Monday, June 1, 2015

Home along the West Valley Fault line

Note:  A slightly different version of this photo-essay appeared in the 29 May-04 Jun 2015 issue of the FilAm Star, the weekly 'newspaper for Filipinos in mainstream America' published in San Francisco, CA. The author/blogger is the Manila-based Special News/Photo Correspondent of the paper.

The Philippines appears bounded by principally 
by the Manila Trench and the  Sulu Trench in
the west, and the Philippine Trench in the east. 
The archipelago is traversed by active faults such
as the Valley Fault System (VFS).
Soon after the Philippine Institute of Volcanology and Seismology of the Department of Science and Technology (PHIVOLCS-DOST) launched and distributed hard copies of the Valley Fault System (VFS) Atlas to local government officials on 18 May 2015, we went to the institute’s webpage and downloaded the online version.

We wanted to verify what we have known in 1998 – that we were building our house in a housing village in barangay Matandang Balara (now Batasan Hills) near the Marikina Fault, which was how the West Valley Fault (WVF) was called before. Political sensitivity appears to have made the government rename the fault to help dispel scary earthquake thoughts among the people of Marikina City.

Our geologist friend also built their house in the next village on the other side of the fault. He was involved in the geological survey during the development of the housing areas. He said not to be scared because our houses do not sit on top of the fault line. After looking at our area map in the Atlas, we estimated that we actually live just about a street block away from the line.

The Atlas is a handbook of 33 large scale map sheets of varying scales, arranged from north to south, showing in detail the areas traversed by the VFS. For the 22 Metro Manila map sheets, the scale is 1:5,000. For Laguna and Cavite (10 map sheets), it’s 1:10,000, and for the sole Bulacan map sheet, 1:50,000.

The map index shows the areas traversed
by the East and West Valley Faults.The
color-coded boxes indicate magnification
scale of the Atlas map sheets.
The VFS mapping is one of the component activities of the Australian Aid (AusAid) Program-funded Greater Metro Manila Area (GMMA) Ready Project under the United Nations Development Programme (UNDP) and implemented by the member agencies of the National Disaster Risk Reduction and Management Council (NDRRMC).  The Atlas can then be used for land-use planning, engineering and construction, scientific research, disaster risk reduction and mitigation programs, and other activities geared towards the promotion of safer and more resilient communities.

PHIVOLCS says that the EVF “can generate an earthquake with a magnitude of 6.2 that may result to a very destructive ground shaking, with intensity VIII on the PHIVOLCS Earthquake Intensity Scale (PEIS), in the epicentral area ... [and the WFV] can generate an earthquake with a magnitude of 7.2 that may result to a very destructive ground shaking, with intensity VIII on the PEIS, in Metro Manila and nearby provinces.”

The PEIS in Roman numerals ranges from I (Scarcely Perceptible) to X (Completely Devastating). V is ‘Strong’ and VIII is ‘Very Destructive’.

PHIVOLCS Director Renato U. Solidum reportedly said that the WFV has moved four times in the past 1,400 years, and on the average, every 400 years, plus or minus 10 to 100 years. The last earthquake from the WFV was in 1658, around 355 years ago.  Earthquakes are not predictable but the possibility of its occurrence should make us adequately prepared for it.

Barangays in Metro Manila transected by the VFS. Cities not transected are also shown.

In a radio interview, Solidum revealed that, in a night time scenario, around 33,500 people will die and around 113,600 will be injured in areas within the vicinity of the WVF when this moves and causes a magnitude 7.2 earthquake. He said that PHIVOLCS and other agencies based their estimates on the population of the said areas, the quality of buildings and houses found near the fault line, and damage percentages in past earthquake records here and abroad. 

Ground rupture resulting from an earthquake may damage buildings and structures built directly above the active fault. PHIVOLCS recommends a minimum distance of at least 5 meters from both sides of the active fault against ground rupture hazard.  This should be a consolation for us who live near the WVF line.

Barangays in Bulacan, Rizal, Laguna and Cavite that are transacted by the VFS.

PHIVOLCS has been distributing the material titled “How Safe Is My House?” to enable people to evaluate the integrity and vulnerability to strong earthquakes of their 1 to 2-story concrete hollow block (CHB) houses. This “House self-check” is based on a “full-scale shaking table experiment on CHB masonry structures conducted in Japan to two types of CHB houses.” Anyone who wants to evaluate their house can download the material from the PHIVOLCS website.

The self-check comprises twelve questions, each with a set of three possible answers, the best scoring a “1” and the two others a “zero”.

This Atlas map shows the West VFS traversing
areas in Marikina City & Quezon City. The 
author lives in the blue-circled area .
The self-check tells that the earthquake-safe house is one that was built or designed by a licensed civil engineer/architect, built in or after 1992, not damaged or was repaired after a past earthquake or disaster, of regular shape (symmetrical, rectangular, box-type, simple), has not been extended/expanded or a civil engineer/architect supervised the extension/expansion, the external walls are 6-inch (150mm) thick CHB, the steels bars are of standard size (10mm diameter) and spaced correctly in the walls, there are no unsupported walls more than 3 meters wide, there is no gable wall or the gable wall is made of light materials or properly anchored CHBs, the foundation is reinforced concrete, the soil under the house is hard (rock or stiff soil), and it is in good condition overall.

A score of 11-12 may indicate that one’s house is safe now, but just the same, these findings should be confirmed by experts.  As we write, there are several classy houses being constructed in our village closer to the WVF line. We suppose that design and construction standards are being followed to ensure that their families are safe when the nearby fault moves.

PHIVOLCS photos of lateral spreading in Bagtic, Catigbian, Bohol (left), and of the ground rupture 
in Anonang, Inabanga, Bohol (right) from the Magnitude 7.2 earthquake of October 2013. 

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