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Tectonic background of disaster zones

2018-04-18  |   Editor : houguangbing  
Category : Natural Sciences

China is among the countries that have most serious seismic activities and hazards.

According to statistics, approximately 35% continental earthquakes of magnitude 7.0 or larger occured in China. In the 20th century, 120 million global fatalities were caused by the earthquake, presenting 59 million deaths in china. Moreover,nearly more than one third of lands,also half of cities are mainly concentrated in earthquake -intensity areas. During 1900 and 2007,the earthquakes with magnitude between 7.0 and 7.9 hit China continent 70 times,including 6 times magnitude(>8.0). The disasters affected 28 provinces,resulting in 59 million fatalities and 76 million injuries. These took hundreds of millions of the toll of the victims. Engaging with earthquake prevention and disaster mitigation can maximally decrease disasters, and ensure people's lives and property. Further, it's of great significance for maintaining social stability and development.

The occurrences of disasters are due to the combination of two indispensable and determing factors, disaster-inducing force and disaster-bearing body. In terms of earthquake disasters, disaster-inducing force is the surface vibrations, fault dislocations as well as various geological disaster events triggered by destructive earthquakes. Disaster-bearing body is understood to be people,buildings and social economy and so on.

Meizoseismic areas are determined by peripheral dynamic environments and internal active tectonics. China continent is located in the southeast of the Eurasian Plate,and also surrounded by Indian Ocean Plate, Pacific Plate and Philippine Sea Plate. The collision of interpolate and dynamics of intraplate are responsible for the spatial distribution of seismic-tectonic zone. In this case, one of the most striking characteristics is a lot of exclusive huge and large active tectonic belts that divide China continent into different levels of active tectonic blocks. The edges of active tectonic blocks are important seismic belt. In historical times,almost all master earthquakes of magnitude 8.0 or larger and more than 80% huge earthquakes of magnitude 7.0 -7.9 took place along the boundaries of active tectonic blocks.

In 2008,Wenchuan Ms 8.0 earthquake struck in the eastern boundary of Tibet plateau, the Longmen shan fault in the middle section of the north-south tectonic belt. The initial rupture is in the mid-south of Longmen shan fault in vicinity of Yingxiu town of Wenchuan county. The rupture extends north-eastward along Beichuan-Yingxiu fault(central fault) and Guanxiu-Jiangyou fault(piedmont fault). Surface rupture zone along Beichuan-Yingxiu fault reaches 240km,showing the effect of the trust faulting of dextral slip component. Maximum vertical displacement is 9m±0.5m. Maximum dextral slip displacement is 4.9km. Surface rupture zone along Guanxiu-Jiangyou fault reaches 90km. It's the typical reverse-fault earthquake surface rupture. Maximum vertical displacement is 3.5m. Besides,Xiaoyudong fault along with thrust from north to west and left-lateral strike slip motion developed between these two faults. It's about 6km.

Based on field surveys, the highest seismic intensity of Wenchuan earthquake is XI degree, of which the area is 0.24 square kilometers, distributing in a stripe shape on the centroids of Yingxiu town, Wenchuan county and Beichuan county,Sichuan province.

The areas of intensityⅩ(MMI), intensity Ⅸ, intensityⅧ, intensity Ⅶ, and intensityⅥ are 0.31 million square kilometers, 0.77 million square kilometers, 2.78 million square kilometers, 8.44 million square kilometers, 31.49 million square kilometers respectively. The total area of intensity above Ⅵ account for 44 million square kilometers. The distribution range compromises Sichuan province, Gansu province, Shanxi province, Yunan province, Ningxia Hui Autonomous Region and Chongqing city. severe and heavy disaster areas(intensity Ⅷ or above) are about 4.10 million square kilometers.

Since Wenchuan surface ruptures mainly gather in Longmen shan district. Strong earthquakes lead to serious secondary damage. In severe disaster areas, especially around surface rupture area, the earthquake induced a large number of geological hazards,such as landslides,collapses, crack stones,debris flows, earth fissure,sand liquefaction, surface subsidence.

Wenchuan earthquake-stricken area is located in the densely populated area,Sichuan province. Strong surface vibration,earthquake faulting,and various geological disasters along or jointly play a role, which primarily causes the mass damaged housing and causalties.

The information is provided by presentation of "Songliao Basin" from China National Petroleum Corporation.

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