“Feeling earthquakes was part of growing up, and also preparing for them: doing earthquake drills, or having earthquake supplies. The looming feeling was part of my life. My experience of earthquakes has always been more the fear of them, or the possibility.” – Karen Thompson Walker
There was nothing unusual last Wednesday night. After dinner, I went immediately to my sala and finished the article I wrote that afternoon. Thirty minutes later, I decided to lay down in my bed. It was 7:30 in the evening already – to early to go to sleep.
I decided to open my Facebook account. Some friends were online so I chatted with some of them. At 7:37 pm, I was jolted by the strong quake. The house was literally swaying. I rushed to the door but it was locked. I tried to open the door when suddenly the room was completely dark.
There was a total blackout. It was good that I was using my iPad; I used the light to see my way out. Now, at the tierra firma, I was all alone. But there was chaos all over.
From nearby house, where my parents and my sister’s family live, there was a lot of shouting. My mother, who was lying in her bed, had a hard time getting out from her bed. So, was my father.
A few minutes after the earthquake, the Philippine Institute of Volcanology and Seismology (Philvolcs) of the Department of Science and Technology (DOST) issued a bulletin that the earthquake was tectonic in origin.
It was felt in almost all part of Mindanao: Intensity VII in Kidapawan City, Intensity V in Tupi, South Cotabato and Alabel, Sarangani; Intensity IV in General Santos City; Kiamba, Sarangani; and T’Boli, South Cotabato; Intensity III in Cagayan de Oro City and Gingoog City; Intensity II in Cagayan de Oro, Misamis Oriental; and Intensity I in Dipolog City and Bislig City.
“The U.S. Geological Survey said the magnitude 6.4 quake was centered about 8 kilometers from Columbio, a landlocked town in the coastal province of Sultan Kudarat,” Associated Press reported. “The earthquake had a depth of only 14 kilometers. Shallow quakes tend to cause more damage than deeper ones.”
“The Ring of Fire” – that is how scientists call the area in the basin of the Pacific Ocean where a large number of earthquakes and volcanic eruptions occur. Unfortunately, the Philippines – a country with 7,107 islands – is located in this rim sometimes called the circum-Pacific seismic belt. About 90% of the world’s earthquakes and 81% of the world’s largest earthquakes occur along the Ring of Fire.
The Encyclopedia Britannica says about 50,000 earthquakes large enough to be noticed without the aid of instruments occur every year globally. “Of these, approximately 100 are of sufficient size to produce substantial damage if their centers are near areas of habitation. Over the centuries, they have been responsible for millions of deaths and an incalculable amount of damage to property,” it adds.
An earthquake (also known as a quake, tremor, or temblor), according to a monograph circulated by the PhilVolcs, “is feeble shaking to violent trembling of the ground produced by the sudden displacement of rocks or rock materials below the earth’s surface.”
The earth has an outermost shell, about 80-kilometer thick, which is solid and rigid. This shell is called lithosphere, which is subdivided into small and large pieces with some pieces large enough to contain continents. These pieces of lithosphere are called tectonic plates.
So-called faults are breaks or zones of weaknesses in rocks along which displacements had occurred or can occur again. They may extend for hundreds of kilometers downward, even down to the base of the lithosphere. Faults showing signs or documented history of recent displacements are called active faults.
The Philippine Archipelago lies between two major tectonic plates: the Philippine Sea Plate and the Eurasian Plate. “Philippine Sea Plate is moving towards the Philippine Archipelago at the rate of about 7 centimeters every year,” Philvolcs explains. “The Eurasian Plate is being subducted along western side of Luzon and Mindoro at the rate of 3 centimeters per year except on Mindoro and northwest of Zamboanga where collision is taking place.”
At the intersection of the two aforementioned plates is the Philippine Fault Zone, “which decouples the northwestward motion of the Pacific with the southwestward motion of the Eurasian Plate.” Movements along other active faults are reportedly responsible for the present-day high seismicity of the Philippine archipelago.
According to Philvolcs, at least 5 earthquakes per day occur in the Philippines. For almost four decades now, the country had been affected by 10 earthquakes with magnitude greater than 7.0. As such, the possibility of destructive earthquakes occurring again in the future “is very strong.”
Hazards posed by earthquakes
The destructive effects of earthquakes are due mainly to intense ground shaking or vibration. “Because of severe ground shaking, low and tall buildings, towers and posts may tilt, split, topple or collapse, foundation of roads, railroad tracks and bridges may break, water pipes and other utility installations may get dislocated, dams and similar structures may break and cause flooding, and other forms of mass movement may be generated,” Philvolcs explains.
Liquefaction, the process where particles of loosely consolidated and water-saturated deposits of fine sand are rearranged into more compact state, can also occur. Liquefaction prone areas can be found in beach zones, sand spits, sand bars, wide coastal plains, deltaic plains, floodplains, and former or existing marshlands and swamplands.
