The province where the earthquake took place, Sichuan, is in western China and its capital is named Chengdu. It is surrounded by the Sichuan Basin. In the Great Sichuan earthquake, the epicenter was in Wenchuan County, Ngawa Tibetan and Qiang Autonomous Prefecture, which is 80 km north-west of Chengdu; and its depth was 19 km. On May 12, 2008, the earthquake occurred at 14:28 (China Standard Time) and the first tremor was felt.
The earthquake was so strong that it was felt by cities like Beijing and Shanghai, and neighbour places like Hong Kong, Macau, Vietnam and Thailand also experienced tremors. The scope of the areas affected by the quake mainly included the north-east China along the Longmen Shan fault.
The Sichuan Basin was created 55 millions years ago, where the Indo-Australian Plate collided with the Eurasian Plate. These endogenetic forces create fold mountains, and therefore the Sichuan basin is bounded by mountains on all sides. As a consequence, areas around the Sichuan Basin are prone to earthquake. In 1933, there was a 7.5-magnitude earthquake occurring in north-west of Sichuan, causing 7500 deaths.
What has caused such a massive earthquake? To be precise, according to the United States Geological Survey, the earthquake occurred because of the motion on a northeast striking reverse fault on the northwestern edge of the Sichuan Basin. Since the crustal materials moved slowly from the high Tibetan Plateau, when they flowed to the strong crust underlying the Sichuan Basin and southeastern China, pressure gradually developed. The energy was then released in the form of seismic waves and thus led to the occurrence of the earthquake. Another possible cause is that the huge amount of water storage in reservoirs in Sichuan
References: http://www.cea.gov.cn/manage/html/8a8587881632fa5c0116674a018300cf/index.html http://geology.com/events/sichuan-china-earthquake/ http://paper.wenweipo.com/2008/05/13/CH0805130019.htm http://www.stnn.cc:82/society_focus/200805/t20080522_783600.html http://tsinghuafoundation.org/earthquake08/index.php http://en.wikipedia.org/wiki/1976_Tangshan_earthquake http://harvardhumanitarian.org/index2.php?option=com_content&do_pdf=1&id=162 http://www.ntdtv.com/xtr/b5/2008/05/21/a144365.html All figures were extracted from: http://en.wikipedia.org/wiki/2008_Sichuan_earthquake