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Tsunami alert!

July 4, 2015 It is good to see the publicity and preparations done with regard to the highly probable occurrence of an earthquake in the West Valley Fault. But there is need to point out that another possible calamitous natural

It is good to see the publicity and preparations done with regard to the highly probable occurrence of an earthquake in the West Valley Fault. But there is need to point out that another possible calamitous natural event can also affect Metro Manila and the western coasts of northern Luzon: a tsunami that can occur at any time and cause havoc particularly on the coastal areas of Metro Manila and the province of Cavite.

A tsunami is made up of giant sea waves often generated by the occurrence of an earthquake and a volcanic eruption below the sea. An earthquake with a magnitude of at least 7.0 on the Richter Scale can cause displacement in parts of the sea floor and subsequent disturbance in the mass of water above it.

It must be pointed out that a tsunami is not like the slow ebb of waters during a low tide cycle. Following the displacement of oceanic waters, there is actually a quick sea-level recession even beyond the lowest tide water marks, after which would come the returning deluge of large waves of the tsunami. With the quick retreat of the water level, interesting sights are revealed, such as stranded fishes and marine organisms as well as beautiful corals and sea grasses. This scene may draw people to the bared shoreline and, in the process, expose themselves to danger when onrushing waves of the tsunami hit the area in less than an hour.

The Philippines is a tsunami-prone country because it is also prone to earthquakes and volcanic eruptions. Studies done by Filipino geologists reveal that most of the tsunami-generating earthquakes take place in the western, eastern and southern coasts of the country. Indeed, these indicate the Philippines’ unfortunate unstable location in the so-called Pacific Ring of Fire. Our country is actually precariously floating on the earth’s mantle between two subduction zones on its western and eastern flanks—that is, it is being squeezed by two oceanic plates that subduct or move down toward the earth’s mantle and, in the process, create oceanic and terrestrial disturbances as well as crustal deformations.

In a 2003 study funded by the Japan International Cooperation Agency and titled “Metro Manila Earthquake Impact Reduction Study” (MMEIRS), one model or scenario assumes the occurrence of a tsunami on the coast of Manila Bay. This model is based on the characteristics of the 1677 7.3-magnitude earthquake that resulted from the subduction tectonics of the Manila Trench. Based on the calculations of the MMEIRS, the location of the event is placed on the subducting Eurasian Plate at a shallow depth of 40 kilometers and with a magnitude of 7.9. The fault length is 255 km and its width is 68 km. In this condition, the propagation distance is about 200 km from the shore, with the mean run-up tsunami height calculated at around two meters and the maximum height at around four meters. The propagation time of the sea waves, from the source area to the coast of Metro Manila, is about 70 minutes. This lead time for escaping from a tsunami is indeed short if people from the coastal areas should move east to higher ground in, say, Quezon City.

According to the model, when the tsunami arrives, the greater middle portion of the metropolis will experience an Intensity 7 or “destructive” scale while the smaller western and eastern portions will experience an Intensity 8 or “very destructive” scale. There are many lowland areas of the metropolis with elevations of less than four meters, and these will be severely affected. There will be flooding in such flood-prone places as the cities of Manila, Malabon, Navotas, Caloocan, Valenzuela, Pasig and Parañaque, as well as Cavite City and the town of Rosario, also in Cavite.

The tsunami may also run up into the Pasig River and possibly flood Malacañang Palace and the Pandacan fuel deposit area. It is estimated that the damage from the earthquake can include 190 heavily damaged residential buildings, 6,600 partly damaged residential buildings, 100 people dead, 300 people injured and 2.8 percent of midrise buildings partly damaged.

The destruction may not be as great as that predicted by the MMEIRS for the West Valley Fault, but what if the earthquake magnitude that occurs is greater than the calculated 7.9? For example, what if the magnitude goes up to 8.9, as what happened in the 2004 tsunami in Banda Aceh, Indonesia, that killed close to 250,000 people in Asian and African countries? Or what if the earthquake assumes a magnitude of 9.0, like the one that occurred in 2011 in Fukushima, Japan, where nine-meter waves killed 15,890 people and damaged several nuclear reactors in the area? Or what if the West Valley Fault and the Manila Trench should move simultaneously, considering their proximity to each other? The possible level of destruction is mind-boggling!

And so there is also need to equally focus our disaster risk reduction and management activities to a possible tsunami generated by an earthquake in the Manila Trench. The Disaster Risk Reduction and Management Council and the Metro Manila Development Authority have focused their drills and preparations only on the West Valley Fault earthquake, which people may have a greater chance of surviving if they follow the government’s guidelines. But can the “duck, cover and hold” mantra keep one alive amid the onslaught of rampaging and swirling waters? Or, based on the MMEIRS scenario, will the lead time of 70 minutes before the tsunami be enough for escaping to places at least two km from the coast? To start with, as usual, traffic would be heavy in the possible escape routes of north Edsa, Quezon Avenue and Aurora Boulevard. With everyone in the coastal areas scrambling to escape the deluge, traffic would crawl along the few and narrow escape routes, and there would be bedlam.

Have the concerned government agencies made preparations in terms of designating escape routes, evacuation areas, and buildings to occupy, as well as survival equipment and supplies to prepare (first aid kits, life vests, food, water, flashlights, whistles, maps, etc.)? Science, despite its rigorous methodology, can only give us probabilities with regard to the occurrence of natural disasters based on the general cyclical behavior of certain natural phenomena. We are in the dark as to the exact time and place of their occurrence. All that we can be certain of nature is its uncertainty. And such uncertainty we can only anticipate, and plan judiciously.

Meliton B. Juanico is a retired professor of the Department of Geography at the University of the Philippines Diliman. He is a licensed environmental planner and is active in consultancy work in urban and regional planning and in part-time teaching as a professorial lecturer.

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