the corner: if we turn right, we will be going to Cubao via P. Tuazon. On your left is a short zigzag road that leads to a subdivision down below. Beneath those houses lie another segment of the Marikina Fault.
 
Let’s go back up to Katipunan and move on to the corner at Santolan. If you turn right, you go to EDSA. If you turn left, the road goes down to Libis. Yes, the fault is at the base of the sloping road. Let’s continue along Katipunan and pass through White Plains. If you look to your left, all roads  slope downward. Ever wondered why? Stop at the Mormon Temple and face eastward. (East is where the mountains are.) You can see where the ground abruptly ends and there’s a sudden drop along a steep slope. Where the steep, sloping ground meets the flat valley is another section of the Marikina fault.
 
If you possess some spatial ability, try this in your mind: connect the fault segments in La Vista, Ateneo, at the ‘zigzag road’, Libis, and White Plains. What you get is the trace of the Marikina Fault, what geologists call a faultline. In fact, the faultline extends to Bulacan in the north and all the way down to Tagaytay Ridge in the south, passing by Pasig, Taguig, Alabang, and Muntinlupa.
 
All this time, I have been calling it by its old name, Marikina Fault, but PHIVOLCS has given it a new name: Valley Fault. And there are two of them, actually: the West Valley Fault (what I’ve been calling Marikina Fault) and East Valley Fault. One is found on the west side of the Marikina Valley and the other is on the east side, in Montalban and San Mateo Together the two faults make up the Valley Fault System. (For more information, please visit the PHIVOLCS website.)
 
(In case you are thinking that this fault does not concern you, maybe I should introduce you to the Philippine Fault. It starts in the Ilocos, climbs up to the Cordilleras, crosses Nueva Ecija and on to the coast of Quezon, down to Bicol, skirting Masbate on the east, slicing Leyte from north to south, cutting eastern Mindanao from Butuan to Mati, and extending under the waters off the coast of Davao Oriental. It’s roughly as long as the San Andreas Fault in California.)
 
A fault is a break in the Earth’s crust along which considerable movement has taken place. Every time a fault moves, an earthquake is generated. According to PHIVOLCS, the Marikina Fault is active. And that it moves every 200 to 400 years. The last time it moved was 200 years ago. That means it could move any time now. 
 
Are we prepared? — TJD, GMA News




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