METRO MANILA’S WEST VALLEY FAULT: EARTHQUAKE RISKS AND HAZARDS

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METRO MANILA’S WEST VALLEY FAULT: EARTHQUAKE RISKS AND HAZARDS

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sundry-thumbby Jes B. Tiron

Proem

Last 19 May 2015 the PHIVOLCS (Phil. Inst. Of Volcanology and Seismology) published a map of the West Valley Fault of Metro Manila and the probable risks and hazards it will bring in case the fault line will cause an earthquake.

The crack or ground surface manifestation runs from the province of Bulacan, then down to Marikina City, Quezon City, Pasig City, Taguig City, Muntinlupa City, and downward to the province of Laguna for a total distance of about 100 km.

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Cause For Alarm

Along and crossing this fault line are already many built structures.  If and when the earthquake will occur, the affected population will be about 20 million people.

According to Phivolcs the expected strength of the earthquake will be Magnitude 7.2 and the damage will be Intensity 8 in Metro Manila.  When I computed the probable earthquake from standard empirical formulas, the expected Magnitude for a 100 km distance is M7.6.  It is stronger than what Phivolcs expected.  (Note: I applied the same formula for the Bohol Earthquake and the result was M7.2.  It was the same as the actual magnitude.)

Bohol had already experienced the damages done by a M7.2 earthquake.  You can just imagine how much damages will be done by a M7.6 earthquake in a densely populated area.

The Phivolcs sounded the alarm because the last time the West Valley Fault produced a strong earthquake was in year 1658.  Their study showed that the return period of the earthquake is about 400 years.  It is about time that the next strong earthquake will occur.

There is yet no accurate formula for predicting when the next earthquake will occur.  The last time the Central Bohol Fault line moved was in 1906.  The return period for the same fault line is 100 years.  So I gave the warning that by year 2006 the next big earthquake will occur.  However, it occurred in 2013 or seven (7) years later.

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Expected Damages

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According to the estimates of the Phivolcs, a M7.2 earthquake will produce 37,000 deaths, 168,000 injuries, and 40% of the houses, buildings, etc. will be destroyed or damaged in the Metro Manila area.  The value of the damages will be about P2.5 Trillion.  It is really a daunting scenario to imagine.

Take note that many of the roads and bridges will be damaged.  It will hamper rescue operations.  Many of the funeral homes and hospitals will be damaged; where will you bring the dead and the wounded?  The telecommunication network will be dead, how will you communicate?  The water supply will be damaged, where will you obtain water?  The government offices will be damaged, who will take charge?

Causes of Damages

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I do not know what are the causes of damages the Phivolcs have considered, but the following are the usual causes of damages during an earthquake:

  1. Ground shaking – Many of the structures are not “engineered”. These will be easily damaged. Even the engineered structures will be destroyed because our Building Code provides only 10% gravitational acceleration (g=32.2 ft/sec/sec: 10%g = 3.22 ft/sec/sec).  In a near-site occurrence, the ground acceleration will reach 2g = 64.4 ft/sec/sec and you design only for 3.22 ft/sec/sec.

Force = mass x acceleration. (F = ma).  Assume a building mass = 10,000 Lbs.

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Code provision: Force = 10,000 x 3.22 = 32,200 Lbs.

Actual Eqk. Force = 10,000 x 64.4 = 644,000 Lbs.

The Code provision design is 32,200 Lbs while the actual earthquake force will be 644,000 Lbs.  The Code provision is too far below the safety limit.  Many buildings will collapse.

Note: When I make designs, I use the Modal Method which is a mathematical model of the actual earthquake forces.  I am proud to say that none of the buildings I designed suffered any structural damaged during the Bohol Earthquake of 2013.

  1. Ground separation – The ground will crack or separate from each other along the fault line by as much a 3 meters. If your building is over the fault line, it will be destroyed.
  2. Fire – In the Metro Manila area, there will be thousands who will be cooking any time of the day. When the earthquake will occur, many of the stoves will be overturned and cause fires. Do not expect the Fire Department to come because the roads are not passable.  The destroyed electrical connections can also cause fire.
  3. Flooding or Inundation – There are huge dams in the area, especially the La Mesa Dam. If the La Mesa Dam will be destroyed, the water will flood the nearby housing projects.
  4. Falling Debris – Falling debris from damaged buildings can cause death or injuries.
  5. Electrocution – When the earthquake will occur, many of the electrical connections will still be live. It will have capability to electrocute.
  6. Landslides – The movement of huge chunks of land can cause death and damage.

The Other Hazard

The Phivolcs has only concentrated its attention on the West Valley Fault.  There is another fault line that is equally hazardous.  It is the Manila Trench found in front of the Manila Bay and located along the western side of Luzon from Cavite to Zambales.

If a M7.2 earthquake will occur there, a tsunami of about 6 to 10 meters high will arrive in ten (10) minutes and engulf the cities along the Manila Bay.  Bear in mind that Manila is only 1.5 m about sea level.  We have seen from television how a tsunami will cause damage.  Imagine if it is a M7.6 earthquake.

At Makati City there is a thrust fault transverse to the West Valley Fault.  If it will move, it will produce an up and down motion.  Many of the tall buildings in Makati will topple over because they are designed only for a side to side shaking.

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