Hazard Assessment Report for the Philippines

Earthquakes are the most fatal natural hazard in the Philippines, but although preventing earthquakes is impossible, there are certain steps that can be taken to ensure the loss of life is minimized.  Earthquakes happen due to tectonic shifts, and because there are plates pushing under the Philippine Islands from both the East and West, that means that there is significant earthquake activity. Also there is a North-South fault that runs through the center of the archipelago (reference my blog September 12, 2016). Because the plate boundaries to the East and West are under water, this also creates serious risk of earthquake-generated tsunamis. Besides tsunamis, secondary hazards that are triggered by earthquakes in the Philippines include landslides, sinkholes, and volcanic eruptions, making earthquakes one of the most dangerous hazards for Filipinos.

Any earthquake mitigation recommendations would have to include the continual support of PHIVOLCS, the Philippine Institute of Volcanology and Seismology, which monitors seismic activity across the islands, spearheads education and outreach efforts, and issues alert warnings for secondary hazards, such as tsunamis and volcanic eruptions. Two major target areas of earthquake hazard mitigation must include monitoring and infrastructure. Infrastructure includes housing, communication, and disaster preparedness plans, but the area of highest concern is housing. Housing is a major concern because the enforcement of housing codes is nearly non-existent, and studies have shown that cinder block houses, which the majority of homes in the Philippines are, do not have the structural integrity to withstand earthquakes of magnitude 7.0 or higher, and as low as M 4.0 can suffer serious damage. Many cinder block houses (concrete hollow block houses) rarely have re-bar, iron bracing, that not only helps the structure remain upright, but allows a certain amount of flexibility during lateral and horizontal shaking. Many roofs are sub-standard, and collapse, even if the walls remain intact. Many homes have additions, (rooms or extra floors) that were constructed after the original homes were built, often with the help of friends and neighbors without consulting an architect or getting any kind of permit or housing inspection.

This picture from the 7.2 magnitude Bohol earthquake in 2013, shows the collapse of typical concrete hollow block houses in the Central Visayas region. (Photo from India TV News)

Because earthquakes are so common in this area, many homes have some sort of previous earthquake damage, and because of heavy rainfall and typhoon activity many homes also have some storm or flood damage. Because over a quarter of the population of the Philippines lives in poverty, homes are not very well maintained or fixed after being damaged. After floods, for example, they are often cleaned but not repaired, nor are they evaluated for safety. Flood damage often undermines the strength of the foundation, which makes it just that much more vulnerable to future hazards, even small earthquakes. It is likely many people live in homes that are not safe to live in, and if those same houses were in the US they would be condemned.

Since a majority of the islands are mountainous, with narrow to wide coastal areas, there are a lot of slope concerns. The picture below shows a landslide that was caused by a 6.8 earthquake near Cebu and Negros in 2012. This makes the question of where is safe to build your home quite complicated. Many people build in areas that are simply incapable of supporting houses due to soft soil, steep slopes, being too close to flood-prone areas or the coast. (Picture from Egagah blogspot)

From the Map below you can see that the safer areas for avoiding earthquake-induced landslides are denoted in gray. Some of those areas are coastal areas, and are therefore subject to other hazards, such as flooding, storm surge, coastal erosion, tsunami, and typhoons. So the most inland gray areas would be the safest places to build a house in the Philippines. You would still need to figure in soil composition, proximity to faults and to water systems (such as lakes and rivers that may be prone to flooding), and access to roads and other resources. As can be seen from my previous blog, there are faults through out the islands, with the notable exception of the western-most island of Palawan, which unfortunately has little gray area on the Map below apart from coastal areas.

(Map from PHIVOLCS)
One of the other concerns on where to build your house has to do with soil liquefaction, so just when you thought building your home in the gray area above was enough, you also need to look at what areas are gray but not yellow, in the map below (also from PHIVOLCS). You will notice, by comparing these two maps that Palawan is still a pretty good area to build your house.

Although volcanoes have been quite fatal in the Philippines, causing serious long-lasting damage to the environment and economy, typhoons are a more dangerous hazard, and are set to be of greater concern with the warming of the ocean near the Philippine islands. As the ocean warms, this provides additional power and strength to typhoons. The Philippines suffers the most typhoons of any other country in the world, with at least 20 a year, half of which make landfall. The Philippines is located in what is called the typhoon belt, an area with warm ocean temperatures (at depth). The latent heat from the ocean feeds monsoons and mixes with the natural wind currents from the East in the summer creating ideal conditions for typhoon formation.

