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Pan J.,State Key Laboratory of Continental Tectonics and Dynamics | Pan J.,Chinese Academy of Geological Sciences | Li H.,State Key Laboratory of Continental Tectonics and Dynamics | Li H.,Chinese Academy of Geological Sciences | And 8 more authors.
Tectonophysics | Year: 2014

On 12 May 2008, the Wenchuan earthquake (Mw 7.9) produced complicated thrust-type co-seismic surface rupture zones, which encompass the dextral-slip thrust of the Yingxiu-Beichuan fault, the approximately pure thrust of the Guanxian-Anxian fault, and the sinistral-slip thrust of the Xiaoyudong rupture zone located between the former two. In order to understand the faulting mechanism, we discuss the rupture process by examining the segmentation and kinematics of the surface rupture zones, together with the co-seismic fault striations at various sites. Based on the two along-strike main displacement peaks (6-6.5. m and 11-12. m) and on the different geometric and kinematic patterns for the southern and northern segments of the surface rupture zones, we find that the Wenchuan earthquake might have consisted of two rupture stages, which is in agreement with seismic wave inversion results. By comparing the kinematics of fault striations occurring in the Bajiaomiao and Beichuan areas, it suggests that during the first stage, thrusting along both the Yingxiu-Beichuan fault and Guanxian-Anxian fault produced the ~ 80-100. km-long Yingxiu-Qingping surface rupture segment and the ~ 80 km-long Guanxian-Anxian surface rupture zone, respectively. Then, faulting was triggered along the Yingxiu-Beichuan fault by the first rupture process, yielding the second rupture stage, which was characterized by dextral strike-slip (or dextral oblique thrusting). Due to the overlap between the two rupture stages, the southern segment (Yingxiu-Qingping) of the Yingxiu-Beichuan rupture zone comprises two different processes while the northern segment (Gaochuan-Beichuan-Shikan) only suggests one rupture phase. © 2013 Elsevier B.V. Source

Zhang Y.,Chinese Academy of Geological Sciences | Cheng Y.,Sichuan Academy of Geological Survey | Yin Y.,China Geological Environmental Monitoring Institute | Wang J.,Chinese Academy of Geological Sciences
Global View of Engineering Geology and the Environment - Proceedings of the International Symposium and 9th Asian Regional Conference of IAEG | Year: 2013

The high-position debris flow is a kind of debris flows characteristic of the Wenchuan earthquake region. As a conception, it graps the main points of the post-earthquake geological disasters there, that is, the long-term lasting and exetremly destructive force of high-position debris flows. This kind of debris flows can be recognized using the following indexes: the height difference between the material source and the debris flow exit or the gully mouth is more than 350 m; (2) the volume of source accumulation is equal to or more than 60 × 104 m3; and (3) the gradient ratio of a debris flow channel way or gully is equal to or more than 270‰. The control of high-position debris flow gullies is a difficult task which people in the earthquake region would confront in the coming several ten years. © 2013 Taylor & Francis Group, London, ISBN. Source

Zhang Y.,Chinese Academy of Geological Sciences | Cheng Y.,Sichuan Academy of Geological Survey | Yin Y.,China Geological Environmental Monitoring Institute | Lan H.,CAS Beijing Institute of Geographic Sciences and Nature Resources Research | And 2 more authors.
Engineering Geology | Year: 2014

