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August, 2006 A new controversy raging in the world is that two of the recent earthquakes could have been triggered by Taipei 101, the tallest building of the world. 508 metres high, Taipei 101 weighs 700,000 tonnes and it is believed that it has put so much stress on the ground below it that some earthquake fault could have been reopened.
Putting forth this theory, Dr Cheng Horng Lin, a geologist from Taiwan, has said that Taipei basin was a stable area before the construction of the tallest building and the number of earthquakes in the area have increased since its completion. Dr Lin’s theory has put a question mark on the future of other skyscrapers under planning or construction in the world.
Earth and Tectonic Plates
Before believing in Dr Lin’s theory, the composition of earth and the causes of earthquakes will have to be looked into again. Earth is made of four layers, inner core, outer core, mantle and crust. Inner core is solid metal of about 2450 km diameter. Outer core is liquid and 2150 km thick. Next comes Mantle, 2900 km thick and made of high temperature rock. The topmost layer is Crust having a thickness of 5 to 40 km, the oceanic crust being 5 to 10 km thick and continental crust 35 to 40 km thick.
High temperature and pressure difference between the Crust and the Core cause masses of the Mantle to slide over Outer Core. These masses of Mantle are called Tectonic Plates. Earth’s surface has seven tectonic plates which keep moving in different directions thereby causing stress on adjoining faces of the plates.
The “faults” lie at the adjoining faces of tectonic plates. Movement of plates causes stress in the faults. Whenever this stress becomes excessive, vibrations travel to the Crust causing earthquakes.
The point inside Earth where an earthquake originates is called Focus. Point on Earth’s surface directly above the Focus is called epicentre. The depth of Focus is generally very high, in the range of 100 km. If less than 70 km, it is called shallow, if 70 to 300 km, it is called Intermediate and the one above 300 km is called deep depth of focus. Though most of earthquakes occur at depth less than 100 km, even the shallowest earthquakes are a few km deep below earth’s surface.
Taipei101, located in Xin-Yi district of Taipei in Taiwan, is built over 80-metre-deep piles, 380 in number, each having 1500 mm diameter. 30 metre depth of piles penetrates into the bedrock. The load of Taipei101 is spread over an area of 3.7 acres.
Whenever a building is constructed, the pressure that its load causes on the ground is calculated and compared with the bearing capacity of the soil below it. The type of foundation is chosen accordingly to keep the pressure less than the bearing capacity.
Settlements allowed in the buildings are also very small. Howsoever high or heavy a building may be, pressure caused by it on the founding stratum will be within the bearing capacity at that level. The pressure distribution takes place in a bulb below the foundation, least of it being at the lowest point of the bulb.
The pressure caused by Taipei101 even at a depth of 500 metres should be small, leave apart its effect at the depth at which earthquakes originate. In support of his argument, Dr Lin has also quoted the number of earthquakes taking place in Taipei101 area. Taipei101 is, in fact, already located in a highly active seismic zone. Relating the earthquakes occurring in that area to Taipei101 therefore is not justified. Moreover, worldwide, about 1.5 lakh earthquakes are recorded annually while 9.0 lakh of them go unrecorded. Under such circumstances, two or three micro-earthquakes occurring last year in Taipei101 area can’t be attributed to it.
The theory put forth by Dr Lin, therefore, doesn’t hold good. Earthquakes originate from within the Earth and in comparison to the size and magnitude of earth, man’s activities are too small and can’t cause earthquakes. Construction of a skyscraper may cause movement of faults only if the stress caused by its load reaches the Focus zone below the earth. Howsoever high skyscrapers we may build, their weight can’t cause stress 10 km below ground surface.
Construction of skyscrapers should, therefore, continue without a thought that these may cause earthquakes.
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