Some scientists were puzzled by the unusual quiet period of quakes before the 8.0-magnitude earthquake that struck southwest China. But others believe there had been precursors, which stood as warnings for a major quake.

"There were no foreshocks and the activity level of minor quakes around the epicenter was low for quite a long time before the earthquake," said Xiu Jigang, deputy director of the China Seismological Bureau (CSB).

He said there were no short-term anomaly of animals, underground water and other typical precursors, which can lead to a prediction of a major earthquake.

Chinese netizens cited tens of thousands of migrating toads before May 12 in Mianyang, a city close to the epicenter of the earthquake in southwestern Sichuan Province, and unusual cloud formations in east China's Shandong Province as quake precursors. But experts said they might not be related to the quake.

"There are complicated reasons for the anomaly of animals and underground water. An earthquake is only one of them along with climate change and weather conditions," said Zhang Guomin, a research fellow with the Research Institute of Seismology under CSB.

Another expert with CSB, He Yongnian, said cloud formation was put forward by Japanese scientists as a way of forecasting earthquakes. But like many other methods of prediction, it is not mature enough.

Besides all those, a Taiwan satellite recorded a sharp drop in ionospheric density above Sichuan before the Wenchuan earthquake, according to a Taiwan newspaper.

The newspaper said that the province's Formosa-3 satellite recorded ionospheric density in the atmosphere of 1.2 million electrically charged particles in some 1,000 square kilometers around Wenchuan six to 15 days before the May 12 earthquake. On May 11, the eve of the quake, ionospheric density had dropped by half to 600,000 charged particles, it said.

"There is absolutely no doubt that there were electronic precursors," said Gary Gilson of the Seismology Research Center at Monash University in Melborne, Australia.

But he said the satellite recording of ionospheric changes may not be practical to use in earthquake forecasting and it would be difficult to do it quickly.

China's earthquake prediction program, which was born with the founding of CSB in 1971, has proved successful at least for two major earthquakes. The bureau made its first successful short-term prediction 13 hours before a 7.3-magnitude quake hit Haicheng in northeast China's Liaoning Province on February 4, 1975. There were frequent foreshocks as well as other anomalies, which clearly pointed to a strong earthquake, the CSB said.