‘Shaking Table’ at Phivolcs helps Pinoys prepare for quakes

Along the driveway of the Philippine Institute of Volcanology and Seismology (Phivolcs) in Quezon City, there is a small green hut with a lamp at the center that swings gently or violently depending on the earthquake’s intensity. Phivolcs personnel call it the Shaking Table, but it’s more than a piece of furniture. The open structure is an earthquake simulator device that replicates the varied intensities of a tremor, the only one of its kind in the country. The simple device was built a decade after a devastating magnitude-7.8 earthquake struck Luzon in 1990, toppling tall buildings in Baguio City and burying hundreds of victims under mounds of rubble. The tremor was stronger than the magnitude-7.2 earthquake that struck Haiti last week, which damaged billions in property and claimed thousands of lives. In the wake of the 1990 disaster, the Philippine government developed the Phivolcs Earthquake Intensity Scale (PEIS) the following year to describe the movements during a tremor, according to Ma. Mylene Villegas, Chief Science Research Specialist of Phivolcs. Villegas said differences in land forms in each country and other geological considerations prompted the development of the PEIS. This is especially crucial in the study of earthquakes in countries like the Philippines that are part of the so-called Pacific Ring of Fire, where 90 per cent of the earth’s tremors are known to occur. The PEIS measures how an earthquake is felt in a certain area based on its relative effect to people, structures, and objects in the surroundings. It was developed specifically for the Philippines, Villegas said. She explained that intensity is different from magnitude, which uses the Richter scale as an international standard to measure an earthquake's strength at its point of origin or epicenter. An earthquake’s intensity, meanwhile, is a measure of its strength from the epicenter. The farther a place from the epicenter, the weaker the measure of intensity. The Shaking Table is based on the PEIS, which uses a 10-point range to describe the intensity of earthquakes. “We have developed this to illustrate the effects of the different intensities of tremors as described in the PEIS," said Villegas. The simulator contains motors underneath the floorboards that duplicate both horizontal and vertical earthquake motions, explains Villegas. The vertical motions represent the primary waves of an earthquake, while the horizontal motions represent the secondary waves. Inside the booth, an intensity meter displays the earthquake intensity being simulated. “We got the idea of the simulator from Kobe, Japan, which developed simulators as big as a whole room," explains Villegas. Japan’s Earthquake-Defense or E-Defense, the largest in the world, was developed after the 1995 Kobe earthquake. Japan’s shaking table measures 65 feet by 49 feet, and can support building experiments weighing up to 2.5 million pounds. Villegas said the device is open to the public for trial, particularly to schools and universities. The simulator has a floor base of 200 cm by 200 cm, and a height of 240 cm. An Earthquake Preparedness Guide and a Filipino version of the PEIS are available on the Phivolcs website. Villegas said leaflets and posters of these guides are being disseminated in schools and universities across the country, and may be obtained in all Phivolcs regional and field offices. New building code With the recent Haiti earthquake, Villegas underscores the importance of the quality of building construction in determining the ability of structures to withstand similar strong tremors. Haiti’s Presidential Palace, which survived a demolition and explosion before it was reconstructed in 1918, collapsed during last week’s tremor. Its dome-shaped center, a hollow area supported by several Ionic columns, was seen as the structure’s weakest point as it received the most severe damage. Recalling the 1990 earthquake in Luzon, Villegas said, “There were high-rise structures that gave way to the tremor, but there were also those that did not. The quality of building construction largely defines whether a structure will weather an earthquake as strong as that in 1990." Villegas added that the architectural make-up of a building also determines its vulnerability to tremors.

“Pakistan and Turkey usually use bricks for their structures," said Villegas, explaining that bricks are more vulnerable to collapse. The 1999 magnitude-7.4 earthquake in Turkey claimed about 14,000 lives. Pakistan’s 2005 magnitude-7.6 tremor killed 79,000, blamed partly on poor construction. “The Philippines’ buildings have a good chance of surviving similar strong tremors if the 2000 National Building Code was followed during the construction," explains Villegas. Department of Public Works and Highways Undersecretary Romeo Momo supports this view. “Our flyovers are guaranteed safe. High- and medium-rise buildings have complied with the design requirements of the National Building Code, upgrading the standards of structures to withstand quakes of up to Intensity VIII," he said in an interview on dzXL radio Tuesday. However, he added that buildings constructed before the year 2000, especially low-budget socialized housing projects, may be vulnerable to damage from strong tremors. - YA, GMANews.TV