Watch out for ‘The Big One’

By Henrylito D. Tacio
(First of Two Parts)
It’s not only Metro Manila that is waiting for the “Big One.” Davao City, the country’s largest city, is ripe for another big earthquake, too.
Last year, EDGE Davao Editor-in-Chief Antonio M. Ajero wrote: “A big earthquake as strong as, if not even stronger than, the so-called ‘Big One’ that Metro Manilans are preparing for is a possibility in Davao City in the immediate future.”
With a land area of 244,000 hectares, Davao City is approximately three times the size of the entire Metro Manila. According to Desiderio Cabanlit, regional director of the Philippine Institute of Volcanology and Seismology (Phivolcs), the city is likely to be destroyed by big earthquake that may be triggered by the Surigao-Mati fault.
The new active fault line was discovered by geologists when Phivolcs updated its 10-year old fault map. It stretches from Surigao City to Mati City in Davao Oriental and has a distance of 320 kilometers.
Based on the recent Phivolcs study, it was found that a 7.2 magnitude earthquake hit Compostela Valley in 1893. On April 15, 1924, another earthquake with 8.3 magnitude happened somewhere in Sigaboy, now known as Governor Generoso in Davao Oriental.
Cabanlit explained that should an earthquake with an intensity of 7.2 may happen again in Compostela Valley, Davao will immediately feel a “7.0 to 7.1 magnitude and ground shaking could reach (the city) in less than one minute.”
Aside from those mentioned earlier, there were other major earthquakes that rocked Mindanao, according to data from Phivolcs. On August 16, 1976, an earthquake with a magnitude of 8.0, with epicenter traced to the Celebes Sea, triggered a tsunami in Moro Gulf. A major aftershock was recorded to have a magnitude of 6.8.
A magnitude of 7.6 earthquake struck Lanao on April 1955 that killed 400 people. The intensity recorded in Dansalan was 8.0; in Dipolog and Ozamiz, 7.0; and in Cagayan de Oro and Malaybalay, 6.0. A 6.8 magnitude earthquake that rocked Southern and Central Mindanao on March 7, 2002 killed 11 people.
 
Destructive earthquakes most likely
“For the last 35 years, the Philippines had been affected by 10 earthquakes with magnitude greater than 7.0,” said a fact sheet circulated by Phivolcs. “Hence, the likelihood of these destructive earthquakes occurring again in the future is indeed very strong.”
Several studies pointed out the Philippines archipelago lies between two major tectonic plates, the Philippine Sea Plate and the Eurasian Plate. Phivolcs says the Philippine Sea Plate is moving towards the Philippine Archipelago at the rate of about 7 centimeters per year. 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 these two plates is found the Philippine Fault Zone which decouples the northwestward motion of the Pacific with the southwestward motion of the Eurasian Plate,” Phivolcs explains. “Movements along other active faults are responsible for the present-day high seismicity of the Philippine Archipelago.”
Every day, at least 5 earthquakes occur in the country. “Based on the distribution of earthquake epicenters, the most seismically active part of the county is its eastern section containing eastern Mindanao, Samar and Leyte with an average of 16 perceptible earthquakes per year.”
 
6,000 earthquakes annually
According to the Grolier Encyclopedia, about 6,000 earthquakes are detected throughout the world each year. Of this, 5,500 are either too small or too far from populated areas to be felt directly. Another 450 are felt but cause no damage while 35 cause only minor damage. The remaining 15, however, can exact great toll sin death and suffering, besides heavily damaging houses, buildings, and other structures.
An earthquake (also known as a quake, tremor or temblor), according to a monograph circulated by the Philvocs, “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 akong 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.
 
Earthquake hazards
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,” explains RED (Reference for Emergency and Disaster), which is published by the Department of Science and Technology (DOST).
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.
“The most common manifestation is a long fissure extending from a few kilometers to tens of kilometers,” RED explains. “Ground rupture may also occur as a series of discontinuous cracks, mounds or depressions. Houses and buildings on top of an active fault can be damaged by ground rupture.”
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. “The main effects of landsliding would be erosion and burial,” RED warns.
Earthquakes can also cause tsunamis. “Tsunamis are giant sea waves generated mostly by submarine earthquakes,” RED says. They occur when the earthquake is shallow-seated and strong enough (magnitude 6.5 or greater) to vertically displace parts of the seabed disturb the mass of water over it.
Other causes of tsunamis include submarine or coastal landslides and submarine volcanic eruptions. Tsunamis can flood low lying coastal areas and drown people.
“Not all submarine earthquakes, however, can cause tsunamis to occur,”Phivolcs points out. (To be concluded)