Many strong earthquakes originate along faults that break the earth’s rigid crust. Called ground rupture, it is a deformation on the ground that marks the intersection of the fault plane with the earth’s surface.
Landslides, the downward movement of slope materials either slowly or quickly, are most likely to happen. Hilly and mountainous areas, escarpments, and steep river banks, sea cliffs and other steep slopes are prone to landslides.
Earthquakes can also cause tsunamis. “Tsunamis are giant sea waves generated mostly by submarine earthquakes,” Philvolcs says. “Not all submarine earthquakes, however, can cause tsunamis to occur.”
Recently, Philvolcs has come up with 10 intensity scales of earthquake. The intensity scale, which is a measure of how an earthquake is felt in a certain locality or area, is “based on relative effect to people, structure, and objects in the surroundings.”
I. Scarcely perceptible: It is perceptible to people under favorable circumstances. Delicately-balanced objects are disturbed slightly. Still water in containers oscillates slightly.
II. Slight felt: It is felt by few individuals at rest indoors. Hanging objects swing slightly. Still water in containers oscillates slightly.
III. Weak: It is felt by people indoors especially in the upper floors of buildings. Vibration is felt like the passing of a light truck. Dizziness and nausea are experienced by some people. Hanging objects swing moderately. Still water in containers oscillates moderately.
IV. Moderately strong: It is felt generally by people indoors and some people outdoors. Light sleepers are awakened. Vibration is felt like the passing of a heavy truck. Hanging objects swing considerably. Dinner plates, glasses, windows and doors rattle. Floors and walls of wood-framed buildings creak. Standing motor cars may rock slightly.
V. Strong: Generally felt by most people indoors and outdoors. Many sleeping people awakened. Some are frightened; some run outdoors. Strong shaking and rocking are felt throughout the building. Hanging objects swing violently. Dining utensils clatter and clink; some are broken. Small, light and unstable objects may fall or overturn. Liquids spill from filled open containers. Standing vehicles rock noticeably. Shaking of leaves and twigs of trees is noticeable.
VI. Very strong: Many people are frightened, many run outdoors. Some people lose their balance. Motorists feel like driving with flat tires. Heavy objects and furniture move or many be shifted. Small church bells may ring. Wall plaster may crack. Very old or poorly built houses and man-made structures are slightly damaged, though well-built structures are not affected. Limited rockfalls and rolling boulders occur in hilly to mountainous areas and escarpments. Trees are noticeably shaken.
VII. Destructive: Most people are frightened and run outdoors. People find it difficult to stand in upper floors. Heavy objects and furniture overturn or topple. Big church bells may ring. Old or poorly built structures suffer considerable damage. Some well-built structures are slightly damaged. Some cracks may appear on dikes, fish ponds, road surfaces or concrete hollow block walls. Limited liquefaction, lateral spreading and landslides are observed. Trees are shaken strongly.
VIII. Very destructive: People are panicky. People find it difficult to stand even outdoors. May well-built buildings are considerably damaged. Concrete dikes and foundations of bridges are destroyed by ground settling or toppling. Railway tracks are bent or broken. Tombstones may be displaced, twisted or overturned. Utility posts, towers, and monuments may tilt or topple. Water and sewer pipes may be bent, twisted or broken.
Liquefaction and lateral spreading cause man-made structures to sink, tilt or topple. Numerous landslides and rockfalls occur in mountainous and hilly areas. Boulders are thrown out from their positions particularly near the epicenter. Fissures and fault rupture may be observed. Trees are violently shaken. Water splashes or slops over dikes or banks of rivers.
IX. Devastating: People are forcibly thrown to the ground. Most children cry and shake with fear. Most buildings are totally damaged. Bridges and elevated concrete structures are toppled or destroyed. Numerous utility posts, towers and monuments are tilted, toppled or broken. Water and sewer pipes are bent, twisted or broken.
Landslides and liquefaction with lateral spreading and sandboils are widespread. The ground is distorted into undulations. Trees are shaken very violently with some toppled or broken. Boulders are commonly thrown out. River water splashes violently or slops over dikes and banks.
X. Completely devastating: Practically all man-made structures are destroyed. Massive landslides and liquefaction, large scale subsidence and uplifting of landforms, and many ground fissures are observed. Changes in river courses and destructive seiches in lake occur. Many trees are toppled, broken or uprooted.
Always be ready
“The only way to avoid disasters caused by earthquakes is to prepare for them,” wrote Maria Elena Paterno in her book Earthquake! After earthquakes, Filipinos should be prepared for the aftershocks. These are usually weaker earthquakes that follow the main shock of an earthquake sequence.
Kathleen Tierney, director of the Natural Hazards Center at the University of Colorado, reminds: “The Philippines is one of the most disaster-prone places on Earth. They’ve got it all. They’ve got earthquakes, volcanoes, floods, tropical cyclones, landslides.” – ###
(Photos taken from the net)