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Metro Manila’s other earthquake risks

(Article published in the Mar 30,2011 issue of Manila Standard Today)  

Except for the Guadalupe formation, which in my column last week I had described as bedrock, the ground of Metro Manila consists mainly of layers and layers, in different proportions in differing places, of silty clay, clay, sandy alluvium and fill.  When a strong earthquake occurs, the response of the ground in whole area will accordingly range from weakest for ground that is on bedrock to strongest for areas atop fill of 30 meters or more.  Since most of Metro Manila is not on a thin overlay of soil over bedrock, it makes sense to understand the dangers most of us we will be facing when, if I may be excused by Ayn Rand, Atlas shrugs.

 Because our soil is made up of loose sand layers, liquefaction is a real danger.  Liquefaction, literally “acting like liquid”, occurs when the shaking of the earth disturbs the sand particles that hem the water in little spaces thereby allowing the water to wet the sand which in turn become more movable.  At that point, the soil is said to behave like water and thus become an unstable base for the heavy buildings above ground. 

 While there is very little direct evidence of earthquakes in Metro Manila having caused liquefaction, it is possible that the ground subsidence and tilting in the Intramuros area, including Binondo and Sta. Cruz, may have been due, to some extent, to liquefaction of loose sand layers under the deltic plain of Manila during the violent earthquakes that rocked the city towards the end of the 16th century.
 










     

In Metro Manila, liquefaction at this time is likely to occur in beds of loose, water saturated, homogenous sand to an average depth of 30 meters or more.  The newly reclaimed areas in the city, not yet compacted by use and weather, are most vulnerable to liquefaction.

 To get an idea of the effect of liquefaction on a standing building, imagine yourself absent mindedly stepping into a pond of quick sand.  The sand around you offer no resistance to your arms and legs as you seek to exploit their resistance o stabilize you and permit you to step out.  Instead of getting anywhere, you trash about in all directions and flailing your arms all around you, unable to grasp even at the weakest blades of grass to secure for your body some badly needed leverage and resistance to stabilize you.  You tilt and are unable to keep yourself upright.

 Another source of concern is the impact of possible displacement of faults due to ground shaking. The root of the English word “fault”, as in “It is my fault”, is the latin “fallare”, to deceive or disappoint.  What appears solid, for instance, disappointingly shows, upon closer examination, minute cracks or imperfections.  If I am to engage in rapping in geological parlance, a fault is a crack on the rock. 

 In Metro Manila, three faults have been identified as significant enough to merit serious consideration, the West Valley Fault, the Manila Trench, and Manila Bay. There are no historical records of fault displacement in the area but there is evidence that some displacement together with the usually associated deformation occurred sometime during the second Glacial Age. Because this was very much earlier than the arbitrary 15,000 year benchmark that scientists use to categorize a fault to be “active”, it has not been given the usual attention.  Nevertheless, since there have been past displacements of more 80 meters in Pasig, movements by any of the three aforesaid faults during an earthquake should be recognized as possible, if not just probable.  

 To determine just how unnerving fault displacement can be, try standing one foot each on two separate bancas kept side by side only by the strength of your inner thighs.  All it takes is for a single wave to force your legs apart and make you unstable, if not completely bring you down into the water in the least ungraceful manner.

 Resonance is a third risk.  Resonance is the movement of the buildings in sympathy with the shaking of the ground surface.  The vibration of the ground has been determined to be replicated in the buildings.  It is this sympathetic movement of the buildings with the motion on the ground that puts great stress on buildings not suited to vibrating and thereby cause damage on buildings’ structure.

 Have you ever observed how the workmen contracted by the power or water concessioners  cut into the cemented or asphalted of the streets in order to etch a narrow area within which to lay out their principals’ pipes?  They use a worthy jack hammer that repeatedly pounds on the cement or asphalt.  Every so often, more frequent than seldom, several workmen take their turns at the jack hammer.  That is because the force on the street generates the opposite reaction of force on the jack hammer which is held by the hand of the workmen.  The hands are connected to the arms, which are connected to the shoulders, which are connected to the head which, predictably, also shake with the pounding of the jack hammer.  And, you can shake the head only for short durations.

 The process of the transfer of the vibration from the jack hammer to the head is called resonance.  And it is almost self-evident to say that the longer the exposure to the resonance, the greater is the stress suffered.   

 It has been observed that wherever there is a thick layer of soft mud there is a correspondingly long period during which certain buildings move in sympathy by way of distributing the stress in the entire structure.  These sympathetic movements have a duration which can range from 0 to 5 seconds or more.  The longer the duration the greater the stress.  On that same principle, the men operating jack hammers take turns frequently, all by way of self-preservation against the risk of resonance.

 When the likelihood of these risks of liquefaction, fault displacement, and resonance highly converge in one place, we have a disaster simply waiting to happen.  Not in our lieu times, please, Lord.

     

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