(First of two parts)
Last year’s killer 7.2-magnitude earthquake in Bohol may have been a precursor of a more powerful temblor.
Central Philippines is not the only area considered to be a high-risk zone. Even Metro Manila can be hit at any time, based on previous patterns.
While science is still unable to predict exactly when earthquakes can strike, where they are going to occur has long been known. New and old data can guide builders in choosing where to build, and what places to avoid.
Boholanos and Cebuanos should take note of a new data affecting their places of work and residence when erecting a house or a building.
Volcanologists from the Philippine Institute of Volcanology and Seismology (Phivolcs) recently announced the discovery of three possible fault lines in the island of Cebu, fault lines that could trigger a series of earthquakes with yet unknown intensities or magnitudes.
One of the fault lines crosses central Cebu, while two lie opposite each other in southern Cebu.
Some of the towns in southern Cebu include Ginatilan, Malabuyoc, and Alegria to the west side of the island, opposite Oslob, Ginatilan, Boljoon, and Alcoy to the east.
Central Cebu includes Toledo City, the mountain areas near the boundary of Cebu City and Balamban, Talisay, Mandaue, Lapu-Lapu, Danao, and the Cebu Metropolitan Area.
These new fault lines are alarming given that the quake that struck Bohol and Cebu last October destroyed or damaged over 50,000 homes.
Residents of the National Capital Region (NCR), however, should not consider themselves “lucky” that temblors of that magnitude will never affect them.
In all likelihood, it will.
Dr. Renato Solidum Jr., Phivolcs director, said the latest collaborative works of geologists show that the West Valley fault—which covers parts of Metro Manila—has moved in the last 1.400 years, and has a cycle of between 400 to 600 years.
Aerial photographs, satellite imagery and topographic maps completed by the institute in 1991 showed movements in the fault.
The next killer quake may not strike until 2058 (the last major movement took place in 1658), but this early government agencies were preparing for the worst, Solidum said.
Intense damage can be expected in Marikina, Quezon City, Pasig, Makati, Pateros, Taguig and Muntinlupa, where the West Valley fault passes.
Based on studies by Phivolcs and the US Geological Services, the next big quake—to hit either Metro Manila or central Visayas (again)—would be between magnitude 6 and 7.
“For planning purposes and based on the data we have, what we are planning for is a magnitude 7.2 earthquake scenario,” Solidum said.
In the case of Metro Manila, the scenario is based on the length of the West Valley fault.
If a big quake hits the metropolis at night, the death toll could be as high as 31,000. Some 78 million square meters would be affected.
Phivolcs studied the buildings in Metro Manila and found that most of them are old and needed retrofitting in order to survive the same type of shock as the Bohol quake.
The national government will need P2.2 trillion to reconstruct Metro Manila. Should that expected quake strike this year, the entire annual budget of P2.268 trillion would be exhausted.
“On the other hand, if the magnitude is lower than 6.5—which is significantly less than 7.2—then the collapsed damaged square meter area would be around seven million. Complete damage would be 58 million sq/m and fatalities would be around 23,000,” he said.
The cost of damage would “only” be P1.7 trillion.
The most important consideration for ongoing and planned construction projects is the strength of materials used. Under the current building code, all new structures should be built to withstand up to a magnitude 9-quake.
An estimated 35 percent of low-rise and 25 percent of mid-rise buildings in the metropolis were not built under the tougher building code now in effect.
A review of the Bohol quake reveals that even newer buildings may not have complied with the higher standards.
Engineers inspecting the damaged structures recovered substandard building materials from the debris. The worse the damage sustained by the structure, the greater likelihood substandard materials were used in their construction. There is clear correlation between the proliferation of substandard building materials in the region and the scale of the damage from the earthquake.
The most usual cause for the structures collapsing is the use of inferior reinforcement steel bars. Many of the reinforcement bars recovered from the debris of concrete houses in the quake zone weighed at least 11 percent less than the required product specification.
It is likely that many of these inferior steel products were fabricated from wire rods instead of billets, causing them to have less tensile strength.
Had the building codes been strictly observed in Bohol, most of the damaged structures should have been able to withstand the tremor.
Besides the likely use of substandard materials, another man-made threat that can worsen the damage caused by the next big quake is the seemingly non-stop reclamation of lands along the coast.
Pasay City and other frequently flooded areas face the danger of liquefaction, which can cause a “quicksand effect” and swallow up structures. With the soil partially or fully saturated, the foundations of high-rise buildings are literally standing on shaky ground.
There are many different kinds of fault lines criss-crossing the earth’s surface, many of which may require extensive studies and the use of sophisticated geologic instruments to identify. For example, creeping faults are those that move in lateral motion very slowly all the time. Although moving, its speed is so slow that it could take decades before an offset becomes noticeable.
Such faults could be identified by comparing photographic evidence of structures on the ground or more clearly in aerial surveys by studying the topography, especially of natural features like ridges, valleys, and streams.
Phivolcs is still conducting studies to determine the possible magnitude or strength of the earthquakes, which could be generated along these fault lines just in case they move.