After the 12 May 2008 Wenchuan earthquake in Sichuan, a special type of long-term active compounded geohazard, namely high-position debris flow, frequently occurred in the seismic area. 136 high-position debris flow gullies were recognized from the 796 active debris flow gullies to investigate distinctive characteristics of such debris flows in terms of their geomorphological conditions, spatial distribution and causal factors. They usually initiate at high steep slopes and have high mobility and long runout. High-position debris flows have significantly larger magnitude and destructive force than those of common debris flows observed in the regions not affected by recent large earthquake. The high-position debris flows have been recognized as the most threatening geohazards among post-earthquake geological disasters in the Wenchuan earthquake region. Detailed investigation on 45 debris flows suggests three quantitative indices for recognizing the high-position debris flows: (i) the height difference between the material source and the debris flow exit or gully mouth should be larger than 350m; (ii) the volume of source debris accumulation should be bigger than 1.0×106m3; and (iii) the gradient ratio of a debris flow channel should be steeper than 270‰. The spatial distribution of high-position debris flows was highly controlled by the seismogenic fault. Due to the abundant loose materials caused by earthquake, the rainfall threshold triggering high-position debris flows was found to have decreased at least 36.4-63.7% when compared to the pre-earthquake one. Three disaster-forming models have been identified for the high-position debris flows in the Wenchuan earthquake region. The long-term post-earthquake effect of high-position debris flow is becoming more concerned in the area of high intensity. The characteristics of long-term activity of high-position debris flows using a case study indicate a 15years of extremely active period after a strong earthquake such as the Wenchuan one. A number of issues regarding the classification, investigation, runout prediction and controlling measures for high-position debris flow are discussed to provide insight into the foundation for high-position debris flow mitigation and its risk assessment. © 2014 Elsevier B.V. Source

Zhang Y.-S.,Chinese Academy of Geological Sciences | Cheng Y.-L.,Sichuan Academy of Geological Survey | Yao X.,Chinese Academy of Geological Sciences | Wang J.,Sichuan Academy of Geological Survey | And 2 more authors.
Geological Bulletin of China | Year: 2013

Since the Wenchuan Ms 8.0 earthquake which occurred on May 12, 2008, the geohazards in the earthquake-stricken region have assumed obvious "post-earthquake effect" characteristics. The great earthquake caused a large amount of rock falls and landslides on the mountain ridges and watershed slopes, and the landslide masses were scattered on the upper-middle part of the mountains. Under intense rainfall, a lot of loose deposits were brought together along the steep gully, and their movement was accelerated to form a devastating high-position debris flow, thus constituting a typical earthquake-landslide-debris flow chain. Based on a brief summarization of the co-earthquake geohazards in the Wenchuan earthquake area, this paper has discussed six types of geohazards during the rain season after the Wenchuan earthquake as well as their appearance styles. The evolution process of earthquake-landslide-debris flow disaster chain can be divided into four stages, i.e., nurtured stage, co-earthquake landslides stage, post earthquake landslide-debris flow development stage, and high debris flow evolution stage. The discrimination indexes of high debris flows are put forward, and their distribution features, motive changing trend and corresponding control countermeasures are discussed. Source

Wang H.,Chinese Academy of Geological Sciences | Li H.-B.,Chinese Academy of Geological Sciences | Si J.-L.,Chinese Academy of Geological Sciences | Sun Z.-M.,Chinese Academy of Geological Sciences | And 15 more authors.
Acta Geoscientica Sinica | Year: 2015

After the 2008 Wenchuan earthquake, questions as to how earthquakes occur and when the next large earthquake happens have been asked. These questions are involved in the scientific questions which have been unsolved by geologists and geophysicists, for examples, how do faults rupture? How do faults heal in preparation for the next earthquake? The authors are trying to answer these questions by investigating the internal structures of the Wenchuan earthquake fault zone, fault friction behavior and healing process. The principal slip zone of the Wenchuan earthquake was determined by detailed research based on the WFSD-1 core and outcrop investigation. The dynamic weakening and friction behavior of the Wenchuan earthquake fault was analyzed by long-term temperature monitoring. Long-term hydrological monitoring reveals permeability variations in the Wenchuan fault zone which indicate fault healing process. Based on the information obtained, this paper discusses the Wenchuan earthquake faulting process and the above questions. It is confirmed that the Wenchuan earthquake fault zone (the Yingxiu-Beichuan fault zone) was 105~240 m wide and consisted of 5 different units with different fault rocks, where seismic events occurred repeatedly with multiple coseismic fault weakening mechanisms. Coseismic graphitization occurred during the Wenchuan earthquake; the lowest dynamic friction coefficient (0.02) was measured, and for the first time, the signals showing quick fault healing were recorded. These results not only directly answer the key questions that have puzzled geologists and seismophysiccal researchers for decades but also have important significance for improving the seismic faulting theory and understanding the Wenchuan earthquake fault mechanisms, thus providing a theoretical basis for seismic prevention and disaster mitigation. ©, 2015, Science Press. All right reserved. Source

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