In 2013, Typhoon Haiyan (Yolanda) was incredibly deadly, necessitating an outpouring of international aid. PAGASA  (the Philippine Atmospheric, Geophysical and Astronomical Services Administration) is the weather bureau, and the main monitoring agency for typhoons in the Philippines. They monitor and rate typhoons, issue alerts and warnings, and store data about past events, such as cyclone track charts. (The track of Super-typhoon "Yolanda" below is from PAGASA)

Unfortunately, there is no way to avoid typhoons in the Philippines, they strike all the islands of the archipelago. Reviewing the typhoon tracks data from PAGASA, between 2008-2015 just about every island has been in the path of a typhoon within the last 8 years. Although the majority of the typhoons come from the East, they often pass all the way through the islands, and in some years, even the Western island of Palawan has been hit by the tail end of a typhoon. Some of the secondary hazards from typhoons include storm surges, such as the deadly one at Tacloban in 2013 during typhoon Haiyan (Yolanda), severe flooding, and landslides (again!).

Like earthquakes, typhoons are very common in the Philippines, and this has led to a certain amount of complacency. You live through enough and you tend to take them less seriously. This is particularly a problem when storms are predicted inaccurately. If PAGASA issues an alert for a serious typhoon, and it turns out not to be as bad as they predicted, then people tend to discredit the alerts. Having accurate prediction models, monitoring, and up-to-date alerts that let people know exactly what to expect, will go a long way to ensure trust in the alert system.

As with earthquakes, a problem with typhoons is infrastructure. In the case of typhoons, the two major priority areas are communication and flood control. Communication before, during, and after storm events in critical. Reliable communication and back-up systems are essential in an emergency. Also, since many people are living in housing with no electricity, getting messages to this at-risk population is a serious concern. Television, radio, and the internet are all good sources, and the use of SMS messaging to cell-phones has proven useful for other emergencies such as tsunamis, but not everyone has access to these modes of communication. Since flooding is a severe secondary hazard of typhoons, having a variety of flood control systems should also be a priority. Many flood channels are blocked with debris, and also during the drier months people set up shantytowns on floodplains, which can have disastrous consequences (in 2009 hundreds of people drowned when a shantytown near Manila was flooded by the Marikina river during Typhoon Ondoy, see photo below). The highly populated coastal areas and lake-side areas are particularly vulnerable to flooding. Deforestation up-slope of urban areas only exacerbates the flooding and increases landslide risk.


(Photo from Seattle Globalist)

In summary, the best place to build your home in the Philippines would be the Western island of Palawan, in an area up from the coast, but not in a landslide-prone area. Being a western island, it should get the least amount of typhoon activity, or at least the storms would have lost much of their power while traversing the other islands of the archipelago. Also the southern part of the island seems the most immune from typhoons. With mountains between 1,000 and 2,000 meters, there should be some nice real estate that should survive sea-level rise for several thousand years. And the view out your front door could look something like this:

(Photo from manbos.com)

Nico

References: http://www.phivolcs.dost.gov.ph/, http://www.phivolcs.dost.gov.ph/images/CHB_Test_Briefing_3_0001.mp4, http://www.phivolcs.dost.gov.ph/images/Flyer-How-Safe-Is-Your-House.pdf, https://www.cia.gov/library/publications/the-world-factbook/geos/rp.html, http://www.indiatvnews.com/news/world/philippines-earthquake-death-toll-rises-to-156-14267.html, http://egagah.blogspot.com/2012/02/68-magnitude-earthquake-strikes.html, http://geography.about.com/od/thephilippinesmaps/a/thephilippinesgeography.htm, https://www.usaid.gov/haiyan, https://www.usaid.gov/philippines/humanitarian-assistance, http://www.pagasa.dost.gov.ph/, http://www.pagasa.dost.gov.ph/index.php/tropical-cyclones/annual-tropical-cyclone-tracks, http://www.seattleglobalist.com/2013/11/10/typhoon-haiyan-a-preview-of-calamities-to-come/17715, http://wwf.panda.org/about_our_earth/ecoregions/palawan_moist_forests.cfm,  Keller/DeVecchio "Natural Hazards" Pearson (page 199).