Author Topic: special report: western valley earthquake fault  (Read 1544 times)

jcabanit

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special report: western valley earthquake fault
« on: March 07, 2009, 08:49:51 AM »
Sunday, March 08, 2009
 
 
SPECIAL REPORT:WESTERN VALLEY EARTHQUAKE FAULT
 
Nightmare on Metro Manila streets
 
Earthquake faults lie under the capital region, the Marikina Valley fault being the most threatening
 
By Paul M. Icamina, Special reports editor
 
IT is just after sunset, and the ground shakes violently. A jolt pushes you forward, up, down and sideways, and the ground motion grows more violent for the longest minute of your life.

There is a booming sound. Screams are heard from houses. Glasses break. Trees and utility poles sway violently. Then the power goes off.

You run outside where the road is heaving like waves. Then the shaking stops and you get up with knees shaking.

Cries and wails add to the panic and confusion everywhere. Houses have collapsed, roads are cracked, water and sewage gush from broken pipes.

You hurry home to find it in shambles and the family scared but safe. The mobile phone has no signal and the landline is off.

But the radio works, and you hear the first news: “A devastating earthquake, with magnitude 7.2 generated by the nearby Western Marikina Valley Fault, has hit Metro Manila. Weak to strong after shocks are expected …”

This is not science fiction but a worse-case scenario by the most extensive study yet of what a magnitude 7 earthquake may do to Metro Manila.

The study was done by the Japan International Cooperation Agency, Philippine Institute of Volcanology and Seismology and the Metro Manila Development Authority.

One of Asia’s most densely populated areas, Metro Manila’s 13 cities and four towns packed in 636 square kilometers is home to about 10 million (when the study was made between 2000 to 2004).

The metro population is expected to grow to 25 million by 2015 in a highly urban sprawl that will spread by then to 1,500 square kilometers.

Since Metro Manila is the country’s only mega urban center of government, finance, commerce and social activities (it generates 35 percent of GDP), the study observes, “the impact of a large earthquake will greatly affect the nation … national functions will be paralyzed, and in the worst case, the earthquake will lead to chaos and disruption of the national economy.”

Faults

Many earthquake faults surround the megapolis: the Western Valley Fault System (Marikina Valley Fault), Philippine Fault, Lubang Fault, Manila Trench (which runs off western Luzon) and the Casiguran Fault.

Because of its proximity, the West Valley Fault, which runs north to south along the west and east edges of Marikina Valley, is considered the greatest threat. The 30-kilometer-long northern half of the fault is only 10 kilometers east of central Manila.

The five strongest earthquakes in Manila occurred in 1658, 1771, 1863, 1880 and 1937. Two earthquakes—in 1677 and 1863—generated a tsunami around Manila Bay. Since 1900, over 30 earthquakes have caused some damage.

Analyzing past earthquakes, the study selected 18 scenarios based on three epicenters originating from the West Valley Fault, Manila Trench and Manila Bay. Ground motion, liquefaction (when the soil turns liquid-like), slope stability and tsunami height were factored in.

The fault has moved at least four times and generated strong earthquakes in the last 1,400 years. The approximate return period of these earthquakes is less than 500 years and no event along the West Valley Fault is known after the 17th century, says the study.

Its conclusion: “the active phase of the Western Valley Fault is approaching.”

National crisis

A magnitude 7 earthquake will cause liquefaction in coastal and floodplain lowlands, including the central district of Makati, resulting in damage to roads, buildings and infrastructure.

If the earthquake originates from the Manila Trench (a fault line that runs off western Luzon, from Ilocos to Mindoro), a tsunami 2 meters to 4 meters high may hit the Manila Bay area one hour after. The Old Manila area—including low areas with elevation less than 4 meters such as that in Malacañang and Pandacan—will be hit.

Such an earthquake will result in 170,000 houses collapsing, 340,000 residential buildings damaged, 34,000 persons dead and 114,000 injured.

So many wooden buildings and hazardous facilities will spread fires across 1,710 hectares. Fatalities: over 19,000.

“This human loss, together with properties and economy losses, will be a national crisis,” the study warns.

The hardest-hit areas would be Navotas, Manila’s Port Area and southeastern portion, Central Manila Bay area, northeastern Quezon City, western Marikina, eastern Pasig, the border between Manda-luyong and Makati and the Muntinlupa-Laguna Bay area.

Overall, fires and evacuation will be most intense around Navotas Bay, the North Port Area in Manila, southeastern Manila and those surrounding the central Manila Bay area. Building collapse and evacuation problems will be greatest in northeastern Quezon City, western Marikina, eastern Pasig, Muntinlupa-Laguna Bay area and the border between Mandaluyong and Makati.

Fires will be a problem in the intersection between Valenzuela, south Caloocan and western Quezon City.

Evacuation will be difficult in the fringes of Metro Manila, especially in the northern Taguig and Las Piñas sides. Western Metro Manila will be isolated by fire and destroyed buildings. The northern and southern part will be separated by rubbles, especially in the border between Mandaluyong and Makati.

All road networks along the Western Valley Fault running east-west will be damaged while roads running north-south in the fault area will be cluttered by collapsed buildings.

Surreal landscape

On Day 1, fires will break out in villages, chemical plants, factories and hospitals. About 4,000 major water distribution points will be damaged.

There is no electricity, telephone lines and cell-phone networks are down; from 31 to 97 kilometers of electricity and telecommunication lines will be affected.

About four out of 10 residential buildings will collapse or be affected. Many schools and business establishments will be heavily damaged or destroyed.

Some hospitals will be heavily damaged and many patients will be evacuated. Hundreds, if not thousands, will be trapped dead or injured.

Abandoned cars will litter the streets. Some bridges will be down, many roads will be impassable and ambulances cannot get through. The light rail systems will be at a standstill.

Within the next few hours, the National Disaster Coordinating Council convenes but “not all member agencies have representatives immediately available,” the study says.

By the second and third day, food and drinking water will be scarce. Moderate to weak aftershocks will occur. There will be no electricity, telephone communication and water. Back-up power generators will be available only in critical offices.

Fires will spread unabated. Haze from burning buildings will darken the horizon.

The President declares a state of calamity while the police contain random acts of looting. The military is mobilized for rescue, clearing of debris and construction of temporary shelters and medical centers.

The homeless—that will swell to as much as 3.15 million—begin to occupy open spaces. Relief goods are distributed while volunteer rescue teams arrive from abroad.

Many survivors rescued from collapsed buildings, especially children, will suffer from shock. Bodies exhumed from the rubbles will line the streets. Disease outbreaks threaten.

Some 33.555 million tons of debris from nearly half a million damaged buildings will litter a surreal landscape.

The air has the smell of decay and death. The Philippine flag, if it is raised, flies at half-mast.
 
   
   
 
 
 
 

 
 
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