According to Wikipedia, the Marikina Valley Fault System (also known as Valley Fault System, West valley Fault, east valley fault, Marikina Fault line) is a group of dextral strike-slip fault which extends from San Mateo, Rizal to Taguig City on the south; running through the cities of Makati, Marikina, Parañaque, Pasig and Taguig. There are two segments of the Valley Fault, the West Valley Fault (WVF) and the East Valley Fault (EVF). The East Valley Fault is about 10 kilometers long and traverses the municipalities of Rodriguez and San Mateo in Rizal. East valley fault is said to move in an “oblique dextral” motion and can generate a magnitude 6.2 earthquake.
Meanwhile, the West Valley Fault is longer, 100 kilometers long that traverses several cities and municipalities in Bulacan, Metro Manila, Laguna, and Cavite. The West valley fault is said to move in a “dextral strike-slip” movement = side to side, The WVF can generate a magnitude 7.2 earthquake.
Other common types of ground movement is the Normal movement (where one side goes down) and a Thrust motion (where one side goes up).
The destruction/damage of an earthquake would depend on the magnitude (the force) that it will produce. Several studies and models already predicted that the West Valley fault would likely produce a minimum of 7.0 Magnitude earthquake compared to the East valley fault which is only predicted to produce a minimum of 6.3 magnitude earthquake. Aside from the magnitude, we should also look at the intensity that the earthquake will produce. It is the shaking of the ground (that we feel) when an earthquake happens. The farther we are from the epicenter (where the earthquake originated), the lesser the ground movement that we feel.
When will the West valley fault move? No one knows. As of posting time, there is no person or instrument in the world that could accurately tell where (which part of the fault) and when an earthquake would strike. They could only predict the occurrence using historical data. And speaking of historical data, the West Valley Fault last moved on August 18, 1658 generating a 5.7 magnitude earthquake while the East valley fault last moved on February 01, 1771 with a magnitude 5.0. According to Dr. Ishmael Narag, officer-in charge of the Seismology Division of the Philippine Volcanology and Seismology (PHIVOLCS), based on data from carbon dating studies, there is movement along the Valley Fault every 200 to 400 years. So it has been 355 years since the last movement. Experts are now saying that the west valley fault is expected to move any moment now- when we least expect it. So what will happen if the West Valley fault moves? If you’re house or building is on top of the fault, there’s no question, it will be damaged. If you are away from the fault, the effects will depend on the strength of your structure. The Government has been trying to predict the effects based on given scenarios. The most recent “report” or output was provided by the National Mapping and Resource Information Authority (NAMRIA) which released risks and Hazard maps for Greater Metro Manila Area (GMMA). The maps shows – on a per Barangay level – the possible property damage, economic loss, and loss of life if the west valley fault would move. The photo below shows a collection of maps showing the damage brought by a 6.5 magnitude earthquake by the west valley fault. The next photo shows the damage from a 7.2 Magnitude earthquake scenario. Notice now that it has a lot of red areas. And the area with the biggest damage (Red color) is Antipolo. All in all, it is predicted that a minimum of 35,000 people will die in Greater Metro Manila alone. This is a good reference for Local Government units, disaster, and relief response groups to plan ahead for the possible scenarios in their areas. But bear in mind that these are scientific guesstimates/predictions, they may or may not be the actual case when an Earthquake happens. The casualties and damage may be more severe (let’s hope this will not happen). If you want to see if your house or building is on top or near the Marikina Valley Fault system, you can check it via Google maps here or here or from www.nababaha.com here.
Disclaimer: I am not an Engineer nor a subject matter expert on this topic. This post is just to share with you what I know and my views about the topic presented.