Sichuan was not known as a risk area - NRC

via Wikipedia

Yesterday the Dutch daily NRC Handelsblad had in interesting piece in its science-section on the Sichuan earthquake and how it has surprised scientists. I have not seen its equivalent in English, so I used Google Translate to make it into English and corrected a few obvious mistakes. Still: do not consider this to be a professional translation.

Houses in the Chinese province of Sichuan required under local regulations by no means to be able to withstand an earthquake as powerful as the beving last Monday. That's not the fault of China's government who did has not properly ensure that its citizens' safety. The Chinese building regulations are based on a scientific survey of the United Nations: the Global Seismic Hazard Assessment Program. According to that assessment in the affected area there is a 10 percent chance of a quake with an intensity of 7 on the Mercalli scale of a subjective experience: it will be felt by people while they sit in the car and they will flee out of buildings, but no it is no tremor where a well-constructed building collapse. The earthquake on Monday with a power of ten on the Mercalli scale, was certainly ten times as heavy. According to the Chinese state newsagency Xinhua is around the city of Chengdu 80 percent of the buildings collapsed.

This underestimation confirms according to Mark van der Meijde affiliated with the International Institute for Earth observation ICT in Enschede, the scientific limitations in predicting earthquakes. Not only the time of the earthquake is unpredictable, also the valuation of the risks still leaves much to be desired. "Extreme, rare disasters change the figures in your model," he says. "The maps with earthquake protection are therefore inaccurate. The only thing you could do in the construction industry is observing a wide margin."
The earthquake occurred on or near the fault line in the Longmenshan-south east side of the Tibetan plateau. This plateau is pushed aside-by India with a few centimetres per year year as it advances northwards. This creates the mountains of the Himalayas and the Tibetan plateau is from the south pushed away. It goes to the east and along the fault line-Longmenshan to the southeast.
The Chinese province of Sichuan was in 1933 also if hit by a major earthquake. But that quake was not analyzed with modern equipment so that the estimated strength, 7.5 on the Richter scale, in reality not very accurate. That is why this earthquake is in the UN models not as prominent as recent large earthquakes, said Hong Hao, professor of structural dynamics at the University of Western Australia in Perth, on the phone. "The first earthquake that was really well registered, was the 1940 earthquake in El Centro found in Imperial Valley in California."

Stricter rules had the effect of the earthquake on Monday in China have not changed. "The money is missing to build houses that can survive such an earthquake," said Hong Hao. "You need reinforced concrete of good quality. Good cheap solutions are not really there. There is little money for scientific research in this area. " The strength of the earthquake in Sichuan was according to the U.S. geological service 7.9 on the Richter scale. This scale gives a measure of the energy released by an earthquake. Hundreds seismometers, spread over the whole earth, are registering vibrations in the earth's crust, caused by pressure by the earth move. Adjusted for the distance is the strength of the earthquake, according to Richter at any place in the world to assess. Unlike the Richter scale the Mercalli scale tries to estimate how the intensity of an earthquake at various places on earth is experienced.The maps of the U.S. geological service, made in the hours after the tremor, show a maximum force of around ten on this scale of Mercalli. 'Particularly devastating, "according to the translation of the KNMI [the Dutch weather institute]: destruction of many buildings, damage to dams and dikes; shifting of soil and cracks in the earth. But messages from the disaster show that even these cards underestimate the actual intensity of the eathquake. "The U.S. Geological Service maps are taking California as a starting point," says Van der Meijde. "But the Sichuan basin is a sedimentary basin in which loose material has collected. An area where the ground is not structurally connected is experiencing more damage of a vibration in the earth's crust. Compare it with a slap on a sturdy table that motion caused less than a slap in a bucket of water. "