Strong seismic intensity, geological conditions, 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 top reason for so many buildings collapsing after the quake," said Liu Jie, the China National Seismological Network director in charge of earthquake forecasting, on Thursday.

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 km below the earth 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 release 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 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 structural makeup of houses was another reason. "With the same earth surface's moving, large rooms are more prone to collapse than small rooms."

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

If a building was constructed strictly in line with the related quake-resistance standard, it could resist a quake to the designed capacity, he added. "If a building collapsed during a quake within a designed capacity, it is very possibly due to bad construction quality."

He stressed, however, "If a building collapsed during a quake beyond the designed capacity, it is very difficult to judge whether the construction quality directly resulted in the collapse."

Since the earthquake is a natural disaster and very difficult to accurately forecast, experts suggest giving more consideration to the above four points in the after-quake reconstruction.

"While choosing the sites for new buildings, geological conditions must be carefully studied and we must avoid building houses on quake faults or areas prone to landslides," Liu said.

Wang Jun, China Academy of Building Research president, called on carefully studying and learning lessons from every earthquake, "so as to timely adjust the designed quake resistance capacity if the economic conditions permit."

Experts also suggested choosing qualified designers, builders and project supervisors to ensure a building's construction quality.

(Xinhua News Agency June 20, 2008)