Then in 1995, scientists used various precursors including foreshocks and variation in water levels and temperature to warn local authorities one day before a major earthquake struck Menglian County in southwestern Yunnan Province.
However, the precursors were still elusive, with the lack of a short-term prediction before the 7.8-magnitude Tangshan earthquake in 1976. So the successful prediction was limited to a small percentage of quakes mainly with frequent foreshocks, said He Yongnian.
"The methods Chinese scientists use for earthquake prediction are mainly empirical," said Gibson, "but they are still useful."
Some Chinese experts said that the long-term and medium-term quake prediction is much more successful in China than short-term prediction.
Deputy director of the Institute of Geology of CSB Xu Xiwei said short-term prediction means a warning of "time, place and magnitude of an earthquake" shortly before it takes place, which is a very complicated issue.
"Earthquake forecasting remains a puzzle for the world," said deputy director of China Earthquake Networks Center Zhang Xiaodong.
However, research fellow with the Institute of Crustal Dynamics of CSB Qiu Zehua said, China would be able to make substantial progress in short-term prediction if more monitoring stations could be set up in areas which have been found prone to earthquakes.
"I believe that there must be precursors before an earthquake, but they might only happen in areas around the epicenter," said Qiu. "The problem is that our monitoring stations are too scattered to observe them. We should focus our monitoring efforts in certain targeted areas."
Researchers in China, a country which suffered 33 percent of the world's inland earthquakes in the 20th century, are catching up with international seismological studies. In the measurement of the earthquake in Sichuan, the first report by CSB set the magnitude at 7.6 on the Richter scale after the quake on May 12. The quake was shortly upgraded to 7.8, based on more statistics from monitoring stations. Then the bureau revised the magnitude to 8.0 on Sunday, with reference to foreign observatories.
The magnitude was revised upward after specialists carried out "real-time and detailed measurements of the quake according to international practices," said Luo Zhuoli, an expert with CSB.
The quake, claiming 34,073 lives as of 4:30 p.m. Monday and leaving 245,108 injured so far, has caused serious damage to buildings, bridges and other public facilities in an area of more than 100,000 square kilometers.
(Xinhua News Agency May 20, 2008)