10/19/2013

LES ZONES À RISQUES !

Aux Philippines peu d’endroits échappent aux risques inhérents aux tremblements de terre. Le pays se trouve sur la ceinture de feu du Pacifique et de plus à une jonction de plusieurs plaques tectoniques.

Cela va de la petite secousse à la catastrophe majeure, mais aucun endroit de l’archipel, sauf l’île de Palawan, n’est à l’abri de ce genre de catastrophe naturelle qui, je le rappelle, à ce jour n’est  toujours pas prévisible.


Le phénomène qui de loin m’effraie le plus.

Imprévisible, violent, la force de la nature à l’état brut, un déchainement de forces colossales, un combat de titans entre les plaques de la croûte terrestre, le frissonnement de la planète Terre.  

Le récent tremblement de terre qui a touché Bohol et cette région des Visayas a fait de nombreuses victimes. Des églises, souvent vieilles de plusieurs siècles, ont également souffertes, des dégâts parfois irrémédiables.





Experts warn that many parts of the Philippines—including the nation's densely-populated capital—are long overdue for major earthquakes, and we can't entirely be sure where the next big one will strike.

But one thing is for certain: it's only a matter of time.

Mario Aurelio, Laboratory Head of the Structural Geology and Tectonics of the UP National Institute of Geological Sciences (NIGS) laments the lack of comprehensive knowledge about the country's numerous geologic faults.

The 7.2 magnitude earthquake that shook Central Visayas on October 15 was a relatively rare event, Aurelio notes, but other parts of the country are also due for similar quakes.


Earthquake-prone areas

“Bohol and Cebu earthquakes have been relatively rare, considering that the entire Philippine archipelago is earthquake-prone except for Palawan,” said Mario Aurelio, Laboratory Head of the Structural Geology and Tectonics of the UP National Institute of Geological Sciences (NIGS).


“('Yung Bohol at Cebu) hindi naman kasing active ng mga considered na very active faults tulad ng sa Siargao and Zambales,” said Ric Mangao, research specialist from the Philippine Institute of Volcanology and Seismology (Phivolcs).


Hidden faults

On Wednesday, Phivolcs said that a newly-discovered fault might be the source of the Visayas earthquake.

Phivolcs identified the quake's real epicenter as being located between the municipality of Catigbian and Sagbayan in Bohol, and not in the town of Carmen, as was previously thought.
However, Aurelio said that immediately associating the recent earthquake in Bohol with an underlying fault is still a tentative hypothesis. He said that there are many unidentified faults in the Bohol region and elsewhere, which makes the identification of the origin of the earthquake “a little bit tricky”.

As a parallel example, he cited the 7.7-magnitude earthquake in Northern Luzon in 1990. There were no recorded faults in the area when this quake hit, killing 1,621 people across the region.

Moreover, Phivolcs' reassessment of the quake's epicenter coincides with initial data from the US Geological Survey (USGS): in its summary page for the Bohol quake, the agency located the epicenter at 2km northeast of Catigbian.


“'Yung mga aftershock lumalabas sa East Bohol Fault,” said Mangao.


Fate of old churches, other buildings

Old churches and shrines in Bohol were not designed to be earthquake-proof.

“Assuming that churches in Bohol were done through sound construction practices, the damage were done not because of material but because (of the) design,” Aurelio said.

Old churches don't have rebars, the metal skeletons that serve as the foundation of a structure and hold it together. Instead, they were built by piling blocks of limestone, one on top of another.
Restoring the churches in Bohol will take years, according to heritage documentation specialist Joel Aldor in a GMA News TV report.

Earlier this year, Phivolcs director Renato Solidum warned that many buildings, particularly in the nation's densely-populated capital, remain unassessed for physical integrity in the event of strong earthquakes.

“The issue though is some of the buildlings are non-engineered and most likely walang permit so they have to focus on that also,” he said.

Major lifelines like water and power supply, and communication means, should be strengthened to withstand fires and earthquakes.


Active fault line under Metro Manila

Meanwhile, hidden under Metro Manila is the West Valley fault line, which experts also believe is long overdue for a large quake.

"Ripe na gumalaw ang fault. Napakataas ng probability na gumalaw ito in the future, hindi lang natin masabi ang exact date and time," Phivolcs deputy director Bartolome Bautista said during a Senate inquiry on the country's disaster preparedness on Wednesday.

The earthquake fault runs from the Sierra Madre mountain range to Tagaytay, and moves every 200 to 400 years. The last time this fault moved was 200 years ago.

Phivolcs presented the prediction in July this year, warning Metro Manila that this quake is due to happen within our lifetimes.

Solidum said that preparations for the next large quake are of paramount importance.
Unlike storms and typhoons, earthquakes cannot be “forecast” and can only be predicted by looking at how often it happens in history. To date, there are no scientific instruments that can predict when an earthquake will occur.


A 7.2-magnitude earthquake, as was seen in Bohol, can shake the ground even a hundred kilometers away, meaning a tremor in Metro Manila can affect surrounding provinces.


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