Sat, Dec 03, 2005 page2

• Taipei 101 faulted for earthquakes

  TOO BIG? A Taiwanese geologist says the building may have reopened an old geological fault, but other experts say there is not enough data to make such a judgement

  • THE GUARDIAN
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  Taipei 101 is a building with a lot to boast about. Standing 508m high, it is the world's tallest. And at 700,000 tonnes, it must be among the heaviest.

  But the sheer size of the skyscraper has raised unexpected concerns that may have far-reaching implications for the construction of other buildings and man-made megastructures.

  Taipei 101 is thought to have triggered two recent earthquakes because of the stress that it exerts on the ground beneath it.

  According to geologist Lin Cheng-horng (林正洪) of National Taiwan Normal University, the stress from the skyscraper may have reopened an ancient geological fault in the Taipei basin.

  If he is right, then it raises concerns about proposals such as Sky City 1000 in Japan, the vertical city that has been proposed to solve Tokyo's housing problems. And it is not just skyscrapers that are a problem. Dams and underground waste deposits may also cause rumblings if they become too large.

  Before the construction of Taipei 101, the Taipei basin was a very stable area with no active earthquake faults at the surface. Its earthquake activity involved micro-earthquakes (less than magnitude 2) happening about once a year.

  However, once Taipei 101 started to rise from the ground, things changed.

  "The number of earthquakes increased to around two micro-earthquakes per year during the construction period [1997 to 2003]. Since the construction finished there have been two larger earthquakes [magnitude 3.8 and 3.2] directly beneath Taipei 101, which were big enough to feel," Lin says in his paper, published in the journal Geophysical Research Letters.

  Using the construction information, Lin has calculated how much pressure Taipei 101 exerts on the ground. The weight of steel and concrete came to more than 700,000 tonnes. This is spread over an area of 15,081m2, meaning that it exerts a huge pressure of 4.7 bars on the ground below.

  "The construction of Taipei 101 is totally different to many other high-rise buildings because it used hybrid structures made of both concrete and steel, to give it added protection from earthquakes and fire. Therefore it has a huge vertical loading on its foundation," Lin says.

  And it is this exceptional downward stress that Lin thinks may have caused the extra earthquakes.

  Other experts are more cautious about blaming the skyscraper for the earthquakes.

  "A building will change the stress on the ground under the building, but this probably won't reach down to around 10km, the level where the earthquakes occurred," says John Vidale, an earthquake expert at the University of California in Los Angeles.

  Zygmunt Lubkowski, an earthquake analyst for the engineering firm Arup, is concerned at the lack of data.

  "Earthquakes occur on time-scales of thousands to millions of years. From just 10 years of earthquake data it is hard to tell if the extra earthquakes are just noise in the signal or due to the building," Lubkowski says.

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