MANILA – State seismologists urged the public Monday to be more wary of the conditions and integrity of their homes following the 6.8 magnitude earthquake that jolted Sarangani and nearby provinces in Mindanao last week.
Philippine Institute of Volcanology and Seismology (Phivolcs) science research specialist Bhenz Rodriguez explained that more than just participating in regular earthquake drills, one must be mindful that the “duck, cover and hold” is also done under the assumption that the structure we are in has good integrity.
“Pag nagkaroon ng isang malakas na lindol, tapos nagkaroon ng hairline crack, kailangan na talagang ipa-check ang bahay o building. Kasi posibleng sa unang lindol, nagkaroon ng damage pero minor lang hindi siya bumagsak. Pero pag nagkaroon ng malakas na lindol o sunod sunod na lindol, posibleng sa lindol na yun yun ang magpabagsak ng bahay,” he said.
(When a strong earthquake happens and a hairline crack appears, then it’s time to have the house or building checked. Because it is possible that in the first quake, it only sustained minor damage. But if another strong earthquake occurs, then it’s possible that the next one can make it collapse.)
Tremors are not rare in the Philippines, which in recent history has recorded damage and destruction at massive proportions.
Phivolcs monitors an average of 20 tremors in different parts of the country per day but a lot of them are not felt.
But while some earthquakes are stronger than others, experts said that it becomes potentially damaging once its magnitude hits 5 on the scale.
Magnitude versus Intensity
A magnitude, often used in media reports following tremors, is the size of an earthquake. This is a number, written either in whole or decimal fractions, that doesn’t change regardless of an area’s distance from the epicenter.
Meanwhile an earthquake’s intensity varies depending on proximity to the epicenter.
“Pag sinabi namang intensity, ito yung effect ng lindol. Kung paano siya naramdaman ng mga tao o yung mga tao at building na nasa ibabaw ng lupa.”
(When we say intensity, it refers to the earthquake’s effect on the public and the structures on land.)
Therefore, the closer one is to an earthquake’s epicenter means a higher intensity.
In the recent Mindanao earthquake, Glan town in Sarangani Province felt the highest intensity at 7 while General Santos City and Koronadal and Polomolok in South Cotabato registered intensity 6.
However, Rodriguez also said that there are also instances that despite recording a high magnitude, an earthquake’s damage remains minimal.
“Pag mas malalim, mas mahina yung mararamdaman. Kahit na malapit ka sa epicenter ng lindol, pero malalim naman yung event, medyo mahina yung mararamdaman. Pero this time, yung event sa Sarangani, malakas yung lindol tapos considerably mababaw yung lalim,” he explains.
(If the depth of the earthquake is significant, it’s not as felt. Even if you’re near the epicenter of the earthquake, but the depth is significant, you won’t feel it as much. The earthquake in Sarangani is strong because the depth of it is considerably shallow.)
It’s also worth noting that an earthquake has more than one center -- epicenter being the most commonly-known. The epicenter is the point on the earth’s surface that is directly above the source of ground movement. The second center is known as the hypocenter or the focus point of the movement.
Aside from ground-shaking, ground ruptures and fissures are among the most common hazards following a strong quake.
According to Phivolcs, this is “characterized by deformation on the ground that marks, the intersection of the fault with the earth’s surface.
Tremors can also cause landslides or the down slope movement of rocks and other debris.
If the movement of the ground is significant, it can also expose people to the threat of tsunamis. This is a series of waves which can cause massive flooding, coastal erosion and worse, drowning and damage to properties.
Another phenomenon that can happen to an area following an earthquake is liquefaction where sediments, especially those near bodies of water, begin to behave like liquid thereby losing its ability to hold structures in place.
In Sarangani, Phivolcs has yet to declare that the fissures in some areas were because of liquefaction.
Rodriguez explains should an earthquake of the same intensity happen in Metro Manila, the damage and destruction is expected to be much worse.
“Mas malaki ang effect sa Metro Manila kasi nandito yung sentro. Maraming mga building, mga bahay, maraming pwedeng maapektuhan unlike sa rural areas, posibleng konti lang yung mareport o makaramdam kasi konti lang din yung tao,” Rodriguez explained.
(The effect will be much bigger in Metro Manila because of the many buildlngs, houses and other structures that can be affected. In rural areas, it’s possible that there are less reported damage because there are less people.)
The country’s unique position in the Pacific Ring and the layout of faults and trenches and, more importantly, the vulnerability of a greater majority of the public to these hazards, make it extremely important for local governments to continue conducting earthquake drills as it is a good way to instill the importance of preparation and readiness especially since earthquake, unlike typhoons, are unpredictable.
“Wala pang science na nakakapredict o nakakaalam kung kailan mangyari ang lindol. Pwedeng malaman kung gaano kalakas yung pwedeng mangyari sa isang area na lindol. Nakikita din kung kailan yung huling gumalaw based on studies. For now yun pa lang ang nagagawa sa earthquakes,” Rodriguez said.
(There is still no way for us to predict or know for sure when an earthquake will happen. We can determine the strength of an earthquake after it’s happened. We can also say, based on data, when and how many times a particular fault line moved. For now that’s all that we can do with earthquakes.)