Experts say that an earthquake of magnitude 8 occurs once in a thousand years, and the largest aftershock of Wenchuan earthquake is magnitude 6.

Special topic: Wenchuan earthquake in Sichuan

　　At the end of the research on "The Impact of Wenchuan Earthquake on Chengdu", experts predicted that the late strong aftershocks would not cause damage to Chengdu.

　　The research on "The Influence of Wenchuan Earthquake on Chengdu" ended recently. Officials and experts from Chengdu Earthquake Disaster Reduction Bureau said yesterday that the Wenchuan earthquake was a "main shock-aftershock" type, and the recurrence period of magnitude 8 earthquakes in Longmenshan fault zone was more than a thousand years. At present, more than 30,000 aftershocks have been recorded, showing a trend of "ups and downs" and having little impact on Chengdu.

　　Long-short axis effect Chengdu main city is safe

　　Yuan Haiquan, deputy director of the Municipal Bureau of Earthquake Prevention and Disaster Reduction, Zhu Jieshou, a professor at Chengdu University of Technology, and Hong Shizhong, a researcher on earthquake prevention in Sichuan Province, disclosed at the 27th special press conference of the municipal government information office yesterday that the research report came to two conclusions: (1) During the Wenchuan M8 earthquake, the earthquake damage in the main city of Chengdu was slight, which was determined by the general law of earthquake intensity attenuation, the structural position of Chengdu and the seismic fortification of buildings, so it was inevitable; (2) After the Wenchuan M8 earthquake, Chengdu is still safe.

　　The so-called "long-short axis effect" means that the distribution of seismic intensity is generally flat and oblong, and the attenuation of seismic intensity along the short axis direction is much faster than that along the long axis direction. Yuan Haiquan explained that it is this law that makes the main city of Chengdu safe and sound: the long axis direction of Wenchuan earthquake intensity distribution is exactly the same as the extension direction of Longmenshan fault zone, which is northeast-southwest direction, while the main city of Chengdu and the whole Chengdu plain are located in the short axis direction which is generally perpendicular to the strike of Longmenshan fault zone. In this direction, the intensity decays very quickly, so the earthquake damage in the main urban area of Chengdu is quite slight.

　　Experts also agree that Chengdu’s adherence to seismic fortification for many years has played an important role in reducing earthquake damage. Yuan Haiquan introduced that since the 1980s, Chengdu has carried out seismic fortification work for buildings and structures. In most areas of Chengdu, general industrial and civil buildings are fortified according to the seismic intensity of 7 degrees. In addition, after the large-scale transformation of the old city, there are few old houses left, and the seismic capacity of urban buildings has been greatly improved.

　　The ups and downs attenuate the future aftershocks with a maximum magnitude of 6.

　　In view of the fact that Chengdu is still safe after the Wenchuan earthquake, experts explain that there are mainly two types of earthquakes that have a great impact on Chengdu: external earthquakes and local earthquakes. The research shows that there are many large-scale active fault zones in the periphery of Chengdu, among which Longmenshan fault zone is the closest and most threatening to Chengdu. A variety of studies show that the recurrence period of the Longmenshan M 8 earthquake is more than a thousand years. Therefore, after the Wenchuan earthquake, there is no need to worry about the occurrence of a major earthquake of magnitude 8 or above in the Longmenshan belt for a long time.

　　Experts predict that the maximum magnitude of aftershocks in the future will be around 6. "Since the intensity of the main city of Chengdu was only 7 degrees during the main earthquake of magnitude 8, even if there are strong aftershocks of magnitude 5 and 6 in the future, it will naturally not cause any damage."

　　At present, the number of aftershocks of the Wenchuan earthquake has reached 37,816, including 8 with magnitude 6 or above. Hong Shizhong said that more than half a year has proved that the intensity of aftershocks is declining, and there have been no aftershocks of magnitude 5 or above for a long time. He believes that the Wenchuan aftershock will be "fluctuating attenuation", and it is also possible to have a strong aftershock of magnitude 5 ~ 6. He reminded that the aftershock area in Yingxiu-Qingchuan area should be especially vigilant, and aftershocks may trigger new secondary disasters in mountainous areas. But for Chengdu, aftershocks will not cause damage, but at most they will have a sense of shock.

　　"After the Tangshan earthquake, there were aftershocks of magnitude 5 or above in 1979; Haicheng earthquake in 1975 and aftershocks in 1977. " Hong Shizhong said that the aftershocks of the Wenchuan earthquake will continue for quite some time.

　　In addition, according to the report, there are some fault zones in Sichuan where major earthquakes may occur. In history, strong earthquakes in these areas have had some impact on the main urban area of Chengdu, but the maximum intensity is only 6 degrees. The calculation shows that even if an earthquake of magnitude 8 occurs in these areas, the intensity of the main urban area of Chengdu will not reach 7 degrees because they are all more than 150 kilometers away from Chengdu.

　　There is little possibility of a major earthquake in Chengdu Plain.

　　With regard to earthquake prediction, experts said that it is still a worldwide problem to predict when and where an earthquake will occur, but it is much easier to judge the trend. Regarding the possibility of an earthquake in Chengdu in the future, Yuan Haiquan said that the Chengdu Plain belongs to the Sichuan Basin, which is a part of the Yangtze block. "The Yangtze block is a fairly hard and stable block. Therefore, the seismic activity of the whole Sichuan Basin, including Chengdu Plain, is much weaker than that of the Longmenshan Belt, and there has not been a strong earthquake of magnitude 6 or above in the basin so far. Although there are some faults in Chengdu, both the scale and the activity of the faults are far from being compared with the Longmenshan fault zone, and there is little possibility of an earthquake of magnitude 8. On the whole, Chengdu Plain is quite safe. "

　　Historical records show that Chengdu suffered a strong earthquake of magnitude 7 or above once every 100 years in more than 2000 years. (Tang Xiaotao)

Editor: Wang Jiaolong