The strong seismic intensity and geological conditions along with structural design and construction quality were the four major reasons why so many buildings collapsed in the May 12 earthquake that rocked Sichuan Province, according to Chinese experts.

"The strong seismic intensity is the main reason why so many buildings collapsed after the quake," said Liu Jie, the China National Seismological Network director in charge of earthquake forecasting.

The death toll of the massive earthquake increased by four overnight to 69,180, with 17,406 still missing, the State Council Information Office said.

Preliminary statistics showed more than 6.5 million houses collapsed and another 23 million were damaged after the devastating earthquake jolted Sichuan. "The seismic intensity of this 8.0-magnitude earthquake is as high as 11.0," said Liu, who immediately traveled to Sichuan following the quake.

According to China's quake-resistance standard, all buildings in Sichuan are built with a designed capacity to resist earthquakes of seismic intensity 7.0.

This quake was a shallow-focus earthquake about 10 kilometers below the earth's surface.

The total seismic energy released was astonishingly huge and felt in nearly the whole of China, except Heilongjiang, Jilin and Xinjiang.

Bo Jingshan, Institute of Disaster Prevention Science and Technology president, said the energy released was equal to 5,600 times that of the atomic bomb the United States dropped on Hiroshima, Japan, in 1945.

"Such a devastating quake (in Sichuan) caused a 300-kilometer crack of the earth's surface and all the mountains, rivers, roads, bridges and houses on the fault were damaged."

According to Liu, the geological condition under the houses was the second reason for their collapse.

"Our field investigation shows that the highest land rise after the quake is six meters," he said. "If the building is on the fault, even the firmest houses are unable to resist the quake."

The final reason was construction quality. "Houses of bad quality are easily destroyed by an earthquake," Liu said.

(Xinhua News Agency June 20, 2008)